The Mineral Basis of Soils

1.1. What is the Earth’s crust and why is it important for soil science?

The Earth’s crust is the upper solid shell of our planet. On the scale of the entire Earth, it is extremely thin: its thickness ranges from 5–10 km beneath the oceans to 30–70 km beneath the continents, while the Earth’s radius is 6370 km. However, it is precisely this thin layer that is the “source” from which all soils of our planet originate (Buol et al., 2011; Weil & Brady, 2017).

Why is the Earth’s crust of interest to a soil scientist? Because:

1. The Earth’s crust is the supplier of raw material. All mineral components of soils come from the rocks that make up the Earth’s crust.

2. The composition of the Earth’s crust determines the potential set of chemical elements that can pass into the soil and become available to plants.

3. The diversity of rocks in the Earth’s crust is the main reason why soils in different territories differ so greatly, even under identical climatic conditions.

1.2. Chemical composition of the Earth’s crust

Let us look into the “chemical kitchen” of our planet. The average chemical composition of the Earth’s crust (Table 1) gives us an understanding of which elements build minerals, and from minerals—rocks (Scheffer et al., 2018).

Table 1. Average chemical composition of the Earth’s crust

Source: Scheffer et al. (2018)

Pay attention to two key points that are extremely important for soil science:

First. Almost half the mass of the Earth’s crust (47%) is oxygen. In volume terms, oxygen occupies almost 90%! This means that the crystal lattices of minerals are built mainly from large oxygen ions, while all other elements are “squeezed” into the voids between them. This is why all silicate minerals—and silicates make up more than 90% of the volume of the Earth’s crust—are based on a tetrahedral structure: four oxygen ions surround one silicon ion (White, 2006; Eash et al., 2016).

Second. By mass, oxygen ranks first, silicon second. Their compounds—silica (SiO₂) and silicates—are the main components of the Earth’s crust. Therefore, it is no coincidence that most soil minerals are silicates and aluminosilicates (Foth, 1990; Scheffer et al., 2018).

1.3. Mineral and rock composition of the Earth’s crust

Let us move from chemical elements to minerals and rocks. According to the data presented in Table 2, more than 90% of the volume of the Earth’s crust is composed of silicate minerals.

Table 2. Mineral composition of the Earth’s crust

Source: Scheffer et al. (2018)

When it comes to the rocks that compose the Earth’s crust, the distribution structure is as follows (from the same source):

• Basalts, gabbros, and other mafic igneous rocks — 42.6% by volume
• Gneisses — 21.4%
• Granites, granodiorites, diorites, syenites — 22.0%
• Crystalline schists — 5.1%
• Clays, argillites — 4.2%
• Carbonate rocks — 2.0%
• Sands, sandstones — 1.7%

This distribution is important to remember: mafic (basaltic) rocks predominate in the Earth’s crust due to oceanic crust, whereas on the continental surface, acidic rocks (granites) and sedimentary formations are more common (Scheffer et al., 2018).

1.4. The Earth’s crust as a source of soil-forming rocks

For soil science, it is important not only what the Earth’s crust consists of as a whole, but also which rocks are exposed at the surface. According to geological data, the land surface is covered by:

• Sedimentary rocks — cover about 75% of the continental surface (Weil & Brady, 2017; White, 2006)
• Igneous and metamorphic rocks — are exposed on about 25% of the land area (Eash et al., 2016)

It is these rocks, when exposed at the surface, that become soil-forming parent materials. They enter the zone of active interaction with the atmosphere, hydrosphere, and biosphere—and the process of weathering begins, which we will examine in detail in the next lecture.

1.5. Why different rocks form different soils — the first answer

Now we can give the first, most general answer to our key question. Different rocks form different soils because:

1. The initial chemical composition of rocks determines which set of chemical elements will enter the soil. For example, granites are rich in quartz and potassium feldspars, while basalts are rich in calcium plagioclases and ferromagnesian minerals. Accordingly, soils on granites will be more acidic and poorer in calcium and magnesium than soils on basalts (Buol et al., 2011; Foth, 1990).

2. The mineral composition of rocks determines which secondary clay minerals will form. On granites in humid climates, kaolinite and aluminum hydroxides form, whereas on basalts—montmorillonite and iron-rich clay minerals (Scheffer et al., 2018; Weil & Brady, 2017).

3. The physical properties of rocks (porosity, fracturing, hardness) affect the rate and depth of weathering. Granites with their coarse-crystalline structure break down more rapidly physically and give deeper profiles than fine-crystalline basalts (Buol et al., 2011).

4. The presence or absence of carbonates determines how long the rock will “resist” acidification and how quickly leaching processes begin. Loess (carbonate-rich rocks) gives Chernozems with a neutral reaction, while carbonate-free sands give Podzols with an acidic reaction (Scheffer et al., 2018; Weil & Brady, 2017).

1.6. Main groups of rocks as sources of soil formation

In soil science, it is customary to distinguish three large groups of rocks, each with its own formation history and its own characteristics as a soil-forming material (Weil & Brady, 2017; Eash et al., 2016):

Igneous rocks — formed when magma solidifies. They are divided into:

• Plutonic (intrusive) — slow cooling in the depths, coarse-crystalline structure (granite, gabbro, diorite)
• Volcanic (extrusive) — rapid cooling at the surface, fine-crystalline or glassy structure (rhyolite, basalt, andesite)

Igneous rocks are the primary, “original” rocks. It is with their weathering that the formation of sedimentary rocks and, ultimately, soils begins.

Sedimentary rocks — formed from the breakdown products of other rocks. They cover most of the land surface and are therefore the most widespread soil-forming rocks. They are divided into:

• Clastic (sandstones, conglomerates, breccias)
• Clayey (argillites, clay shales)
• Chemical (limestones, dolomites, gypsum, rock salt)
• Organogenic (shell limestones, chalk, coals)

Metamorphic rocks — formed by recrystallization of igneous and sedimentary rocks under the influence of high temperatures and pressures. They are close in composition to the original rocks but have a different structure (gneisses, schists, quartzites, marbles).

Each of these groups, as we will see later, has its own characteristic set of minerals and chemical elements, which ultimately determines the properties of the soils that form on them.

Conclusion to the section

Thus, we have become acquainted with what the Earth’s crust is, its composition, and why this knowledge is important for a soil scientist. We have learned that:

1. The Earth’s crust is composed mainly of silicates and aluminosilicates—this determines the mineralogical composition of future soils.

2. Oxygen and silicon are the main elements of the Earth’s crust; they form the basis of the crystal lattices of minerals.

3. The three types of rocks (igneous, sedimentary, and metamorphic) are the three main sources of soil-forming materials.

4. Differences in the chemical, mineralogical, and physical composition of these rocks are the main reason why different soils form on different rocks.

In the next part, we will move on to a detailed examination of each group of rocks and the minerals that compose them.

2.1. Origin and general characteristics

Igneous rocks are the primary rocks of the Earth’s crust. They form by the solidification and crystallization of molten magma that rises from the Earth’s depths toward its surface (Weil & Brady, 2017; Eash et al., 2016). In this sense, igneous rocks can be called the “progenitors” of all other rocks, because it is their destruction that provides the material for sedimentary rocks, and their transformation—for metamorphic rocks.

Depending on the cooling conditions of the magma, two main groups of igneous rocks are distinguished (Scheffer et al., 2018; Weil & Brady, 2017):

Plutonic (intrusive) rocks are formed by the slow cooling of magma within the Earth’s crust. Since cooling proceeds very slowly—thousands and millions of years—the minerals have time to grow into large, well-formed crystals. Such rocks have a holocrystalline, coarse-grained structure. The classic example is granite, which can be seen as pink or gray granular masses.

