Biotite weathering soils

Blum J. D. and Erel Y. (1997) Rb-Sr isotope systematics of a granitic soil chronosequence the importance of biotite weathering. Geochim. Cosmochim. Acta 61, 3193-3204. 

Biotite (Pe2+. bearing micas) Phyllosilicates Granitic and high-grade metamorphic rocks R Stable in only the youngest or least weathered soils precursor of other 2 1 soil clay minerals and Fe-oxides source of K... 

Knauss KG, Wolery TJ (1988) The dissolution kinetics of quartz as a function of pH and time at 70 °C. Geochim Cosmochim Acta 52 43-53 Kodama H, Schnitzer M, Jaakkimainen M (1983) Chlorite and biotite weathering by fulvic acid solutions in closed and open systems. Can J Soil Sci 63 619-629 Krauskopf KB (1967) Introduction to geochemistry. McGraw-Hill, New York, 617 pp Krumbein WE, Werner D (1983) The microbial silica cycle. In Krumbein W E (ed) Microbial geochemistry. Blackwell Scientific, Oxford, pp 132-143 Kummert R, Stumm W (1980) The surface complexation of organic acids on hydrous a-AI2O3. J Colloid Interface Sci 75 373-385... 

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. 

Wilson, M.J. Farmer,V.C. (1970) A study of weathering in a soil derived from a biotite-... 

The over-all picture of what happens to the soil waters, as illustrated by Tables II and IV, is that initially they rapidly attack the rocks, kaolinizing chiefly plagioclase plus biotite and K-spar. As they penetrate more deeply, the reaction rate slows down, and both kaolinite and mont-morillonite are weathering products. Also, an important part of the Ca2+ comes from solution of small amounts of carbonate minerals. 

Trioctahedral illites have been reported by Walker (1950) and Weiss et al.(1956). Walker s analysis, which he considers only a rough approximation, is given in Table XI. The clay biotite occurs in a Scottish soil and is believed to be authigenic however, it weathers so easily to vermiculite that unweathered material is difficult to find. Due to its instability, it is not likely that much clay-sized biotite exists although trioctahedral biotite-like layers may occur interlayered with dioctahedral illite layers. Such interlayering has been reported by Bassett (1959). 

All of the saponites that have been discussed were grown from solution or were found by the alteration of non-micaceous minerals or glasses. Similar type clays can form by the weathering of biotite and phlogopite. MacEwan (1954) has described such a clay (cardenite) from Scottish soils. The formula he gives is ... 

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. 

Wallander, H. (2000b). Use of strontium isotopes and foliar K content to estimate weathering of biotite induced by pine seedlings colonised by ectomycorrhizal fungi from two different soils. Plant and Soil, 221, 215-29. 

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). 

Many of the above observations can be illustrated through discussion of an example of weathering taken from Goldich (1938) (see also Krauskopf 1967). The parent rock in this humid, temperate climate example is a quartz-feldspar-biotite gneiss. The mineralogic and oxide composition of this rock and its weathered product soil is given in Table 7.2. 

Of total silica in the rock, 22% has been lost to the soil solution, chiefly from weathering of the plagioclase feldspar, biotite, and hornblende. All or most of the quartz has evidently been unaffected by weathering. 

Weathering of the feldspars, biotite, and hornblende has also released their cations to the soil solution, as apparent from the high percentage losses of MgO, CaO, Na20, and K2O. 

Weathering has led to oxidative loss of FeO in the biotite and hornblende and a relative increase in Fc203 in the soil. A comparison of the total iron as Fe in the parent rock and the soil shows that a 44% loss of Fe has occurred on weathering. 

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. 

Vermiculite is a widespread hydrated clay mineral of lesser abundance than smectite. Understanding of its diagenetic behavior is complicated by the fact that most of the laboratory measurements on vermiculite have been made on the hydrothermal alteration products of coarse-grained biotites, whereas most soil vermiculites that would be fed into a sedimentary pile like that of the Gulf of Mexico coast are weathered dioctahedral Ulites containing a lot of interlayer Al- and Fe-hydroxides. 

Chlorites in soil occur as primary minerals derived from mafic rocks and as secondary minerals from the weathering of biotite, hornblende, and other amphiboles and minerals (Bamhisel, 1977). Chlorites are 2 1 1 minerals consisting of 2 1 mica structure in addition to an interlayer hydroxide sheet. Chlorites have low CEC and surface areas. 

The concentration of magnesium in soils generally lies in the range between 0.5 g/kg for sandy soils and 5 g/kg for clay soils. The levels of magnesium are higher in clay soils due to the presence of weatherable ferromagnesian minerals, such as biotite, serpentine, and olivine and also the carbonate mineral dolomite. It is also present in secondary clay minerals, such as chlorite and vermiculite. 

A higher content of magnesium is characteristic of basic and ultrabasic magmatic and metamorphic rocks (e.g., basalt and serpen-tinite) and their weathering products. Mg-rich rock-forming minerals are biotite and other dark-colored silicate minerals, as well as serpentine. In the case of the serpentinite soils, a Mg-adapted natural vegetation has developed. In the case of sediments, dolomite is a Mg-rich limestone young sediments deposited from seawater are also Mg-rich, as can be seen in marsh soils. 

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