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
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 calcium carbonate in the form
of calcite. Two other polymorphs of calcium carbonate, aragonite and vaterite,
also exist; however, neither is common in soils. Other carbonates occur either
from the parent material (such as dolomite), in sediments (siderite), or as
evaporites (magnesite, nesquehonite, and soda).
Calcite is sparingly soluble in soil solution, with higher solubilities at
lower pH values. Free calcium carbonates typically do not occur in soils with
pH values below 5.0. 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.
Dolomite also occurs in soils, mainly as a primary mineral inherited from
the parent material, notably from dolostone or its weathering products. It
is quite common in soils in many regions. There is some discussion as to the
potential for its formation in soil environments, but that hypothesis has
not yet been proven. Regardless, the vast majority of dolomite occurring in
soils is of primary origin.
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 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 |
Gypsum is the most common evaporite mineral occurring in soils. Its occurrence
and behavior is similar to that of calcite, but it is significantly more soluble;
hence, it is generally found in more arid environments than calcite. It can
be inherited from gypsiverous 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.
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.
Nitrate Minerals
Nitrate minerals are relatively uncommon except in very arid areas and occasionally
in seeps. 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 ET >> 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.
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 (S2-) 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
sulfate 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 chemcial 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 |
| 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 g-1), 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 g-1), where they sink.
The majority of minerals present in soils have densities on the order
of 2.65 to 2.75 g g-1, 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.
Author: Ed Nater
Department of Soil, Water, and Climate
Copyright: Ed Nater
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.
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