Carbonates, Evaporites, and Accessory Minerals

Readings

Doner, H.E., and W.C. Lynn. 1989. Carbonate, Halide, Sulfate, and Sulfide Minerals. Ch. 6, p. 331-378. In: J.B. Dixon and S.B. Weed (ed.), Minerals in Soil Environments, 2nd Edition.

Carbonates

Carbonate minerals come in a variety of forms with a variety of cations. The two most common polymorphs of calcium carbonate, CaCO₃ give their names to two different isotypes, the calcite isotype and the aragonite isotype, which differ in their crystal structure. The calcite isotype is trigonal of space group R-3c, with a rhombohedral unit cell, while the aragonite isotype is orthorhombic of space group Pmcn. The difference in "packing" of Ca and CO₃ into the two polymorphs creates a slight but significant difference in density, expressable as molar volume. Cations smaller than Ca²⁺ can fit into the calcite structure in which 6 oxygens encompass the central metal cation but cations larger than Ca²⁺ fit into the denser aragonite structure that surround each metal cation with 9 oxygens. (Shades of Pauling's Ratio Rule!!)

 

Oddly, yet a third, relatively rare calcium carbonate polymorph is known--vaterite. It too is orthorhombic, with space group Pnma, and packs 8 oxygens around each Ca²⁺. In recent years, a fascinating method of producing vaterite crystals has been reported. A thin layer of stearic acid is placed over a supersaturated solution of calcium bicarbonate and compressed to form a highly structured monomolecular layer, oriented with the ionized carboxylate groups of the stearate forming a negatively charged layer in the solution. The spacing of the cations in vaterite does not coincide with the spacing between the charged groups in the stearate layer so it is presumed that the stearate does not serve as a template as much as simply "condensing" Ca concentration in the vicinity of the layer to make formation of vaterite favorable.

In addition to the single metal species of the calcite isotype, i.e., Co (sphaerocobaltite), Mg (magnesite), Ni (gaspeite), Zn (smithsonite), Fe (siderite), Mn (rhodocrosite), Cd (otavite), and Ca (calcite), regular interlayering of Ca and Mg produce both dolomite, CaMg(CO₃)₂, and huntite, CaMg₃(CO₃)₄.

Isomorphous Substitutions among Carbonates

With so many different metal carbonates that can be formed all within the same space group, it is not surprising that many "impurities" are found. For example, calcite will often contain as much as 5 or 10% Mg substitution, sometimes even more if the calcite is of biogenic origin, and is known as magnesian or magnesium calcite. Changes in the specific x-ray spacings of calcite have been used to quantify the degree of Mg substitution in calcites. Magnesite (MgCO₃) has a solid solution series to siderite FeCO₃which has a solid solution series to rhodocrosite (MnCO₃). Interestingly, dolomite (CaMg(CO₃)₂) has relatively little deviation from an ideal 1:1 Ca:Mg ratio, except when the Mg is substituted by Mn, leaving the Ca layer intact.

Carbonate Occurrence

The table below provides compositions and occurrences of a variety of common carbonates:

Mineral	Composition	Occurrence
Calcite	CaCO3	soil, limestone, igneous
Aragonite	CaCO3	biological (shells, molluscs)
Siderite	FeCO3	sediments
Nesquehonite	MgCO3 · 3H2O	evaporite
Magnesite	MgCO3	evaporite
Dolomite	CaMg(CO3)2	dolostone
Soda	Na2CO3	evaporite

 

The most common carbonate mineral in soils is calcite, present in ~30% or more of the world's soils. Calcite is sparingly soluble in soil solution, with higher solubilities at lower pH values. Free calcium carbonates typically do not persist in soils with pH values below 7.0, except as unweathered fragments, and pH values of 7.5 to 8.2 are not at all uncommon for calcareous soils. In calcareous soils, calcite is often found in the silt and the clay (<2 µm) size fractions, often as a result of secondary precipitation, if the method of sample preparation has not removed carbonates before analysis. Sorption of phosphate on calcite is well-known. Calcite may occur in soils from a variety of sources: as rock or sand-sized grains inherited from parent rock; as diffuse secondary carbonates of pedogenic origin; as threadlike or soft masses of secondary carbonates; as casts around roots; or as moderately to highly cemented petrocalcic horizons. Aeolian origins are also common in semi-arid to arid environments.

Of the two other polymorphs of calcium carbonate, aragonite is occasionally present in soils, usually inherited from parent material or fresh shells. Vaterite has not been found in soils.

