Muscovite weathering soils

RICH (C.I.), 1958. Muscovite weathering in a soil developed in the Virginia Piedmont. Clays and Clay Min. 5, 203-213. 

The most abundant fine-grained mica in soils is aluminous as well as dioctahedral and, thus, is more similar to muscovite than to any other large-grained specimen-type mica. Biotite and phlogopite, like fine-grained micas, are probably next in abundance, but these minerals are present only in slightly weathered soils. 

Rich, C. I., 1956. Muscovite weathering in Virginia piedmont soil. Proc. 5th Nat. Conf. Clays Clay Min. 203. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

Forster (1961, 1963) reported a similar release and uptake of K by the mycelium of A. niger and a variety of other soil fungi when incubated with orthoclase and oligoclase. Weed et al. (1969) demonstrated that fungi weathered biotite, muscovite and phlogopite to vermiculite by acting as a sink for the K released from these minerals. Wheat plants apparently function in this manner during the alteration of biotite to vermiculite (Mortland etal., 1956). 

The rates of dissolution of carbonates and aluminosilicates as a function of pH are generalized in Fig. 2.11. Calcite and dolomite dissolution rates are generally 10 to 1 O -fold faster than rates for the silicates and decrease with pH up to saturation with the carbonates, usually between pH 8 and 10. Dissolution rates among the silicates range widely and are greatest for rapidly weathered minerals such as nepheline and olivine and slowest for quartz, muscovite (illite) and kaolinite, important products of chemical weathering in soils, discussed in more detail in Chap. 7. 

The formation and survival of unstable or metastable micas and clays in sediments and soils at low temperatures reflects kinetic as well as thermodynamic factors. First, the rates of reactions involving solid-aqueous and especially solid-solid transformations in dilute solutions are very slow at low temperatures (most natural waters are dilute )- The slow kinetics of clay transformations reflects small differences in free energy between stable and metastable clays. Also, the occurrence of specific clays is related to the chemistry and crystal structure of source minerals. Thus, illite often results from the weathering of muscovite, and vermiculite results from the weathering of biotite (cf. Drever 1988), consistent with the similar chemistries and structures of these pairs of T 0 T minerals. 

The watershed is underlain by the Lower Pelitic Schist of the Wissa-hickon Formation. The minerals constituting the rock, in order of abundance, consist of quartz, muscovite, oligoclase (An24Ab7c), biotite, and staurolite with minor amounts of garnet, tourmaline, zircon, apatite, chlorite, and pyrite. The bulk chemical composition of the unweathered rock calculated from the modes is shown in Table I. The mantle of weathered rock and soil consists primarily of quartz and muscovite in the sand sized fraction and kaolinite and illite in the clay sized fraction. Gibbsite was observed in samples from the tops of the drainage divides. 

Soil consists of weathered material (clay) of the bedrock and other material. The main clay minerals are silicates and aluminosilicates derived from muscovite, bio-tite, olivine, and pyroxene. Iron oxide and aluminum hydroxide also constitute clay minerals. For example, olivine wiU be slowly eroded by the effect of air and water. The chemical reactions involved are summarized as follows ... 

Mica is included in the weatherable minerals in the 7th approximation [1967]. But there is recognition of the resistance of muscovite to weathering, for in the case of oxic horizons, less than 3 % of the soil should be weatherable minerals, but muscovite may constitute as much as 6 % of the fraction. 

An extensive chemical and microscopic study of mica separated from 21 Piedmont soils from the Piedmont of southeastern United States was made by Denison et al. [1929]. The fraction analyzed was greater in size than that passed through a 200-mesh sieve. Weathering was evident in even these sand-size particles. In all soil profiles, biotite seemed to be altered to the same extent, the K2O content usually approximated 4%. On the other hand, muscovite, as described by the authors, had a wide variance in K2O content. This muscovite in some profiles contained less than 1 % K2O, whereas in other profiles, the mineral contained as much as 9% K2O. The authors proposed that there were two kinds of muscovite present—some secondary, having formed from the products of potassium-feldspar weathering, and some primary, which muscovite inherited from the parent material. The identification of material with 1 % K2O as muscovite would be questioned today. The use of X-ray diffraction for soil clay identification occurred after this microscopic work was done. It would have been useful to examine the < 200-mesh particles to supplement the work done with the microscope on particles of a size greater than 200 mesh. 

Mica is a very common constituent of soils other than laterites. The earth s crust contains 1.4% muscovite and 3.8% trioctahedral micas (Ahrens [1965]). On the average, soil clays and silts probably contain more mica. The percentage of mica in the silt and clay fraction of the average soil is probably closer to 10%, and this is largely dioctahedral mica. Soils are derived more from sedimentary rocks than from other rock types. Since mica is a common component of shale, and shale is a common sedimentary rock, natural rock weathering and soil-forming processes have tended to favor the concentration of mica in the silt and clay fractions of soils. 

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