Volcanic (extrusive) rocks are formed by the rapid cooling of magma at the Earth’s surface. The cooling rate is high, so crystals do not have time to grow to large sizes; the rock is fine-crystalline, cryptocrystalline, or even glassy (obsidian). An example is basalt, which often has a dark gray to black color and very dense texture. If such a rock contains individual large crystals against a fine-grained groundmass, the structure is called porphyritic (Scheffer et al., 2018; Weil & Brady, 2017).

Why is this distinction important for soil science? Plutonic rocks (granites), due to their coarse-grained nature, break down more rapidly by physical weathering (thermal expansion of different minerals creates internal stresses), so they develop thicker layers of loose material (regolith) (Buol et al., 2011). Extrusive rocks (basalts) are more resistant to physical breakdown, but their minerals are chemically more active, so chemical weathering proceeds faster and deeper (Scheffer et al., 2018).

2.2. Classification of igneous rocks by silica content

The main classification criterion for igneous rocks is the content of silicon dioxide (SiO₂). This indicator determines which minerals will predominate in the rock and, consequently, which chemical elements will enter the future soil (Scheffer et al., 2018; White, 2006).

Four groups are distinguished (Table 3):

Table 3. Classification of igneous rocks by SiO₂ content

Compiled from data: Scheffer et al. (2018); Weil & Brady (2017); Eash et al. (2016); White (2006)

Note the color scheme: acidic rocks (granites) are light, pink, gray because they contain many light-colored minerals (quartz, feldspars). Mafic and ultramafic rocks are dark, almost black or dark green (gabbro, basalt, peridotite) due to the abundance of dark ferromagnesian minerals (pyroxenes, amphiboles, olivine). This link between color and composition is an important diagnostic feature in the field (Eash et al., 2016; Weil & Brady, 2017).

2.3. Mineral composition of the main igneous rocks

2.4. Significance of igneous rocks for soil formation

Igneous rocks are unevenly distributed on the Earth’s surface. According to geological data, they are exposed on about 25% of the land area, but in mountainous regions (Urals, Caucasus, Siberia, Scandinavian Mountains, Canadian Shield) they can occupy large areas (Weil & Brady, 2017; Eash et al., 2016).

In the context of our key question—why different rocks give different soils—igneous rocks provide vivid examples:

1. On acidic rocks (granites) in humid climates, acidic, base-poor soils with low fertility form. Example: Podzols and Sod-Podzolic soils on granitic moraines in the northern taiga. The clay minerals are dominated by kaolinite, hydromicas, and iron and aluminum hydroxides (Scheffer et al., 2018; Buol et al., 2011).

2. On mafic rocks (basalts, gabbros) even in humid climates, more fertile soils form—Brown soils, Ferrallitic soils, rich in iron and clay. They contain more bases and have higher cation exchange capacity (Buol et al., 2011; Foth, 1990).

3. On ultramafic rocks peculiar soils with unique chemistry arise, often with toxicity problems for plants (Serpentine soils). They have high concentrations of Mg, Ni, Cr, which strongly affects vegetation composition and limits agricultural use (Buol et al., 2011; Weil & Brady, 2017).

In addition, it is important to remember that the grain size of the rock matters for soil formation. As noted by Scheffer et al. (2018), coarse-grained granites are more susceptible to physical weathering and form thick weathering crusts than fine-grained basalts, although the latter are chemically more active. This creates an interesting contradiction: on granites, loose material accumulates faster, but it is poorer in bases, while on basalts, weathering is chemically deeper, but physical breakdown is slower.

Igneous rocks are only the beginning of the journey. In the next section, we will see how these rocks break down and are redeposited to form sedimentary rocks, which cover 75% of the land surface and serve as the main source of soil-forming materials in most agricultural regions of the world.

3.1. Origin and general characteristics

Sedimentary rocks are the most widespread rocks on the land surface. They cover about 75% of the continental area (Weil & Brady, 2017; Eash et al., 2016), and therefore they serve as the main source of soil-forming materials for most agricultural territories. According to geological data, sedimentary rocks make up only about 8% of the volume of the Earth’s crust, but their proportion increases sharply at the surface (Scheffer et al., 2018).

The key difference between sedimentary rocks and igneous rocks lies in their origin. Sedimentary rocks are formed from the breakdown products of other rocks (igneous, metamorphic, or even pre‑existing sedimentary rocks) by sedimentation—the deposition of material on the bottom of water bodies or on the land surface, followed by compaction and cementation (diagenesis). As textbook authors vividly express it, “sedimentary rocks are former rocks that have passed through a cycle of weathering, transport, and deposition” (Weil & Brady, 2017; White, 2006).

This fact has fundamental significance for soil science. Sedimentary rocks are already products of weathering to some degree. Many minerals in them have already been transformed, some soluble compounds have been lost, and resistant minerals (quartz, clay minerals) are enriched relative to the original igneous rocks. Therefore, soils forming on sedimentary rocks often inherit properties that are not directly related to processes occurring under current conditions, but reflect the history of weathering and transport of the material (Buol et al., 2011; Scheffer et al., 2018).

3.2. Classification of sedimentary rocks

In soil science and geology, it is customary to divide sedimentary rocks by genesis (mode of formation) into three large groups (Scheffer et al., 2018; Weil & Brady, 2017):

1. Clastic (detrital) — formed by mechanical accumulation of fragments of various sizes, later cemented by natural “glue” (clay material, calcite, silica, iron oxides).

2. Clayey — composed mainly of clay minerals formed by chemical weathering and deposited in quiet water environments.

3. Chemical and organogenic — formed by chemical precipitation from solutions (salts, carbonates) or by accumulation of organism remains (calcareous skeletons, siliceous tests, plant residues).

In addition, in soil science, much attention is paid to unconsolidated sedimentary deposits—sands, clays, loess, glacial deposits, alluvium, etc.—which are often the direct substrate for soil formation, bypassing the stage of solid rock (Mukha et al., 2003; Ganzhara et al., 2002).

3.3. Clastic rocks and deposits

Clastic rocks are classified by the size of their constituent particles (Table 4) and the degree of cementation.

Table 4. Classification of clastic rocks by particle size

Compiled from data: Weil & Brady (2017); White (2006); Scheffer et al. (2018)

Sandstones are the most widespread cemented clastic rocks. They consist mainly of quartz grains (more than 75%—quartz sandstones), but may also contain feldspars (arkoses) or rock fragments (graywackes). The cement—calcite, iron oxides, silica, or clay material—determines the sandstone’s resistance to weathering: sandstones with carbonate cement break down faster, while those with siliceous cement are very hard (Buol et al., 2011; Scheffer et al., 2018).

Soils on sandstones, especially quartz sandstones, are usually light in texture (sandy, loamy sand), have low water-holding capacity, are poor in mineral nutrients, strongly acidic in humid climates, and prone to podzolization (Ganzhara et al., 2002; Weil & Brady, 2017). On graywackes (rich in feldspars), soils may be more fertile.

Loose sands are very widespread as soil-forming deposits (dune sands, fluvioglacial sands, ancient alluvial sands). They consist almost entirely of quartz, so their chemical potential is extremely low. Soils on sands—Podzols, Sod-Podzolic sandy soils—are poor, require high fertilizer inputs, but have good permeability and aeration, which in conditions of excessive moisture can be an advantage (Mukha et al., 2003; Weil & Brady, 2017).

3.4. Clayey rocks and deposits

Clayey rocks consist predominantly of particles < 0.002 mm, i.e., clay minerals. They form in quiet water conditions—on lake bottoms, lagoons, deep‑sea areas, where suspended particles settle in the absence of turbulence (Scheffer et al., 2018; White, 2006).

Argillites and clay shales are cemented clayey rocks. Argillites are massive and upon weathering give a fine-platy or splintery structure. Clay shales have pronounced bedding (fissility), which facilitates their physical breakdown. The mineral composition of clayey rocks is diverse: most often dominated by hydromicas (illite), kaolinite, montmorillonite, chlorite, and mixed‑layer minerals (Scheffer et al., 2018; Mukha et al., 2003).

