In soils, the mineral assemblage may be more complex than often found in rocks. Soil may contain certain primary minerals, weathered minerals, minerals transported from diverse parent materials and locations, relict features from previous soil forming processes, and minerals formed in situ through pedogenesis. The mineral suite may yield important information concerning mineral stability in the current environment and direction of pedogenesis. The following is a brief list of the more common minerals found in soil, and a few optical properties used for identification.

Quartz: is the most common soil mineral identified in soil thin-sections. It may exhibit euhedral morphology (well formed crystal faces) but usually is anhedral (without regular crystal faces) in soil, lacks twinning and cleavage, and is colorless and transparent in plane polarized light. The interference colors are of low order (gray) but are bright and warm. Quartz shows weak birefringence and refractive index is 1.54. Extinction ranges from sharp within a small angle of rotation (blink extinction) to the more common undulatory (wavy or band) extinction. Undulatory extinction is attributed to distortion or deformation of the crystal structure largely due to mechancial strain. Inclusions are common, either solids (rutile, tourmaline, epidote), liquids or gasses (bubble trains). Quartz is quite stable in most soil environments, and not easily weathered. Chert is a very fine-grained quartz (micorcrystalline), it is often considered to be chemically precipitated. Chert should not be confused with siltsone Siltstone will generally show a cementing of individual grains. Chalcedony is cryptocrystalline quartz having birefringence about that of quartz. It is invariably fibrous in character, often including radiating or spherulitic aggregates. Chalcedony and chert are usually colorless, but may exhibit weak color without pleochroism.

Feldspars: Feldspars are common minerals, but due to weathering, are not often observed in abundance in soils, and usually absent in strongly weathered soils. Feldspars are usually colorless in thin section, have low relief and lack pleochroism. Orthoclase (K-feldspar)may resemble quartz, but the refractive index is about 1.52, birefringence is lower, and the mineral may show cleavage in two directiions. Orthoclase may show twinning (carlsbad twinning), but may be untwinned and be mistaken for quartz. Microcline has a refractive index of 1.53, and exhibits a characteristic twinning intergrowth producing a plaid or grid effect (polysynthetic twinning) in crossed polarized light. The refractive indexes in the plagioclase group (Na-Ca feldspars) increases with the proportion of calcium. The refractive index of albite (Na-feldspar), 1.53, is below that of quartz, but the refractive characteristic twinning that appears as alternating dark and light bands in crossed polarized light (pericline twinning). Cleavage is good. Lath and prismatic shapes are common yielding parallel or inclined extinction. Perthite is the intergrowth of K and Na feldspars, commonly albite in microcline or orthoclase. Perthite may give the appearance of twinning, but the intergrowths often are irregular and not as sharp as twins. Since this is an intergrowth of two feldspars, differential weathering may result in one phase being altered. Feldspars may weather to sericite (microcrystalline mica), hydrous mica, kaolinite, or gibsite depending on environmental conditions.

Carbonates are commone to many soils, but are easily weathered. Calcite is the most common carbonate and may be pedogenic, lithogenic (cementing agent) or inherited (limestone clasts, fossil shells). Dolomite may occur pedogenically under certain conditions, but is still considered as a lithogenci mineral. It usually occurs as rhombohedral crystals, but this is not always diagnostic. Carbonates are colorless in thin-section (plane polarized light) and are readily recognized by their high interference color, rhombohedral cleavage and common twinning in dolomite. In soils, calcite may occur as cementing material in aggregates, microcrystalline coatings (calcitans) and other fine-grained segragations often mixed with clay and other minerals. The extremely high interference color always is the clue to identification; it has bright colors in crossed polarized light and marked change in relief when the stage is rotated in plane polarized light. Recent shell fragments may be recognized by their unique microsturcture and limestone fragments may have included fossils. Distinguishing between pedogenic and lithogenic calcite is not always easy. Staining techniques are often necessary to distinguish calcite and dolomite.

Gypsum occurs in numerous habits, but usually as platy, prismatic, or lenticular grains with refractive index about the same as orthoclase, low relief and weak birefringence. In some instances it may look very similar to quartz. It is colorless in thin section, commonly twinned, and identification is primarily by its prismatic or lenticular morphology. Its morphology, however, is often determined by the environment of formation. Gypsum is often a pedogenic mineral, precipitated as a result of the oversaturation of the soil solution, and commonly occurs in voids and channels of arid and semi-arid soil. Gypsum may form in some environments due to oxidation of pyrite. Gypsum may be confused with barite, but barite has a higher relief in thin section.

Barite is colorless and non-pleochroic with moderate relief and weak interference colors. It is prismatic to lenticular in shape with cleavage in one direction. Not too common in soils, but may be found in saline soils or saturated conditions.

Mica occurs as platy grains that often are thin. All the micas are closely related structurally and crystallographically. In thin section, micas are characterized by distinct relief, strong birefringence, single cleavage, low to moderate relief, positive elongation, and parallel extinction. Twinning is rarely observed. All micas show a characteristic granular or speckled surface texture ("bird's eye" effect) in cross polarized light. Biotite shows substantial variability in chemical composition, and thus, variable optical properties. The interference color is often masked by the mineral's color. Biotite is highly colored and strongly pleochroic in shades of green or brown while muscovite is colorless in plane polarized light. Biotite often has Zr halos as radioactive disintegration products. As micas weather, they become more difficult to identify.

