Sabine Grunwald

Primary Mineral Components of Soils

	Primary Minerals
	Parent Rocks
	Transported Parent Material

Primary Minerals

The thickness of the earth's crust varies from 10 km under the ocean to 30 km under the continents. Of the 88 naturally occurring elements on earth, only 8 make of most of the crust. The earth's crust and soils are dominated by the silicic acid in combination with Na, Al, K, Ca , Fe and O ions. In Table 1 the mean elemental content of soil and crustal rocks, and the soil enrichment factors are listed. Elements with high enrichment factors (EF) are C, N, S , and elements with low EF are Na, Mg , Al, P, Cl, K, Ca, Mn and Fe. The latter ones are important nutrients for plant growth.

Table 1. Mean elemental content of soil and crustal rocks, and the enrichment factors.

Element	Mean elemental content of soil [mg/kg]	Mean elemental content of crust [mg/kg]	Enrichment Factor (EF)
Li	24	20	1.2
Be	0.92	2.6	0.35
B	33	10	3.3
C	25,000	480	52
N	2,000	25	80
O	490,000	474,000	1.0
F	950	430	2.2
Na	12,000	23,000	0.52
Mg	9,000	23,000	0.39
Al	72,000	82,000	0.88
Si	310,000	277,000	1.1
P	430	1,000	0.43
S	1,600	260	6.2
Cl	100	130	0.77
K	15,000	21,000	0.71
Ca	24,000	41,000	0.59
Sc	8.9	16	0.56
Ti	2,900	5,600	0.52
V	80	160	0.50
Cr	54	100	0.54
Mn	550	950	0.58
Fe	26,000	41,000	0.63
Co	9.1	20	0.46
Ni	19	80	0.24
Cu	25	50	0.50
Zn	60	75	0.80
Ga	17	18	0.94
Ge	1.2	1.8	0.67
As	7.2	1.5	4.8
Se	0.39	0.05	7.8
Br	0.85	0.37	2.3
Rb	67	90	0.74
Sr	240	370	0.65
Y	25	30	0.83
Zr	230	190	1.2
Nb	11	20	0.55
Mo	0.97	1.5	0.65
Ag	0.05	0.07	0.71
Cd	0.35	0.11	3.2
Sn	1.3	2.2	0.59
Sb	0.66	0.20	3.3
I	1.2	0.14	8.6
Cs	4.0	3.0	1.3
Ba	580	500	1.2
La	37	32	1.2
Hg	0.09	0.05	1.8
Pb	19	14	1.4
Nd	46	38	1.2
Th	9.4	12	0.78
U	2.7	2.4	1.1

Those elements are components of primary minerals, whereas primary minerals are components of parent rocks. There are almost 3000 known minerals, but only 20 are common and just 10 minerals make up 90 % of the earth's crust. Primary minerals are defined as minerals found in soil but not formed in soil. This definition is different from that of secondary minerals , which are defined as minerals formed in soils.

Most of the primary minerals (primary silicates) have a crystalline structure, i.e., a structure in which ions are arranged in an orderly and repeated spatial pattern. The fundamental unit in silicates is the silicon-oxygen tetrahedron, which is composed of a central silicon ion surrounded by four closely-packed and equally-spaced oxygen ions. The four positive charges of Si⁴⁺ are balanced by four negative charges from the four oxygen ions (O₂^- ), one from each ion, thus each discrete tetrahedron has four negative charges. The central ion may be either Al³⁺ , Fe²⁺, or Mg²⁺. When in six-folded coordination, oxygens form an eight-sided octahedron. If larger Ca²⁺, Na⁺, or K⁺ ions are present, they occur at the center of clusters of tetrahedra, with each tetradedron supplying a part of all the oxygens needed for eight-fold or twelve fold coordination. In this arrangement, the larger cations provide a center of positive charge that attracts and holds the clusters of tetrahedra together. The cations occuring in this position, i.e., outside or between neighboring tetrahedra are called accessory cations. Si⁴⁺ and Al³⁺ ions are small and have a high charge (valence). In general, the smaller the cation and the higher its valence the stronger the bond between it and the oxygen.