Unconsolidated clays (loess‑like clays, lacustrine‑glacial clays, marine clays) are direct soil‑forming substrates. They are characterized by high dispersivity, large specific surface area, high cation exchange capacity, low permeability, high water‑holding capacity, and a tendency to swell and shrink.

Soils on clayey rocks (Sod‑Podzolic clayey, Gray Forest, Chernozems clayey, Solonetz, Vertisols) differ sharply from sandy ones: they are more fertile (if not saline), have high absorption capacity, large moisture reserves, but are poorly aerated, difficult to work, and prone to crusting and pudding (Mukha et al., 2003; Weil & Brady, 2017). The mineralogy of the clay fraction is especially important: the presence of montmorillonite makes the soil highly swelling and shrinking, forming Vertisols, while kaolinite is less active, typical of Ferrallitic soils (Scheffer et al., 2018).

3.5. Chemical sedimentary rocks

These rocks form by chemical precipitation from aqueous solutions. The most important for soil formation are:

Carbonate rocks — limestones (CaCO₃), dolomites (CaMg(CO₃)₂), marls (mixture of carbonate with clay material). They make up about 25% of all sedimentary rocks. Pure limestones upon dissolution leave very little insoluble residue (less than 5%), leading to the formation of thin but clay‑ and organic‑rich soils (Rendzinas, Terra‑fusca) on carbonate rocks (Scheffer et al., 2018; Weil & Brady, 2017). Marls give thicker soils (Pararendzinas, Chernozems, Gray Forest soils) due to the greater clay content.

Carbonate rocks exert a strong buffering effect: they neutralize acidity for a long time, prevent the development of acidic weathering, maintain high calcium and magnesium contents in the soil solution, which promotes the formation of water‑stable aggregates and high fertility (Foth, 1990; Eash et al., 2016). In humid climates, carbonates are gradually leached, but as long as they are present (even in lower horizons), they influence the entire profile.

Gypsum (CaSO₄·2H₂O) and anhydrite (CaSO₄) — occur in arid and semi‑arid regions. Soils on gypsum rocks may accumulate significant amounts of gypsum in the profile, affecting physical properties (gypsum concretions) and chemical composition (high sulfate content). Gypsum can also serve as a source of calcium and sulfur for plants (Weil & Brady, 2017; Scheffer et al., 2018).

Salts (chlorides, sulfates of sodium, potassium, magnesium) — accumulate in extra‑arid regions. They are not soil‑forming rocks in the usual sense, but may be incorporated into loose deposits, forming saline soils (Solonchaks). Their solutions alter osmotic potential and affect vegetation (Buol et al., 2011; Weil & Brady, 2017).

3.6. Organogenic sedimentary rocks

This group includes rocks composed of organism remains:

• Shell limestones — composed of mollusk shells, corals, foraminifera. Properties are similar to chemical limestones but often more porous.
• Chalk — fine‑grained, loose limestone from coccolithophore skeletons.
• Diatomites and tripoli — composed of amorphous silica (opal) from diatom frustules. Upon weathering they give siliceous soils rich in opaline silica.
• Coals (peat, lignite, bituminous coal, anthracite) — accumulations of plant remains in waterlogged conditions. Coals are not soil‑forming rocks in the classical sense (except peat), but in areas where they are exposed, specific soils with high acidity and toxicity may form (Scheffer et al., 2018; Weil & Brady, 2017).

3.7. Special continental deposits important for soil formation

A significant part of the world’s soils forms not on solid rocks but on unconsolidated Quaternary deposits formed by glaciers, rivers, wind, and gravity. These deposits have enormous agronomic importance.

Loess and loess‑like loams — aeolian (wind‑blown) deposits composed mainly of silt‑sized particles (0.05–0.01 mm), rich in carbonates, porous, with a high content of primary minerals. Loess covers vast areas in China, Europe (e.g., Ukraine, the Danube Valley), the USA, and Argentina. Soils on loess (Chernozems, Gray Forest, Kastanozems) are among the most fertile in the world, thanks to favorable texture, high base content, and good water‑physical properties (Scheffer et al., 2018; Weil & Brady, 2017; Mukha et al., 2003). The clay content in loess varies from 10–25%, but they are rich in fine silt, which provides high water‑holding capacity.

Glacial deposits (moraines, fluvioglacial sands, lacustrine‑glacial varved clays) — widely distributed in northern Europe, North America, and Siberia. In Russia, they occupy huge areas. Their main types (after Ganzhara et al., 2002; Mukha et al., 2003):

• Basal till — unsorted mixture of clay, sand, gravel, and boulders, often calcareous. Soils on moraines (Sod‑Podzolic, Gray Forest, sometimes Chernozems) are diverse.
• Glaciofluvial sands — well‑sorted, often carbonate‑free, quartz sands. Soils on them are poor Podzols and Sod‑Podzolic sandy soils.
• Lacustrine‑glacial varved clays — thinly laminated clays, often with high silt content, fertile when drained.

Alluvium — river deposits found in river valleys in all climatic zones. They are stratified, diverse in texture (from sands to clays), and often rich in nutrients washed from catchments. On alluvium, floodplain soils (Alluvial Meadow, Bog, etc.) form, which are very productive when water regime is regulated (Weil & Brady, 2017; Mukha et al., 2003).

Colluvium — slope deposits moved by gravity (landslides, soil creep). This is unsorted, often stony material; soils on it are thin, but may be fertile in the lower part of slopes due to concentration of fine earth (Scheffer et al., 2018).

Eolian sands — dune sands of deserts and coasts. Usually quartz, poor, with poor water properties. Soils on them are weakly developed, often saline in arid conditions (Weil & Brady, 2017).

3.8. Significance of sedimentary rocks for soil formation

To summarize, sedimentary rocks play a key role in soil formation for several reasons:

1. They are the main substrate for most agricultural soils (75% of continental area). Understanding their properties is critical for agronomy.

2. They are already partially “weathered” — clay minerals have already formed, some soluble compounds have been lost, which accelerates the process of soil formation compared to igneous rocks, which start weathering from scratch (Buol et al., 2011; White, 2006).

3. The diversity of sedimentary rocks (from limestones to sands) determines a wide range of soil properties: from acidic and poor (on sandstones and sands) to fertile (on loess, alluvium, carbonate rocks). This is a direct answer to our key question.

4. Carbonate rocks and loess serve as sources of calcium and magnesium, maintain high buffering capacity, prevent acidification, and contribute to the formation of a stable structure (Foth, 1990; Weil & Brady, 2017).

5. Clayey rocks cause high absorption capacity, accumulation of humus and nutrients, but can create problems with water and air regimes.

6. Unconsolidated Quaternary deposits (moraines, loess, alluvium) are “young” in geological time; soils on them are often still developing, allowing us to trace the initial stages of soil formation (Scheffer et al., 2018; Mukha et al., 2003).

Thus, sedimentary rocks are not just “products of destruction,” but active participants in soil formation, transferring inherited properties to soils and creating the basis for many soil types, from poor Podzols to fertile Chernozems.

4.1. Origin and general characteristics

Metamorphic rocks are rocks formed by the transformation (metamorphism) of igneous, sedimentary, or previously existing metamorphic rocks under the influence of high temperatures, pressures, and chemically active fluids (aqueous solutions, gases) in the deep zones of the Earth’s crust (Scheffer et al., 2018; Weil & Brady, 2017). Metamorphism is a process of “recrystallization” in the solid state: the rock does not melt, but minerals change, rearrange, new crystals grow, and sometimes the chemical composition changes (metasomatism). As a result, the rock acquires new structure, texture, and often a new mineral composition that reflects the conditions of metamorphism (temperature, pressure, presence of fluids) (White, 2006; Eash et al., 2016).

For soil science, metamorphic rocks are important for several reasons:

1. They are exposed over significant territories — about 20–25% of the continental area, especially in ancient shields and mountain regions. For example, in Russia, these include the Baltic Shield, Ukrainian Crystalline Shield, Aldan Shield, as well as the Urals, Altai, and Caucasus. Worldwide—Canadian Shield, Brazilian Shield, West African Shield, Scandinavian Mountains, Alps, Himalayas.