Precipitated calcite crystals, scanning electron microscope x1250. Nater, Sherman, and Barak, 1996.	Ground dolomite crystals, scanning electron microscope x1250. Nater, Sherman, and Barak, 1996.

Other soil carbonates occur either from the parent material (such as dolomite), in sediments (siderite), or as evaporites (magnesite, nesquehonite, and soda). There are scattered reports of pedogenic dolomite in the literature but are often discounted a priori because of the absolute failure of all attempts to form dolomite under laboratory conditions at earth surface temperatures, i.e., without significant heat and pressure. In fact, little evidence exists of the contemporary formation of dolomite under any natural circumstances, which leads to "The Dolomite Question", where did all the tens and hundreds of meters of solid dolostone come from? Considering the possibility of dolomitization of calcite as an explanation, it has been calculated that the amount of time required for solid diffusion of Mg²⁺ from seawater into limestone deposits to convert calcite to dolomite exceeds the life-span of the earth! Also, difficult to imagine how the miscibility gap between magnesian calcites and dolomite is bridged. Magnesite is similarly difficult to impossible to form in the laboratory under earth surface conditions. Large amounts of dolostone (="dolomitic limestone") are used as agricultural limestone in acid soils.

Siderite (FeCO₃) is reported to be formed in significant amounts in brackish and freshwater swamp sediments but is chemically unstable if the sediments are drained and therefore oxidized. Although it is held by Donner and Lynn (1989) that siderite does not form as a secondary mineral in soils, explanations of the chemistry of paddy soils--a seasonally reducing environment in which decomposing organic matter produces significant concentrations of dissolved CO₂/bicarbonate and Fe²⁺--usually invokes the formation and dissolution of siderite as a factor.

The carbonate contents of soils are typically measured by measuring CO₂ evolved either upon addition of acid or heat (>900°C). Distinction between dolomite and calcite has been made by differences in rate of acid dissolution. Staining techniques have long been used to identify calcite, aragonite, dolomite, and magnesian calcite. The presence of calcium carbonates is well-known to interfere with standard methods of determining exchangeable Ca²⁺ by displacement with buffered salt solutions, e.g., 1 M NH₄OAc at pH 7, and determinations of cation exchange capacity or "effective" cation exchange capacity by such techniques are inherently flawed when applied to calcareous soils.

All carbonates weather fairly readily; the evaporites are readily soluble in water, but the calcium carbonates and dolomite have relatively low solubilities in circumneutral soils and can persist or even form in soils in semi-arid to arid environments. Secondary calcium carbonates or gypsum are one of the most common indicators of soil development in semi-arid and arid environments.

Evaporites

Evaporites are minerals that form readily by precipitation during the evaporation or desiccation of a solution and that have solubilities equal to or higher than that of gypsum. The term does not describe a specific group of minerals based on composition or structure, but rather on their behavior in soils and sediments.

The most commonly occurring evaporites in soils are members of one of the three groups given below. The following is only a partial list of the many species that can be found in soils, near seeps, or in evaporite basins.

Sulfate Minerals

Sulfate Species	Composition	Occurrence
Gypsum	CaSO4 ·2H2O	soils, gypsiferous rock
Anhydrite	CaSO4	evaporite
Mirabilite	Na2SO4·10H2O	evaporite
Thenardite	Na2SO4	evaporite
Epsomite	MgSO4 · 7H2O	evaporite
Bloedite	Na2Mg(SO4)2 · 4H2O	evaporite
Barite	BaSO4	evaporite
Jarosite	KFe3(OH)6(SO4)2	acid sulfate soils and mine drainage waters

 

 

Gypsum is the most common evaporite mineral occurring in soils. A saturated solution of gypsum is 15 mM CaSO₄; hence, it is generally found in environments more arid than those required by considerations of calcite persistence, and is often associated with soils with a high soluble salt content as well. It can be inherited from gypsiferous parent materials or formed as a secondary mineral. It can occur as fibrous masses, small dispersed crystals, concretions, or in petrogypsic horizons where it cements soil materials together. It is moderately soluble in water and can be easily leached from soils. Parent materials high in gypsum can form pseudo-karst landscapes where the gypsum has been leached, producing caves and subsurface porous features. Even simple irrigation of highly gypsiferous soils can produce significant pseudo-karst features, including sinkholes. Gypsum--either from geological deposits or industrial by-products--is occasionally used to ameliorate sodic soils; dissolving gypsum supplies not only Ca²⁺ to replace exchangeable Na⁺ but also increases the total electrolyte concentration to prevent dispersion of clays.