Figure 1. Silicon tetrahedron.

Figure 2. Aluminum tetrahedron.

Stability in minerals requires their structure to be electrically neutral, i.e., the negative charge of the O^2- in the structure must be equally balanced by the positive charge of the cations. Isomorphous substitution is the replacement of an ion with higher valence by some other kind of cation. This process is supported by a high concentration of substituting ions in a mineral-forming medium so as to increase their chance of entering the mineral structure in place. The pattern of substitution is generally the following: Al³⁺ substitutes for Si⁴⁺, and Fe²⁺ and Mg²⁺substitutes for Al³⁺. An electrical imbalance occurs because the valence of the substituting ions is lower than that of the ions in replace. Neutralization of the excess negative charge is accomplished by the inclusion of accessory cations in the structure. In the primary silicates, Ca²⁺, Na⁺ , K⁺ are the principal accessory cations that neutralize the negative charge resulting from ion substitution.

The most important primary silicates are discussed in the following:

Framework Silicates: They are composed of tetrahedra linked trough their corners into a continuous 3D-structure. Quartz is a framework silicate composed entirely of silicon-oxygen tetrahedra. The bulk density of quartz is 2.65 g/cm³ and quartz is highly resistant to mechanical abrasion and chemical weathering. Quartz is very common in most igneous, metamorphic and sedimentary rocks. In feldspars the Si⁴⁺ is partly replaced by Al³⁺, which results in a positive charge balanced by Na⁺, K⁺ or Ca²⁺ ions. In the alkali feldspars Na⁺ and K⁺, and in the plagioclase, Na⁺ and Ca²⁺ are the dominant accessory cations. Feldspars are the most abundant minerals in the earth's crust; they make up 50 - 60 % of the crustal rocks.

Figure 3. General structure of framework silicates.

Chain Silicates: The amphibole and pyroxene are chain silicates, whereas the Si⁴⁺ is also partly replaced by Al³⁺, but the chains are hold together by Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Al³⁺ and /or Ti³⁺ ions. The tetrahedras are linked to each other by sharing two of the three basal corners to form continuous chains. The pyroxene are composed of silica tetrahedra which form single chains, whereas the amphiboles are composed of silica tetrahedra double chains. Because the bonding between chains is not strong the amphibole and pyroxene are easily weatherable.

Figure 4. General structure of chain silicates.

Ortho- and Ring Silicates: They include the olivines, zircon, and titanite. In olivines the silicon-tetrahedra is arranged in sheets and linked by Mg²⁺ and / or Fe²⁺ ions. Olivine is found in basalt and volcanic rocks.

Figure 5. General structure of ring silicates.

Sheet Silicates: They are composed of three basic sheets:

Silicon tetrahedral sheet: Composed of silicon-tetrahedra linked together in a hexagonal arrangement with the three basal oxygen ions of each tetrahedron in the same plane and all the apical oxygen ions in a second plane. Thus the silicon tetrahedral sheet is a hexagonal planar pattern of silicon-oxygen tetrahedra.

Aluminum hydroxide sheet: The basic unit of this sheet is the aluminum-hydroxyl octahedron in which each ion is surrounded by six closely packed hydroxyl groups, in such a way that there are two planes of hydroxyl ions, with a third plane containing aluminum ions sandwiched between the two hydroxyl planes. In order that all the valences of the structure be satisfied only two out of every three positions in the aluminum hydroxide sheet are occupied by aluminum ions forming what is known as a dioctahedral structure.

Magnesium hydroxide sheet: This has a similar structure to the aluminum hydroxide sheet but the aluminum is replaced by magnesium, and because magnesium is divalent all the sites in the middle plane are occupied, forming a trioctahedral structure.