2. Their composition is generally close to the original rocks, but the structure and sometimes mineral composition are substantially altered, affecting physical weathering and chemical stability.

3. Metamorphic rocks often contain specific minerals (garnet, staurolite, kyanite, sillimanite, andalusite, epidote, actinolite, serpentine) that are not found or are rare in igneous and sedimentary rocks. Their presence can influence soil chemistry (e.g., high iron, magnesium, aluminum, and sometimes rare elements) and weathering rates.

4.2. Types of metamorphism and their influence on rocks

In soil science, it is important to distinguish two main types of metamorphism that lead to the formation of different rocks (Scheffer et al., 2018; Weil & Brady, 2017):

Regional metamorphism — large‑scale transformation of thick rock sequences due to tectonic movements (mountain building, subsidence) and the influence of high pressures and temperatures in the crust. It is characteristic of folded regions and ancient shields. Regional metamorphism creates schistose (foliated, with parallel orientation of minerals) textures—schistosity, gneissosity. This facilitates physical weathering along foliation planes.

Contact metamorphism — local transformation of rocks near hot magma intruded into the crust. Here the main factor is high temperature, pressure is often low. Contact metamorphism creates massive, often fine‑grained rocks—hornfels, which are difficult to weather.

For soil formation, regional metamorphic rocks are more important because they occupy large areas and have pronounced schistosity, promoting deep penetration of water and roots.

4.3. Main metamorphic rocks and their soil‑forming significance

4.4. Soil‑forming significance of metamorphic rocks (summary)

Metamorphic rocks as a whole play an important, though somewhat smaller in area than sedimentary rocks, role in soil formation. Their key importance for our course can be reduced to several points:

1. They often have pronounced schistosity or banding, which facilitates physical weathering and penetration of water and roots into the rock mass. Therefore, they develop thicker weathering crusts than massive igneous rocks of the same composition.

2. Their chemical and mineral composition is close to the original rocks, but may be modified by metamorphism. For example, gneisses are close to granites, amphibolites to gabbros, quartzites to sandstones. However, recrystallization may produce new minerals (garnet, staurolite, sillimanite, andalusite, cordierite), which upon weathering release additional elements (aluminum, iron, magnesium) and affect soil buffering and acidity.

3. Some metamorphic rocks have a unique composition (serpentinites, eclogites), leading to the formation of special soils with extreme chemical properties (high Mg, low Ca, toxic heavy metals). This is a vivid example of how different mineral compositions of rocks determine sharply different soil properties, even under the same climatic conditions.

4. Metamorphic rocks are often exposed in mountainous and hilly areas with dissected relief, which promotes the development of catenas and the formation of soils with varying degrees of erosion and redeposition.

4.5. Comparative table of main metamorphic rocks and their soil‑forming properties

4.6. Conclusion to the section

Metamorphic rocks represent an important, though less widespread at the surface, link in the chain of soil‑forming materials. They differ from igneous and sedimentary rocks by the presence of schistosity and specific minerals, which directly affect physical and chemical weathering, and therefore the depth, fertility, and type of soils formed. The differences between ortho‑ and para‑metamorphites, as well as between gneisses and schists, give us another confirmation of the key thesis: the composition and structure of the rock predetermine the main features of the future soil.

In the next part, we will move on to a detailed examination of the main minerals that compose all these rocks and their geochemical significance.

5.1. Minerals: definition and classification

Recall that a mineral is a naturally occurring, predominantly inorganic, chemically and structurally homogeneous crystalline substance formed by geological processes (Scheffer et al., 2018; White, 2006). Most minerals have an ordered crystal lattice, though amorphous ones also occur (e.g., opal, allophane).

In the Earth’s crust and in soils, thousands of mineral species occur, but only about 20–30 minerals are rock‑forming—that is, they compose the bulk of rocks (Buol et al., 2011; Weil & Brady, 2017). It is these minerals that we will study.

From the point of view of soil formation, all minerals are divided into two large groups (Scheffer et al., 2018; Mukha et al., 2003; Ganzhara et al., 2002):

Primary (lithogenic) minerals — formed during crystallization of magma (high temperature, high pressure) or during metamorphism. They make up the bulk of igneous and metamorphic rocks, as well as many sedimentary rocks. In soils, primary minerals predominate in the sand and silt fractions. Their main feature is that they are not in equilibrium with the conditions of the Earth’s surface (atmosphere, water, organic acids, biota), so they actively weather.

Secondary (pedogenic) minerals — formed during weathering and soil formation at low temperatures and pressures. These are clay minerals, oxides and hydroxides of iron and aluminum, carbonates, sulfates, chlorides (forming in arid conditions), as well as allophane and imogolite (in volcanic ashes). They predominate in the clay fraction of soils. We will examine secondary minerals in detail in later lectures; for now, we focus on primary minerals.

5.2. Crystal‑chemical basis of silicates: the silicon‑oxygen tetrahedron

The vast majority of primary minerals (and many secondary ones) are silicates—compounds of silicon and oxygen with various cations (Fe, Mg, Ca, Na, K, Al, etc.). Silicates make up more than 90% of the mass of the Earth’s crust (Scheffer et al., 2018; White, 2006). Therefore, we will start with them.

The basic building block of all silicates is the silicon‑oxygen tetrahedron (SiO₄)⁴⁻ (Fig. 1). This is a structure in which one Si⁴⁺ ion is surrounded by four O²⁻ ions at the vertices of a tetrahedron (a regular four‑sided pyramid). The radius ratio allows this coordination (White, 2006; Weil & Brady, 2017). In the crystal lattice, tetrahedra can link through shared oxygen atoms, forming various structures:

• Island (isolated tetrahedra) — O:Si ≈ 4:1.
• Chain (tetrahedra linked into chains) — O:Si ≈ 3:1.
• Layer (tetrahedra form continuous sheets) — O:Si ≈ 2.5:1.
• Framework (three‑dimensional network) — O:Si ≈ 2:1.

These structural types determine physical properties (hardness, cleavage, brittleness) and resistance to weathering.

Furthermore, in the structure of silicates, isomorphous substitution often occurs—replacement of one ion by another of similar radius without changing the structure. For example, Al³⁺ may replace Si⁴⁺ in the tetrahedron, and Fe²⁺, Mg²⁺ may replace Al³⁺ in octahedral positions. These substitutions create an excess negative charge, which is compensated by incorporation of other cations (K⁺, Na⁺, Ca²⁺) into the structure or into the interlayer space (in clay minerals) (Scheffer et al., 2018; White, 2006).

Now let us move on to specific minerals.

5.3. Quartz (SiO₂)

Quartz is silicon oxide, pure silica. Its structure is framework, all tetrahedra are linked to each other through all four vertices, forming a strong three‑dimensional network (White, 2006; Weil & Brady, 2017). Quartz contains no other cations (except Si), so it lacks plant nutrients (except silicon itself, which is not essential for most plants). Quartz is very hard (7 on Mohs scale), has no cleavage, is chemically inert at pH 2–9. Its solubility is very low: at 25 °C, the solubility of amorphous silica is _{120 mg/L (as SiO₂), and of crystalline quartz only} 6–10 mg/L (Scheffer et al., 2018; White, 2006).

In soil formation, quartz acts as an inert filler. During weathering of rocks, quartz practically does not change—it remains as grains of varying sizes, accumulating in the sand and silt fractions. It is quartz that gives soils their sandy texture. Soils rich in quartz (on quartzites, sandstones, granites with high quartz content) are usually poor in nutrients, have low cation exchange capacity, are strongly acidic in humid climates, but are well aerated and warm up quickly (Buol et al., 2011; Eash et al., 2016).