Other sulfate minerals are of more restricted occurrence. For example, mirabilite and thenardite have been found in evaporation ponds receiving drainage waters from soils of the Kesterton Reservoir project in the San Joachin Valley of southern California. Jarosite is an oxidation product of iron sulfides in acid sulfate soils and in acid mine drainage water.

Measurement of the gypsum content of soil usually involves adding sufficient water to a sample to dissolve the gypsum entirely and then determining the amount of calcium sulfate or additional salt in that water extract. Standard methods for determination of exchangeable Ca²⁺, cation exchange capacity, and soluble salts in soil solution are of dubious value for gypsum-containing soils and special modifications are required to compensate for the tendency of gypsum to dissolve into the extracting solutions until exhausted.

Chloride Minerals

Chloride Mineral Species	Composition	Occurrence
Halite	NaCl	evaporite
Sylvite	KCl	evaporite

Chloride minerals are highly soluble in water and can be readily leached from soils. They typically occur only in areas, such as playas, seeps, or discharge areas, that collect subsurface flow.

Among the evaporites are to be found potassium ores, extensively mined primarily for fertilizers. Among them, sylvite (KCl) above, but also "double salts" with magnesium, such as carnallite (KCl·MgCl₂·6H₂O), kainite (KCl·MgSO₄·2¾H₂O), and langbeinite (K₂SO₄·Mg₂(SO₄)₂).

Nitrate Minerals

Nitrate minerals are relatively uncommon except in very arid areas and occasionally in seeps and dry caves. They are all highly soluble.

Nitrate Mineral Species	Composition	Occurrence
Niter	KNO3	evaporite
Nitratine	NaNO3	evaporite
Nitrocalcite	Ca(NO3)2 · 4H2O	evaporite
Nitromagnesite	Mg(NO3)2 · 6H2O	evaporite

 

General Characteristics

Because evaporite minerals are all soluble in water to some degree, they only occur in relatively arid sites or locations in the soil where desiccation occurs. They are generally associated with areas where evapotranspiration (ET) is much greater than precipitation, and leaching does not occur to great depths. They may also occur in seeps where water reaches the surface from subsurface flow. Consequently, evaporite minerals are seldom observed in soils that are not in ustic, aridic, or xeric moisture regimes. Deposits of nitratine, mined as caliche in Chile, was marketed for many years as "Chilean saltpeter" was an important nitrogen fertilizer source before the industrial fixation of N₂ by the Haber-Bosch process.

When present, evaporite minerals have a major influence on the behavior of the soil. They greatly affect physical and chemical properties such as swelling behavior, pH, and soil aggregate stability. Their greatest effect, however, is on the biological processes occurring in soils, as the presence of quantities of evaporite minerals more soluble than gypsum can cause significant osmotic potentials in the soil that are detrimental to plant growth.

Methods used to study evaporite minerals

Because evaporites are very soluble, they will dissolve if samples are exposed to the normal pretreatments used prior to XRD analyses. Consequently, other techniques must be used. The most obvious technique is to use random powder XRD on dry, ground samples of whole soil. This method is good if the evaporite mineral constitutes more than about 5 % of the total mass of the sample; otherwise the peaks associated with the evaporite minerals may be lost in the background noise. If the evaporite constitutes a smaller fraction, it can sometimes be removed from the soil by hand picking of macroscopic crystals, which is often the case. However, this doesn't work for fine-grained crystals, and other methods must be employed.

For minerals for which these techniques will not work, it is sometimes considered acceptable procedure to dissolve the evaporites by placing the sample in an excess of water, and then reform them by allowing the extract to desiccate. This method may produce artifacts, especially when dealing with minerals that form several hydrated polymorphs. Selective dissolution treatments (or even water) can be used in conjunction with chemical methods. One can also dissolve all but one kind of mineral if one uses solutions that are saturated with respect to the mineral of concern.

Likewise, excessive sample drying (especially at elevated temperatures) can cause the transformation of some hydrated minerals into less hydrated forms, thus changing the mineral structure and providing false information.

It turns out that one of the easiest ways to identify many of the evaporite minerals is by SEM morphology in conjunction with energy dispersive x-ray (EDX) analysis. The combination of compositional analysis and morphology are often conclusive, although there is some problem with the carbonate minerals, as C and O are not detectable by most EDX analyzers. In modern instruments one can also determine structural parameters by rocking beam techniques and the patterns thus produced.