In pyrophyllite one aluminum hydroxide sheet is lying between two silicon tetrahedral sheets and is known as a 2 : 1 type mineral. Micas are the most common primary sheet silicates, such as muscovite (white mica) and biotite (black mica). The micas contain oxygen in octahedra as well as in tetrahedra, with both occuring in a sheet like arrangement. Because of the ratio of two tetrahedral sheets to each octahedral sheet, the micas are called 2 : 1 layer minerals. Micas are generally found in granitic pegmatites, which are coarsely crystalline, igneous rocks. In muscovite one-quarter of the silicon ions is substituted by aluminum ions in the silicon tetrahedral layers. This imbalance in charges is satisfied by potassium, which bonds the composite sheets together. In biotite within the Mg-hydroxide sheet about one-third of Fe²⁺ is substituted for Mg²⁺. The negative charges occurring in this sheet are neutralized by potassium, which bonds the composite sheets together. The potassium is positioned in the interlayer space between neighboring layers. The potassium bonding is weak, where splitting may occur.

Figure 6. Diagrammatic structure of muscovite (mica).

Table 2. Primary minerals.

Primary minerals	Chemical formula	Importance
Quartz	SiO₂	Abundant in sand and silt
Feldspar	(Na,K)AlO₂[SiO₂]₃ CaAl₂O₄ [SiO₂]₂	Abundant in soil that is not leached
Mica	K₂Al₂ O₅[Si₂ O₅]₃ Al₄(OH)₄ K₂Al₂ O₅[Si₂ O₅]₃ (Mg,Fe)₆(OH)₄	Source of K in most temperate zones
Amphibole	(Ca,Na,K)_2,3(Mg,Fe,Al)₅(OH)₂[Si,Al₄O₁₁]₂	Easiliy weathered to clay minerals
Pyroxene	(Ca,Mg,Fe,Ti,Al)(Si,Al)O₃	Easily weathered
Olivine	(Mg,Fe)₂SiO₄	Easily weathered
Epidote	Ca₂(Al,Fe)₃(OH)SI₃O₁₂	Highly resistant to chemical weathering, used as 'index mineral'
Tourmaline	NaMg₃Al₆ B₃Si₆ O₂₇(OH,F)₄	Highly resistant to chemical weathering
Zircon	ZrSiO₄	Highly resistant to chemical weathering
Rutile	TiO₂	Highly resistant to chemical weathering

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Parent Rocks

The nature of parent material profoundly influences the characteristics of even highly weathered soils. Important for soil development is the chemical and mineralogical compositions of the parent material as well as the resistance of the material. Regolith is the loose, unconsolidated material (including soil) at the surface of the lithosphere. Most of the world's soils have developed from sediments (transported unconsolidated material) that were originally derived from rocks such as glacial till, colluvium or loess. Soil development often occurs in a mixed heterogeneous material comprised of weathered bedrock and unconsolidated transported material. Saprolite is ancient residual soil and weathered rock formed by alteration of rock materials to clays and other residual material. A distinctive feature of saprolite is that the arrangement of the alteration products preserves the original rock structure and that the material has not been transported. For example, Saprolite is widespread on the Piedmont Plateau, where it has developed on various kinds of metamorphosed Paleozoic rocks (225 to 570 million years in age).

Other residual deposits comprise organic deposits, that are areas of marsh, swamp, and peat. Poorly drained areas in humid climates collect water and support vegetation adapted to wet environmental conditions (e.g. sedges). The term swamp usually applies to wet areas having trees. Either marshes or swamps may develop into peat bogs. Clayey or silty layers beneath such poorly drained ground are without oxygen, and that anaerobic condition weathers the mineral matter to sticky mud. Peat is the partly carbonized organic residue produced by decomposition of roots, trunks of trees, seeds, shrubs, grasses (reeds), ferns, mosses, and other vegetation. The decomposition is slowed because the ground is saturated with water, i.e., oxygen is excluded.

The mineral composition of parent rocks is responsible for the development of different soils. In general, the higher the content in calcium and magnesium and the lower the content of silicium in a parent rock the more likely soils with high base saturation are formed. Those soils are very productive, because of their high cation exchange capacity, which increase crop growth. Furthermore, calcium and magnesium form aggregates with clays, iron oxides and aluminum oxides, and organic matter. They improve soil structure, which is less prone to erosion.