Quartz may partially dissolve during very prolonged weathering in tropical conditions (at pH > 9 or < 4), but in temperate climates it is practically not destroyed. Therefore, quartz is a standard of stability for assessing the degree of rock weathering (the quartz/feldspar ratio is often used as a weathering index).

5.4. Feldspars

Feldspars are a group of framework aluminosilicates in which some Si⁴⁺ in the tetrahedra is replaced by Al³⁺, and the resulting excess negative charge is compensated by large cations (K⁺, Na⁺, Ca²⁺) located in cavities of the framework (Scheffer et al., 2018; White, 2006; Weil & Brady, 2017). Feldspars are the most abundant minerals in the Earth’s crust: they make up about 50% of its volume (Scheffer et al., 2018).

By chemical composition, feldspars are divided into two large groups:

1. Potassium feldspars (orthoclase, microcline, sanidine) — general formula KAlSi₃O₈. They contain potassium, almost no sodium or calcium. They occur in acidic igneous rocks (granites, rhyolites) and metamorphic rocks (gneisses). Upon weathering, potassium feldspars release potassium—the most important plant nutrient. The hydrolysis process can be schematically represented (after Weil & Brady, 2017; Foth, 1990):

That is, ultimately clay minerals (kaolinite) and dissolved silica are formed, while potassium is released into the soil solution, where it can be taken up by plants or fixed in the interlayers of clay minerals (illite) (Scheffer et al., 2018).

2. Plagioclases — solid solutions from sodium (albite NaAlSi₃O₈) to calcium (anorthite CaAl₂Si₂O₈). Between them there is a continuous series: oligoclase (Na > Ca), andesine (Na ≈ Ca), labradorite (Ca > Na), bytownite (Ca predominant). Plagioclases are widespread in igneous rocks from acidic to mafic (the more mafic the rock, the more calcic the plagioclase) (White, 2006; Scheffer et al., 2018).

Upon weathering, plagioclases release sodium and calcium, as well as aluminum and silicon. Sodium and calcium are important for plants, especially calcium (a structural element of cell walls, regulator of many processes). Plagioclases weather faster than potassium feldspars, especially anorthite (the most calcic). As a result of hydrolysis of plagioclases, clay minerals (kaolinite, smectites) form, and under intense weathering—aluminum hydroxides (gibbsite) (Buol et al., 2011; Scheffer et al., 2018).

Significance for soils: feldspars are one of the main sources of potassium, calcium, and sodium in soils. The released cations not only nourish plants but also affect the reaction of the soil solution (when Ca²⁺ or K⁺ is released, pH rises; when they are leached, it falls). Soils on rocks rich in feldspars (granites, gneisses, loess) are generally more fertile than those on rocks poor in feldspars (quartzites, sandstones).

5.5. Micas

Micas are layered (phyllosilicate) minerals with a characteristic 2:1 structure: two tetrahedral sheets with an octahedral (aluminum‑, magnesium‑, iron‑bearing) sheet between them (Scheffer et al., 2018; White, 2006). The layers are joined through the interlayer space, which is occupied by large cations (most commonly K⁺) that fit into the so‑called “ditrigonal” cavities in the tetrahedral sheets and form strong bonds. Due to this strength, micas have perfect basal cleavage (easily split into thin flakes) and moderate chemical resistance.

Two main groups of micas are distinguished:

1. Muscovite (potassium mica) — KAl₂(Si₃Al)O₁₀(OH)₂, dioctahedral (Al³⁺ occupies 2 of 3 octahedral positions). Muscovite is light‑colored, contains up to 9–11% K₂O. It is fairly resistant to chemical weathering (slower than biotite), but upon prolonged weathering, K⁺ is gradually displaced and muscovite transforms into hydromica (illite) or kaolinite (Scheffer et al., 2018; Weil & Brady, 2017).

2. Biotite (iron‑magnesium mica) — K(Mg,Fe²⁺)₃(Si₃Al)O₁₀(OH)₂, trioctahedral (all three octahedral positions occupied by Mg²⁺ or Fe²⁺). Biotite is dark‑colored, contains less potassium (up to 6–10% K₂O), but much Fe and Mg. Biotite is less resistant to weathering than muscovite because Fe²⁺ readily oxidizes to Fe³⁺, disrupting the structure, and K⁺ is more rapidly lost from the interlayer space (Scheffer et al., 2018; Weil & Brady, 2017).

Upon weathering of micas, potassium is released, as well as iron, magnesium, and aluminum. Micas are an important source of potassium in soils, especially biotite, which releases K⁺ more easily. With partial leaching of potassium, micas transform into vermiculite or smectite (with expanding structure), and upon complete weathering—into kaolinite (Scheffer et al., 2018; White, 2006). Soils formed on micaceous rocks (gneisses, schists, granites with biotite) often have a good potassium regime, although in humid climates potassium may be leached.

5.6. Pyroxenes and amphiboles (ferromagnesian minerals)

This is a group of chain silicates containing much iron and magnesium, as well as calcium. They are characteristic of mafic and ultramafic rocks (gabbro, basalt, peridotite) and some metamorphic rocks (amphibolites, green schists).

Pyroxenes have single chains of tetrahedra (one chain of SiO₄)—general formula M₂Si₂O₆, where M = Ca, Mg, Fe, Al. The most common in soil‑forming rocks is augite — (Ca, Mg, Fe)₂Si₂O₆, as well as hypersthene, enstatite, diopside. Pyroxenes are dark green to black, often found as short prismatic crystals with good cleavage in two directions (White, 2006; Scheffer et al., 2018; Weil & Brady, 2017).

Amphiboles have double chains of tetrahedra (two parallel chains linked through shared oxygen atoms)—general formula M₇Si₈O₂₂(OH)₂, where M = Ca, Mg, Fe, Al. The most common is hornblende (amphibole of intermediate composition) — Ca₂(Mg,Fe,Al)₅(Si₈O₂₂)(OH)₂. Amphiboles are also dark green, prismatic crystals with perfect cleavage at 124° (unlike pyroxenes, which have cleavage at 90°). Amphiboles contain a hydroxyl group (OH), making them structurally closer to layered silicates (White, 2006; Scheffer et al., 2018).

Weathering resistance: pyroxenes and amphiboles weather significantly faster than feldspars and quartz, especially pyroxenes (olivine is even faster—see below). Upon weathering, they release Fe, Mg, Ca into solution, while silicon and aluminum partially go into solution or participate in the formation of secondary clay minerals (smectites, vermiculites, chlorite) (Buol et al., 2011; Weil & Brady, 2017). Under oxidizing conditions, Fe²⁺ is rapidly oxidized to Fe³⁺ and precipitates as iron oxides and hydroxides (goethite, hematite), which color soils yellow, brown, and red.

Significance for soils: ferromagnesian minerals are the main source of magnesium, iron, calcium, and micronutrients (Mn, Zn, Cu) in soils. Soils formed on rocks rich in pyroxenes and amphiboles (basalts, gabbros, amphibolites, green schists) are usually neutral to slightly alkaline, have high CEC, contain many clay minerals, and are fertile. They are often colored red and brown tones due to iron oxides. However, in humid climates with intense leaching, they may become strongly depleted in bases and turn into acidic Ferrallitic soils.

5.7. Olivines

Olivine is an island silicate (isolated tetrahedra), chemical formula (Mg,Fe)₂SiO₄. This is a solid solution between forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). Olivine has an olive‑green color, often occurs as rounded grains in ultramafic rocks (peridotites, dunites) and sometimes in basalts (White, 2006; Scheffer et al., 2018).

Olivine is one of the most rapidly weathered silicate minerals. Upon hydrolysis and oxidation, it breaks down to form iron and magnesium hydroxides and silica. During weathering, serpentine (a layered magnesium silicate) often forms—we have already discussed this in the section on serpentinites (Buol et al., 2011; Weil & Brady, 2017).