Sulfides and Sulfide Oxidation Products

Sulfides form under strong reducing conditions in the presence of sulfates. Sulfate is reduced to sulfide (S^2-) ion and complexes with reduced metal ions, particularly Fe²⁺. Many other metals can form sulfides; Fe is just more common than most of them. Most sulfides are relatively impure, scavenging available metals from solution. Additionally, sulfides often form so rapidly that the minerals produced are poorly crystalline, making identification difficult if not impossible.

Numerous types of sulfides can be present as inclusions in primary minerals or igneous or sedimentary rocks, but they oxidize rapidly once exposed to aerobic soil environments. The oxidation process is typically driven by sulfide oxidizing bacteria. Oxidation of sulfide releases huge quantities of hydronium ions, driving the pH very low. Acid mine drainage, caused by oxidation of high sulfide coal wastes, can have pH values significantly lower than pH 1.0.

 

Sulfide oxidation products are minerals formed during the rapid oxidation of sulfides, typically by sulfur oxidizing bacteria. The two most common minerals are jarosite and natrojarosite. Both have yellow crystal form and typically have a strong sulfurous smell.

Compositional and occurrence data are given below.

Sulfide Mineral Species	Composition	Occurrence
Pyrite	FeS2	reduction product
Marcasite	FeS2	reduction product
Mackinawite	FeS	reduction product
Greigite	Fe3S4	reduction product
Jarosite	KFe3(OH)6(SO4)2	oxidation product
Natrojarosite	NaFe3(OH)6(SO4)2	oxidation product

 

Accessory Minerals

Accessory minerals are those minerals that occur in small quantities in soils. They are generally inherited from the parent materials, and many of them are very resistant to weathering (i.e., they are relatively inert from a chemical perspective).

Igneous rocks are commonly dominated by a few to several minerals from one or more of the silicate classes. Quartz and feldspars dominate most mineral assemblages, but some more basic rocks may be dominated by feldspars, pyroxenes, olivines, and even chlorites. However, many other minerals are usually present, occurring in such small quantities and/or small crystal sizes that they are easily overlooked. These minerals commonly occur as very small crystals, usually of silt size, and are often present as inclusions in other, larger crystals.

 

Accessory Mineral Species	Composition	Occurrence
Rutile	TiO2	primary
Anatase	TiO2	primary
Apatite	Ca5(PO4)3(F,Cl,OH)	primary
Magnetite	Fe3O4	primary, sediments
Ilmenite	FeTiO3	primary
Tourmaline	(Na,Ca)(Li,Mg,Al)(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4	primary
Sphene	CaTiO(SiO4)	primary
Zircon	ZrSiO4	primary
Garnets	numerous species	primary

 

Some of these minerals are the main suppliers of one or more trace elements to the soil. For example, almost all of the phosphorous in soils is originally derived from the weathering of apatite, while tourmaline is usually the major supplier of boron. However, most of the trace elements in soils are supplied from the weathering of more common minerals, such as the feldspars or hornblendes, where the elements are present as trace constituents. Although their concentrations are low in these minerals, the relative quantities of these minerals in the soil are quite high compared to the accessory minerals, so the overall contribution from the more common minerals is often significant.

Most of the accessory minerals described above have high densities (> 2.9 g cm^-3), and are referred to as the heavy mineral fraction of soils. They can be easily extracted from soil samples by centrifugation in heavy liquids (density > 2.9 g cm^-3), where they sink. The majority of minerals present in soils have densities on the order of 2.65 to 2.75 g cm^-3, so they float on the heavy liquids, leaving a clean separation. (Note: clay-sized and very fine silt-sized minerals are more difficult to separate because Brownian motion interferes with their Stoke's settling rates). These minerals are very useful in the study of soil genesis because many of them are resistant to weathering, thus providing information about the parent materials and the relative amounts of more weatherable minerals that have been removed from the parent rock.

In particular, different parent materials usually have different proportions of different heavy minerals, thus making identification of the presence of more than one parent material possible. If two sediments had the same source rock origin, we would assume that they would have similar heavy mineral fractions; i.e., the kinds and relative proportions of the different heavy minerals would be similar. Thus, the analyses of the heavy mineral fraction can sometimes provide a useful tool for determining if two sediments were derived from the same or a different source.

Authors: Ed Nater and Phillip Barak
Dept of Soil, Water, and Climate, U of Minn. and Dept of Soil Sci, U of Wisc.

Copyright: Ed Nater and Phil Barak
Copyright for mineral models held by the Minerals & Molecules Project

The opinions expressed herein are those of the authors and do not necessarily represent those of their respective universities or their Regents.