Igneous Rocks

Igneous rocks are formed by the solidification (hardening) of molten magma in the Earth's crust. They vary in their composition of quartz and the light-colored Ca or K/Na silicates. Acidic rocks are those relatively rich in quartz and Ca or K/Na silicates with light colors (e.g. granite), basic rocks are those low in quartz but high in Ca or K/Na silicates (e.g. gabbro, basalt). Since the minerals in basalt weather more easily than those in granite, a finer-textured soil will develop from basalt. Major areas covered by igneous rocks are in Scandinavia, Canada and the Pacific coast of North America. The complete weathering of minerals in basalt in humid tropical regions produces soils composed only of clay-sized particles. Based on the mineral composition of the rocks soils formed on granite develop less fertile soils, whereas soils formed on basalt develop soils with more bases and therefore a higher fertility.

Table 3. Mineral composition of some igneous rocks [%] (Scheffer et al., 1989).

	Granite	Gabbro	Basalt
SiO₂	73.9	48.4	50.8
TiO₂	0.2	1.3	2.0
Al₂O₃	13.8	16.8	14.1
Fe₂O3	0.78	2.6	2.9
FeO	1.1	7.9	9.0
MnO	0.05	0.18	0.18
MgO	0.26	8.1	6.3
CaO	0.72	11.1	10.4
Na₂O	3.5	2.3	2.2
K₂O	5.1	0.56	0.82
H₂O+	0.47	0.64	0.91
P₂O₅	0.14	0.24	0.23

Sedimentary Rocks

These are composed of the weathering products of igneous and metamorphic rocks and are formed after deposition by wind and/or water. Cycles of geologic uplift, weathering, erosion and subsequent deposition of eroded materials in rivers, lakes and seas have produced superimposed strata of sediments. Under the weight of the overlying sediments, deposits gradually consolidate (diagenesis) to form sedimentary rocks. Often calcium (Ca) and magnesium (Mg) cement sedimentary rocks. A characteristic of sedimentary rocks is their stratification. Sedimentary rocks cover approximately 75 % of the earth's surface.

There are three major groups of sedimentary rocks:

Clastic Sedimentary Rocks

They are composed of fragments of the more resistant minerals. Dependent on the size of the deposited material (conglomerates, sand, silt, clay) different clastic sedimentary rocks are developed. The percentage of quartz is higher than 75 % in sandstones and they weather to produce sandy soils. Graywackes are high in mica and chlorite and quarzites are high in silicium (Si). Sedimentary rocks with a high fraction of clay were originated by deposition of clays in slow running water. The clays are shaped like flat leaflets and therefore under pressure they become stratified to form clay slate (shale). Soils formed on clay slate do have a high content of bases, but they are less permeable due to the high clay content and difficult to use in agricultural production. Clay slate is not very resistant to weathering.

Chemical Sedimentary Rocks

They are formed by precipitation or flocculation from solution, most commonly limestone and chalks. Limestones are high in calcium, whereas dolomites are high in calcium and magnesium. Soils developed from limestone are usually fine-textured soils.

Biogenic Sedimentary Rocks

They develop when organisms living in oceans or lakes die, sink down and become consolidated. For instance, shell limestone and marl with mollusk shells are biogenic sedimentary rock high in calcium.

Metamorphic Rocks

Igneous rocks and sedimentary rocks that are subjected to intense heat and great pressure are transformed into metamorphic rocks. Changes in mineralogy and rock structure generally render the metamorphosed rock more resistant to weathering. Examples for metamorphic rocks are marble, schist, and gneis. Marble, the metamorphic equivalent of limestone or dolomite, maybe either coarse- or fine-grained. Gneiss, which is metamorphosed igneous or sedimentary rock, consists of layers of coarse grains, usually quartz or feldspars, alternating with layers of fine-grained minerals. Schist, metamorphosed shale, consists of micaceous layers. Metamorphic rocks are common in Africa, Brasil, western Australia and India.