Significance for soils: because of its low surface abundance (olivine breaks down rapidly), soils directly on olivine rocks are rare (mainly in areas of young mafic rocks). But its weathering products (serpentine, smectites, iron hydroxides) participate in the formation of soils on ultramafic rocks, as already discussed.

5.8. Carbonates (calcite, dolomite)

Calcite (CaCO₃) and dolomite (CaMg(CO₃)₂) are carbonate minerals, widely distributed in sedimentary rocks (limestones, dolomites, marls) and some metamorphic rocks (marbles). They are not silicates; they are salts of carbonic acid. Their crystal structure is based on CO₃²⁻ ions bonded with Ca²⁺ and Mg²⁺ cations (White, 2006; Eash et al., 2016; Weil & Brady, 2017).

Carbonates readily dissolve in acidic media, especially in the presence of carbon dioxide, which forms carbonic acid:

This process underlies karst phenomena (caves, sinkholes) and leaching of carbonates from soils. In humid climates, carbonates are leached from the upper horizons, and the soil becomes acidic. However, as long as carbonates are present (even in deep horizons), they buffer pH around 7–8.3 (Foth, 1990; Weil & Brady, 2017). The reaction may be slightly alkaline due to hydrolysis of carbonates.

Significance for soils: carbonates are the main source of calcium and magnesium, and provide neutral or slightly alkaline reaction. Soils on carbonate rocks (Rendzinas, Chernozems on loess, Kastanozems) often have high structure (due to calcium bridges between particles), good physical properties, and high fertility (Buol et al., 2011; Eash et al., 2016). In addition, carbonates neutralize acidity, preventing podzolization and ferrallitization. In arid conditions, carbonates may accumulate in the profile as concretions, pseudomycelium, or hard horizons (calcretes) (Scheffer et al., 2018).

5.9. Other important minerals (briefly)

In addition to those listed, soil‑forming rocks also contain:

• Iron oxides (magnetite Fe₃O₄, ilmenite FeTiO₃) — part of the heavy fraction, stable, can serve as a source of iron.
• Titanium oxides (rutile, anatase, brookite) — very stable, used as weathering indicators.
• Sulfides (pyrite FeS₂) — occur in sedimentary rocks; upon oxidation they give sulfuric acid, causing strong acidification (acid sulfate soils) (Scheffer et al., 2018; Weil & Brady, 2017).
• Apatite Ca₅(PO₄)₃(OH,F,Cl) — an important source of phosphorus in soils, occurs as an accessory mineral in igneous rocks and in sedimentary phosphorites.
• Gypsum CaSO₄·2H₂O — occurs in arid zones, source of calcium and sulfur, can form hard horizons (gypsum crusts).

5.10. Weathering resistance of minerals: Bowen’s reaction series and its soil significance

In igneous rocks, minerals crystallize from the melt in a certain order (Bowen’s reaction series), which is simultaneously a series of resistance to chemical weathering at the surface. Schematically, this series looks like this (after Scheffer et al., 2018; Weil & Brady, 2017; Foth, 1990):

Least resistant (crystallize first, weather first):

• Olivine → pyroxenes → amphiboles → biotite
• ↑
• (at surface temperatures they are unstable and break down rapidly)

Intermediate resistance:

• Plagioclases (from calcic to sodic: anorthite → albite) → potassium feldspars → muscovite

Most resistant (crystallize last, survive longest):

• Quartz (most resistant)

This series explains why in the sand fractions of soils, especially mature ones, quartz predominates, while less resistant minerals (olivine, pyroxenes) completely disappear at early weathering stages. The longer and more intense the weathering, the more quartz and other resistant minerals (zircon, rutile, tourmaline) accumulate—these are called weathering‑indicator minerals (Buol et al., 2011; Weil & Brady, 2017).

5.11. Relationship between rock mineral composition and soil fertility

We can now formulate a direct dependence:

• Rocks rich in quartz and poor in feldspars and ferromagnesians (quartzites, sandstones, acidic granites) → soils poor, acidic, sandy, with low CEC, low humus (Podzols, Sod‑Podzolic sandy).
• Rocks containing many feldspars (especially potassium and calcium) → soils of medium fertility, with good potassium and calcium regimes, loamy (Gray Forest, Brown soils, Chernozems on loess).
• Rocks rich in ferromagnesian minerals (basalts, gabbros, amphibolites) → soils fertile, rich in Ca, Mg, Fe, clayey, neutral to slightly acidic (Eutric Cambisols, Brown soils on basalts, Ferrallitic soils).
• Rocks with carbonates → soils with neutral to alkaline reaction, rich in calcium and magnesium, with good structure and high fertility (Chernozems, Kastanozems, Rendzinas).
• Rocks with excess Mg and low Ca/Mg (serpentinites) → soils with magnesium toxicity, specific, often low‑productivity.

Thus, understanding the mineral composition of the rock gives us the key to predicting the main properties of the future soil: its acidity, nutrient supply, texture, structure, and even its tendency to salinization or podzolization.

5.12. Conclusion to the section on minerals

We have examined the main rock‑forming minerals, their structure, chemical composition, resistance, and influence on soils. These minerals are “individuals,” each with its own character. It is their combinations in rocks that create the infinite diversity of soils we observe on the planet.

In the next part, we will move on to the mineralogical composition of soil‑forming rocks as a whole and to the geochemical significance of individual minerals—that is, how specific minerals affect chemical elements in soils.

6.1. Regularities in the distribution of minerals among particle‑size fractions

One of the fundamental regularities of soil science is that different minerals concentrate in different particle‑size fractions (Scheffer et al., 2018; White, 2006; Weil & Brady, 2017). This regularity has enormous practical importance because particle‑size composition determines many physical and chemical properties of soils.

Sand fraction (2–0.05 mm) consists almost entirely of primary minerals resistant to weathering. Here predominate:

• Quartz (up to 80–95% in mature soils) — the most resistant;
• Potassium feldspars (orthoclase, microcline) — relatively resistant;
• Acidic plagioclases (albite, oligoclase) — if preserved;
• Muscovite — as flat flakes, often in coarse sands;
• Heavy minerals (zircon, rutile, tourmaline, garnet, magnetite, ilmenite, apatite) — as rare grains, constituting the “heavy fraction” with density > 2.9 g/cm³.

The sand fraction contains almost no clay minerals (they are too small) and is very poor in nutrients, except for potassium (from feldspars and micas) and sometimes phosphorus (from apatite) (White, 2006; Scheffer et al., 2018).

Silt fraction (0.05–0.002 mm) occupies an intermediate position. It contains:

• Quartz and feldspars — as fine grains (mainly primary);
• Micas (muscovite, biotite) — as fine flakes;
• Carbonates (calcite, dolomite) — especially in loess and carbonate rocks;
• Secondary minerals (clayey) may be present, but often as aggregates.

The silt fraction determines many water‑physical properties of soils: capillarity, water‑holding capacity, plasticity (White, 2006; Weil & Brady, 2017).

Clay fraction (< 0.002 mm) is already the world of secondary (pedogenic) minerals, though very fine fragments of primary minerals (especially quartz and micas) may also occur. Main components:

• Layered silicates (clay minerals): kaolinite, montmorillonite, illite (hydromica), vermiculite, chlorite, mixed‑layer formations;
• Oxides and hydroxides of iron (goethite, hematite, ferrihydrite), aluminum (gibbsite), manganese, titanium (anatase, rutile);
• Amorphous and poorly crystalline compounds (allophane, imogolite, ferrihydrite) — especially in young soils and soils on volcanic ashes;
• Carbonates (in calcareous soils) — often as microcrystals;
• Gypsum (in arid soils).

It is the clay fraction that determines cation exchange capacity, water‑holding capacity, swelling and shrinkage, aggregation ability, and chemical buffering of soils (Foth, 1990; White, 2006; Weil & Brady, 2017).

Thus, in any soil, the sand fraction is the “skeleton” of stable primary minerals, while the clay fraction is the “active matrix” of secondary clay minerals and oxides (Ganzhara et al., 2002; Mukha et al., 2003).