Pyroxenes are a complex, single-chain, silicate mineral group which are characterized by good prismatic cleavage, the two directions of which make an angle of about 90°, distinguishing them from amphiboles. Birefringence is generally high and moderate and high relief is typical. Many pyroxenes exhibit parting, commonly parallel to the 100 or 001 planes. Twinning may also be present. Refractive indices (1.65-1.79) are somewhat higher than amphiboles. Enstatite and hypersthene (orthopyroxenes) are prismatic and have parallel or symmetrical extinction. Enstitite is usually colorless, but hypersthene is frequently pleochroic in shades of pink to blue-green. Augite and diopside (clinopyroxenes) have inclined extinction, colors usually in shades of green, and weak pleochroism. Color and pleochroism increase with iron content. In the absence of diagnostic criteria, it is preferable to use the general term orthopyroxene or clinopyroxene rather than the more specific mineral name. These minerals are not common in soils because they weather easily.

Amphiboles constitute the most chemically complex mineral group in nature, of which hornblend is the most common. Ohters include tremolite and actimolite. These minerals are commonly euhedral to subhedral columnar, irregular prismatic, bladed or acicular crystals. The platy or prismatic minerals exhibit parallel to slightly inclined (<30°)extinction depending on orientation, and moderate to high positive relief. However, extinction angles are variable and do not appear to be diagnostic in identifying mineral species. the minerals generally have good cleavage at angles of about 56 and 124°. Hornblend is generally strongly colored and pleochroic in shades of yellow, green and brown. It has moderate relief and strong birefringence. Twinning is not observed. Refractive index in the group ranges from 1.61 to 1.73. Hornblend is moderately resistant to weathering but commonly weathers to chlorite.Olivine is colorless (Mg-rich Fosterite) to olive (Fe-rich Fayalite) in plane light. colored varieties may show moderate pleochroism; twinning is not common. The most characteristic features are the high relief (higher than most pyroxenes), the lack of well developed cleavage, irregular fracture pattern, and the warm interference colors (blue, green). Olivine is common in basic igneous rocks and alters very rapidly, in fact it is much more commonly found altered than fresh. Iddingsite is a common alteration product. Cracks may be filled with serpentine or goethite.

Garnet. This is a complex group of minerals with wide compositional variations. They are a common detrital mineral. Garnets are isometic and usually isotropic, although some varieties may exhibit weak birefringence. colors range from colorless to pink to dark red, and yellow to dark brown. Darker garnets may show zoning and cleavage is lacking. Garnet commonly occur as euhedral six-sided to eight-sided crystals. Their high refractive index (about 1.8) sets them apart from other minerals.

Kaolinite. Kaolinite is one of the few clay minerals that may be observed in soils. It is because of its common larger size and in unaltered rocks may show a characteristic vermiform morphology. It has weak interference colors and good cleavage. Other minerals may have a similar morphology, so other identification means such as X-ray diffraction are needed for positive identification.

Opaques. These minerals are opaque because they do not pass light. Most are opaque due to their high content of iron or manganese. Many opaques are pedogenic minerals. They are also found as inclusions in other minerals. These minerals are usually studied in reflected light. Magnetite and Ilmenite are difficult to tell apart, both appear dull gray in reflected light. Ilmenite often has an opaque white alteration product (leucoxene) on its surface or border. Pyrite gives a pale brassy color with a metallic luster. It often occurs as rectangular crystals. Pyrite is mainly associated with acid sulfate soils. Hematite has a metallic luster with a color ranging from dark steel gray, dull red, to iron black. Large crystals of hematite are a main constituent of most laterites. Goethite is one of the primary pigmenting agents in soils usually occurring as small grains dispersed throughout the matrix or as grain coatings. Identification is difficult because of the small crystal size.

Lithorelicts include pieces of sandstone, siltstone, shale, limestone, and fine-grained igneous or metamorphic rocks. These clastics are derived from an outside source and incorporated into the soil parent material. These clastics are derived from an outside source and incorporated into the soil parent material. Identification depends on recognition of structure and individual components, and consideration of possible sources. Shales and siltstones frequently show bedding and limestone may have fossils. The presence of lithorelicts often gives clues to the origin of the soil such as glacial outwash.

Artifacts are items that may be observed in thin section but have been introduced during sample preparation. Foreign objects introduced in the section include lint fibers, grinding compound, and bubbles. Other artifacts may include grain fracturing, shrinkage cracks caused by sample desiccation or resin shrinkage, void areas due to grain plucking or imcomplete impregnation, improper or uneven polishing, and slight birefringence of the impregnating medium caused by strain or improper curing.

Orgainic Matter. Most soils support both plant and animal life, both of which may reach considerable depths and leave evidence of their passage. Organic matter may occur in shades of brown or black and may show evidence of cellular structure. Near the surface, organic litter and decomposed products may be common. Roots (either fresh or decomposed) may be distributed throughout the soil, but decrease in frequency with depth. Fresh organic material may be identified by the cellular structure and high interference color. Decomposed organic material may retain some cellular characteristics, but is often isotropic. Fecal material is commonly recognized by its characteristic morphology, usually granular or vermicular. Fecal material may commonly contain both mineral and organic material. Some void patterns are caused by roots or burrowing animals. Soil material and fecal pellets may infill these chambers.

This page (http://www.ces.ncsu.edu/plymouth/programs/useful.html) created by
Vera MacConnell, Research Technician, I on November 21, 1997.
Last Updated on December 1, 1997.