Reference

Scheffer F., and Schachtschabel P. , 1989. Lehrbuch der Bodenkunde, Ferdinand Enke Verlag.

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Transported Parent Material

Transported parent material is unconsolidated material (the regolith) that has undergone transport processes by wind, water or gravity.

Eolian (Wind)

Wind moves rock fragments by processes of rolling, saltation and eolian dispersion. The transport is different for clay, silt , and sand, dependent on the size of particles moved. For example, wind deposits are the sand dunes of shorelines and desert areas. Material swept away from dry periglacial regions has formed eolian deposits called loess in North America, central Europe, and China. It covers eastern Nebraska and Kansas, southern Wisconsin, southern and central Iowa and Illinois, northern Missouri, and parts of southern Ohio and Indiana, besides a wide band extending southward along the eastern border of the Mississippi River. During the glaciation, much fine material was carried miles below the ice sheets by streams. This sediment was deposited over wide areas by the overloaded rivers. Those areas without vegetation were prone to wind erosion when the climate became drier. This blown material is called loess. Loess is usually silty in character and has a yellowish color. Quartz seems to predominate, but large quantities of feldspar, mica, and pyroxene are found, too. Most loess is highly calcareous. Soils formed in loess are highly productive soils. Soils formed in loess material are susceptible to erosion, which might be a problem in hilly areas. Especially, in the Midwest where a combination of humid climate, prairie vegetation and loess material formed one of the best soils in the world.

Figure 7. Approximate distribution of loess in the United States (after Brady, 1984)

Figure 8. Loess depositions in the vine growing area Kaiserstuhl, Southern Germany.

Alluvial (Water, River)

Transport by water produced alluvial, terrace and footslope deposits. During the transport the rock material is sorted according to size and density and abraded, so that fluviatile deposits characteristically have smooth, round pebbles. The larger particles are moved by rolling or are lifted by the trubulence of the water. The smallest particles are carried in suspension. Colloidal material may not be deposited until the stream discharges into the sea, when flocculation occurs and estuarine deposits form.

Alluvial deposits are scattered along the borders of streams during flooding, whereas the material is stratified. When a flooding stream overflows its banks, coarse particles are settled close to the stream, whereas finer particles are deposited further away from the stream. Terraces are developed from floodplains as streams cut deeper channels. Several terraces may be found along a stream. Streams flowing from hills or mountains into dry valleys or basins drop their sediments in a fan-like deposit as the water spreads out. These alluvial fans are usually well drained and coarsely textured, being composed of sands and gravels. Sediments not deposited as floodplains are carried to the lake, gulf, or other bodies of water into which a stream empties. The decrease in velocity at the stream's mouth results in the deposition of much of the suspended material, thus producing deltas, which are poorly drained. Floodplains as well as deltas are generally rich in plant nutrients and comparatively high in organic matter content. Terraces and alluvial fans, on the other hand, are more likely to be less fertile.

Figure 9. Alluvial deposits: Deposition of fine and coarse sized particles if flooding occurs.

Lacustrine (Water, Lake)

When particles settle down in lakes lacustrine deposits occur. Mixed with organic material (e.g. skeleton of organisms) they may become cemented and form biogenic sedimentary rock. Glaciolacustrine deposits are the most common. Other kinds of lake deposits are small. One common kind is found in karst terrain, where limestone roofs over cavernous limestone have collapsed (e.g. in Florida). Still other kinds of lakes develop cut-off meanders at river beds. Crater lakes are formed in volcanic craters or calderas.

Marine (Water, Ocean)

It is common to find marine deposits along coastlines. This material was derived from sediments carried by streams and deposited in the ocean and gulf through decreased current velocity. Also, considerable debris is torn from the shoreline by the pounding of the waves and the undertow of the tides. If there have been changes in shoreline, the alternation of beds will show no regular sequence and considerable variation in topography, depth, and texture. These deposits have been extensively raised above sea level along the Atlantic and Gulf coast of the United States. Marine sediments are generally sandy and low in mineral nutrients.