6.2. Mineralogical composition of the main types of soil‑forming rocks

Now let us consider which minerals predominate in specific types of rocks exposed at the surface and serving as substrates for soil formation.

6.3. Soil mineralogical composition as a reflection of rock and soil formation

The mineralogical composition of soil is formed under a dual influence:

1. Inheritance from the rock — especially in the sand and silt fractions, as well as in the clay fraction of young soils (at early weathering stages).

2. Pedogenic transformations — formation of secondary minerals, destruction of unstable primary minerals, transformation of one clay mineral into another.

Table 5 presents typical associations of clay minerals that form on different rocks in different climatic conditions (after Scheffer et al., 2018; Weil & Brady, 2017; Mukha et al., 2003).

Table 5. Typical associations of clay minerals depending on rock and climate

Sources: Scheffer et al. (2018); Weil & Brady (2017); Mukha et al. (2003); Ganzhara et al. (2002)

The table shows how climate “overrides” the influence of the rock. For example, on the same granite rock, Podzols with illite‑vermiculite clays form in the taiga, while in the tropics, Ferrallitic soils with kaolinite and gibbsite form. However, the rock still leaves its imprint: even in the tropics, on basalts, soils will be richer in iron oxides and smectites (in early stages) than on granites.

6.4. Agronomic significance of the mineralogical composition of rocks

For agronomy, knowledge of the mineralogical composition of the soil‑forming rock allows one to predict a number of soil properties critical for agriculture.

Fertility (nutrient content):

• Potassium: rocks containing potassium feldspars and micas (granites, gneisses, loess, clay shales) — potential source of K, though its availability depends on weathering. Soils on loess and shales are often well supplied with potassium (Weil & Brady, 2017; Mukha et al., 2003).
• Calcium and magnesium: rocks containing carbonates (limestones, dolomites, loess) — main source of Ca and Mg. Soils on them often do not need liming and have good structure (Foth, 1990; Scheffer et al., 2018).
• Phosphorus: rocks containing apatite (granites, some sedimentary rocks) may be a source of P, but often phosphorus in soils is strongly bound and poorly available (especially in acidic and ferruginous soils) (Foth, 1990).
• Iron and micronutrients: mafic and ultramafic rocks provide high contents of Fe, Mn, Zn, Cu, Co, Ni, Cr. This can be both a benefit (when sufficient) and a problem (toxicity of Cr, Ni on serpentinites) (Buol et al., 2011).

Soil acidity:

• Rocks rich in carbonates → neutral or slightly alkaline reaction.
• Rocks poor in bases (quartzites, sandstones, acidic granites) → strongly acidic soils in humid climates, requiring liming.
• Presence of smectites (swelling clays) may increase CEC and buffering against acidification (White, 2006; Scheffer et al., 2018).

Physical properties:

• Rocks yielding much quartz sand → light, well‑drained but poor and drought‑resistant soils.
• Rocks with high clay mineral content, especially montmorillonite → heavy, poorly aerated, prone to slaking, but with high water‑holding capacity.
• Loess and loess‑like loams → optimal balance (loamy texture) → best soils for agriculture (Weil & Brady, 2017; Mukha et al., 2003).

Erosion resistance:

• Soils on loess and loams are more susceptible to wind and water erosion than clayey or sandy soils, due to weak aggregation of silt particles.
• Soils with high smectite and humus content on carbonate rocks are better aggregated and more erosion‑resistant (Scheffer et al., 2018).

6.5. Summary: what we have learned about the mineralogical composition of rocks

1. Different minerals concentrate in different fractions: sand — mainly primary minerals (quartz, feldspars, micas); silt — mixture of primary and secondary; clay — secondary clay minerals and oxides.

2. The composition of the soil‑forming rock is not a random set of minerals, but a regular combination depending on the genesis of the rock (igneous, sedimentary, metamorphic) and its history (weathering, composition).

3. Mineralogical composition directly determines:

• nutrient content (K, Ca, Mg, Fe, P, micronutrients),
• soil acidity,
• texture,
• cation exchange capacity,
• water‑holding capacity,
• structure and aggregation,
• tendency to salinization, podzolization, ferrallitization.

4. Agronomically most valuable soil‑forming rocks are those that combine:

• moderate clay mineral content (for CEC and moisture retention),
• presence of feldspars and micas (for potassium and other bases),
• presence of carbonates (for buffering and structure),
• optimal texture (loamy).

Such rocks include loess, loess‑like loams, marls, alluvial deposits, calcareous moraines, and mafic igneous rocks.

5. Climate is the main “director” that determines which secondary minerals will form from primary ones even on the same rock. But the rock sets the “script”—the set of starting materials with which the climate works.

7.1. Bio‑philic elements and their sources in minerals

Plants require chemical elements, which are divided into macronutrients (consumed in large amounts) and micronutrients (consumed in small doses but essential). All of them, except carbon, hydrogen, and oxygen, enter plants from the soil, and into the soil from weathering minerals (Foth, 1990; Weil & Brady, 2017). Therefore, knowledge of which minerals contain these elements is of paramount importance.

Table 6. Main plant nutrients and their principal mineral sources

Compiled from data: Foth (1990); Weil & Brady (2017); White (2006); Scheffer et al. (2018)

Note: each mineral is a “concentrator” of a specific set of elements. For example, quartz provides only silicon (and very slowly), while apatite provides phosphorus, which is vital for plants. Therefore, the diversity of minerals in a rock means diversity of nutrients in the future soil.

7.2. Geochemical mobility series of elements

During weathering, different elements behave differently: some rapidly pass into solution and are leached, others remain in place and accumulate, still others migrate short distances and become fixed as secondary minerals. To systematize these processes, mobility series of elements are used.

Academician A.E. Fersman and B.B. Polynov developed the classical mobility series for elements under oxidizing weathering. In simplified form, this series looks like this (after Scheffer et al., 2018; Weil & Brady, 2017; Mukha et al., 2003):

Most mobile (easily leached):

Cl⁻, SO₄²⁻, Na⁺, Ca²⁺, Mg²⁺

Moderately mobile:

K⁺, SiO₂ (as orthosilicic acid), Mn²⁺

Low‑ and immobile:

Fe³⁺, Al³⁺, Ti⁴⁺, P (as phosphates), Ni²⁺, Co²⁺, Cr³⁺

This series explains why in mature soils of humid climates, the upper horizons are depleted in calcium, sodium, magnesium (they are leached), and enriched in quartz and iron and aluminum oxides (they remain in place) (Foth, 1990; Weil & Brady, 2017).

However, it is important to consider that mobility depends on pH and redox conditions. For example:

• In acidic media (pH < 5), Al³⁺ becomes mobile and can be leached, whereas in neutral media it precipitates as hydroxide (Foth, 1990; Weil & Brady, 2017).
• Under reducing conditions (in waterlogged soils), Fe³⁺ is reduced to Fe²⁺ and becomes mobile, leading to the formation of gley horizons and concretions (Scheffer et al., 2018; Weil & Brady, 2017).

Thus, element mobility is determined not only by the properties of the element itself but also by the chemical environment in the soil.

7.3. Geochemical role of individual mineral groups

Now let us systematize which elements enter the soil from various minerals and how this affects soil processes.

7.4. Geochemical barriers and element accumulation

In soils, there are geochemical barriers—zones where a sharp change in conditions (pH, Eh, concentration, sorption) occurs, causing precipitation or fixation of mobile elements. Barriers play a key role in horizon formation and element distribution in the profile.

Main types of geochemical barriers in soils (after Scheffer et al., 2018; Weil & Brady, 2017):

1. Carbonate barrier — when pH rises (e.g., transition from acidic to carbonate environment), carbonates of calcium and magnesium precipitate. This is characteristic of soils on carbonate rocks and leaching zones (calcium illuvial horizons).

2. Iron‑manganese (oxidizing) barrier — when conditions change from reducing to oxidizing, Fe²⁺ and Mn²⁺ are oxidized to Fe³⁺ and Mn⁴⁺ and precipitate as oxides and hydroxides. This leads to formation of concretions, ortsteins, plinthite horizons, and soil coloring (Scheffer et al., 2018; Foth, 1990).

3. Sorption barrier — fixation of cations and anions on the surfaces of clay minerals and organic matter. Particularly important for potassium (fixation in interlayers of illite and vermiculite), phosphorus (adsorption on iron and aluminum oxides in acidic soils), ammonium (fixation in interlayers), and heavy metals (binding with organic matter and oxides) (Foth, 1990; Weil & Brady, 2017).

4. Acid‑base barrier — with changing pH, many elements change their mobility. In acidic media (pH < 5), Al³⁺, Fe³⁺, Mn²⁺ become mobile and may be toxic; in alkaline (pH > 8), Mo, Se, As become mobile, while Ca, Mg, P lose availability. This barrier is critical for agronomy because it determines the availability of almost all nutrients (Foth, 1990; Weil & Brady, 2017).

7.5. Practical significance of mineral geochemistry for agronomy

Now let us turn to the most practical conclusions. What does knowledge of the mineral basis of soils give to an agronomist?

1. Prediction of fertility and fertilizer needs

• Soils on rocks rich in potassium feldspars and micas (granites, gneisses, loess, clay shales) — potentially well supplied with potassium (K). However, K availability depends on weathering and acidity. In acidic soils, potassium may be leached, requiring potassium fertilizers (Foth, 1990; Weil & Brady, 2017).
• Soils on carbonate rocks (limestones, dolomites, marls) — well supplied with calcium and magnesium, usually do not need liming. Conversely, they may suffer from iron, zinc, manganese deficiencies (due to high pH) (Foth, 1990; Weil & Brady, 2017).
• Soils on mafic rocks (basalts, gabbros, amphibolites) — rich in iron, magnesium, calcium, often contain micronutrients. However, under acidic conditions (tropics) they may become strongly depleted in bases and require liming and phosphorus fertilizers (P is strongly fixed on Fe and Al oxides) (Buol et al., 2011; Weil & Brady, 2017).
• Soils on sandstones and quartzites — extremely poor, require almost all nutrients, especially potassium, phosphorus, magnesium, and micronutrients. However, they are easily drained and can be cultivated with intensive fertilization (Weil & Brady, 2017).

2. Diagnosis of acidity and reclamation

• Presence of carbonates in the rock or soil (effervescence with HCl) — indication of neutral or alkaline reaction and no need for liming. If carbonates are absent and the climate is humid—the soil is most likely acidic, requiring liming and organic amendments (Foth, 1990; Weil & Brady, 2017).
• Soils on pyrite‑bearing rocks (some marine clays, coal‑bearing deposits) may, upon drainage, give a strongly acidic reaction (acid sulfate soils) and require large‑dose liming (Scheffer et al., 2018; Foth, 1990).

3. Prediction of water regime and irrigation reclamation

• Texture and type of clay minerals directly affect permeability and water‑holding capacity. Soils on montmorillonitic clays (smectites) swell strongly, shrink, and have slow water infiltration, requiring special tillage and drainage (White, 2006; Weil & Brady, 2017).
• Soils on kaolinitic clays (weathered tropical soils) — less swelling, better drained, but may have low CEC.

4. Prediction of phytotoxicity

• Soils on serpentinites and ultramafic rocks may have high contents of Ni, Cr, Co, toxic to many crops. In such areas, selection of tolerant species or agromelioration (e.g., calcium application to improve Ca/Mg ratio) is required (Buol et al., 2011; Weil & Brady, 2017).
• High Fe and Mn under reducing conditions (waterlogged soils) may lead to Fe²⁺ and Mn²⁺ toxicity for rice and other crops (Foth, 1990; Weil & Brady, 2017).

5. Assessment of soil resistance to degradation

• Soils on light rocks (sands, sandstones) are more prone to erosion than clayey ones. Knowledge of mineralogy helps choose erosion control measures (windbreaks, mulching) (Weil & Brady, 2017).
• Soils with high montmorillonite content may be subject to strong shrinkage and cracking, affecting root systems and water supply (White, 2006; Weil & Brady, 2017).

7.6. General answer to the key question of the lecture

We have come to the main point—to the answer to the question posed at the beginning of the lecture:

Why do different rocks form different soils?

The answer can now be formulated at several levels:

1. Mineralogical level. Different rocks consist of different sets of minerals. Each mineral is a source of specific chemical elements and has a certain resistance to weathering. Accordingly, the soil forming on the rock “inherits” its chemical composition (set of elements) and physical properties (structure, dispersivity, porosity), which directly depend on the minerals (Buol et al., 2011; Weil & Brady, 2017; Scheffer et al., 2018).

2. Geochemical level. Upon weathering, nutrients (K, Ca, Mg, Fe, P, micronutrients) are released from different minerals at different rates and in different amounts, and secondary minerals (kaolinite, smectite, illite, Fe and Al hydroxides) form. These secondary minerals determine cation exchange capacity, acidity, buffering, water‑holding capacity, aggregation—that is, all key agronomic properties of the soil (Foth, 1990; Weil & Brady, 2017; White, 2006).

3. Physicochemical level. The presence or absence of carbonates, excess or deficiency of bases, the nature of clay minerals (swelling or not), iron and aluminum oxide content—all this is determined by the initial composition of the rock and predetermines soil reaction (pH), its water‑air regime, structure‑forming ability, and resistance to anthropogenic pressures (Buol et al., 2011; Weil & Brady, 2017).

Thus, the rock serves as the “starting capital” of soil formation. Climate, relief, organisms, and time are “investors” and “managers” that can increase or decrease this capital, but they cannot create it from nothing. A poor rock (quartz sand), even under the most favorable climate, will not give a Chernozem, while a rich rock (basalt, loess) even under unfavorable conditions can give a more fertile soil than a poor rock under the same conditions.

7.7. Final summary of the entire lecture

We have come a long way: from the Earth’s crust, through igneous, sedimentary, and metamorphic rocks, to the main minerals and their geochemical role. Let us summarize.

1. The Earth’s crust is composed mainly of silicates and aluminosilicates, where the main elements are oxygen and silicon. They create the basis for most soil minerals.

2. Igneous rocks are the primary material from which everything begins. Their composition (acidic, mafic, ultramafic) determines which minerals and elements will enter the soil.

3. Sedimentary rocks are the most widespread on the land surface. They are already partially “weathered” and carry clay minerals and carbonates, making them the basis of many fertile soils.

4. Metamorphic rocks are rocks altered by temperature and pressure. They often have a foliated structure that facilitates weathering and may contain unique minerals (serpentine, garnet) affecting soil chemistry.

5. The main minerals are the building blocks of rocks. Quartz is inert, feldspars supply K, Na, Ca, micas supply K, Mg, Fe, ferromagnesians supply Fe, Mg, Ca and micronutrients, carbonates supply Ca, Mg and buffering. Each mineral is a “factory” for producing specific nutrients.

6. The mineralogical composition of rocks is a regular combination of minerals that directly determines texture, cation exchange capacity, acidity, water‑holding capacity, and fertility of the future soil.

7. The geochemical significance of minerals is the practical embodiment of the link “rock → soil → plant.” Elements released from minerals become available to plants, shape acidity, participate in structure formation, and determine the agronomic value of the soil.

Final words to the listeners:

You now possess fundamental knowledge about the mineral basis of soils. By understanding which minerals compose the rock, you can predict soil properties: whether it will be acidic or neutral, fertile or poor, sandy or clayey, easily workable or heavy.

In the next lecture, we will move on to weathering and soil formation processes—to how from minerals and rocks, under the influence of climate and organisms, the fertile medium we call soil is formed. Remember: minerals are not just “dead stones”; they are the reservoir of elements from which life in the soil is built.