Glacial (Ice)

During the Pleistocene (1.5 - 10,000 B.C), northern America and northern and central Europe, and parts of northern Asia were invaded by a succession of great ice sheets. As the glacial ice pushed forward, it conformed to the unevenness of the areas invaded. The mantle of soil was swept away and the underlying rocks were severely ground and gouged. Thus, the glacier became filled with rock fragments, carrying much of its surface and pushing great masses ahead. Finally, when the ice melted and the region was free, a mantle of glacial drift remained, a new regolith and fresh parent material for soil formation. The area covered by glaciers in North America is estimated at 10.4 million km² and about 20 % of the U.S. is influenced by the deposits.

Glacial till is the material deposited directly by the ice. It is a mixture of rock debris of great diversity, especially the particle size range is very large. Glacial till is found mostly as deposits called moraines composed of unassorted material. The terminal moraines characterize the southernmost extension of the various glacial lobes. The ground moraine, a thinner and more level deposition laid down as the ice front retreated rapidly. It has the widest extent of all glacial deposits. An outstanding feature of glacial till material is their variability. This is because of the diverse ways by which the debris was laid down, of differences in the chemical composition of the original materials, and of fluctuations in the grinding action of the ice. The soils formed in such material are most heterogeneous.

The outwash plains are formed by streams heavily laden with glacial sediments. This sediment is usually assorted. Such deposits are particularly important in valleys and on plains, where the glacial waters were able to flow away freely.

In many cases the ice front came to a standstill where there was no such ready escape for the water, and ponding occurred as a result of damming action of the ice. Often very large lakes were formed that existed for many years. The deposits that were made in these glacial lakes range from coarse materials near the shore to fine silts and clay in the deeper and stiller waters (glaciolacustrine deposits). As a consequence, the soils developed from these lake sediments are most heterogeneous.

It is customary to designate all the material deposited by glaciers and their melt water as glacial drift.

Figure 10. Glacial deposits: Glacial till, outwash, and loess.

Figure 11. Areas in the U.S. covered by the continental ice sheet and the deposits either directly from, or associated with, the glacial ice (after Brady, 1984).

Mass movement

Erosion by water may be divided into four categories: (1) splash, (2) sheet, (3) rill, and (4) gully erosion. Sheet erosion refers to the uniform removal of soil from the surface, whereas rill erosion occurs concentrated in rills (or gullys), i.e., unevenly distributed across an area. The deposited material is called colluvium. Colluvium includes only those deposits that are or have been moving slowly downslope by ground creep. It is generally a chaotic mixture of coarse and fine-grained materials. Thickness of colluvium are generally less than 3 m and rarely more than 8m. Solifluction is a process which occurs on partly frozen soils, where the lower part of the soil profile is frozen and the upper part is highly saturated by water. The upper part of the profile flows slowly downhills. Mudflows (fine-grained debris) and avalanche (coarse-grained debris) are extreme events of mass movement acting upon gravity force.

Figure 12. Colluvium deposits.

Volcanic Activity

Volcanic ash is an amorphous (non-crystalline), fine, dustlike material thrown out of volcanoes. Ash falls on the surrounding land to form thick sediments for soil development. Other materials resulting from volcanic events are scoria, pumice, and bomb.

Significance in Pedology

The symbol 'w' is used for horizons with development of color and structure. For example, a 'Bw' denotes a B horizon that has developed structure or color different, usually redder, than that of the A and C horizons but do not have apparent illuvial accumulations. Buried horizons which might develop due to erosion and deposition are designed by a 'b'. A ' r' denotes weathered bedrock. This symbol is only used with a master C horizon. It designates saprolite or dense till that is hard enough that roots only penetrate along cracks, but which is soft enough that it can be dug with a spade or shovel.

Reference

Brady N.C., 1984. The Nature and Properties of Soils. MacMillan Publishing Company. Inc., New York.