Biotite, layered silicate

In general, weathering conditions tend to favor the gradual conversion of trioctahedral layer silicate clays (with dominantly Mg " and Fe " in octahedral sites) to dioctahedral layer silicates (with dominantly AP and Fe in octahedral sites). Layer charge reduction accompanies this process. Expected weathering sequences of the two common micas, biotite and muscovite, in leaching environments are ... 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

Puziewicz J, Joharmes W (1990) Experimental study of a biotite-bearing granitic system under water-saturated and water-undersatrrrated conditions. Contrib Mineral Petrol 104 397-406 Radoslovich EW (1960) The structrrre of muscovite, KAl2(Si3Al)Oio(OH)2. Acta Crystallogr 13 919-932 Radoslovich EW (1961) Strrface symmetry and cell dimertsions of layer lattice silicates. Nature 191 67-68 Radoslovich EW, Norrish K (1962) The cell dimensiorts and symmetry of layer-lattice silicates. I. Some stractural consideratiorrs. Am Mineral 47 599-616... 

The weathering rate of insoluble silicate minerals is very low. Field balance studies showed that their actual values are by several orders of the magnitude (up to 5) lower than those observed in lab experiments. This may be due to a thick diffusion layer and rather high content of dissolved orthosilldc add. Nevertheless, as observations show, at weathering first to disappear are plagioclases, biotite and hornblendes, whereas quartz and orthoclase remain. Sequential disappearance of different silicate minerals (Table 2.25) shows their difference in the rate of chemical weathering. 

Chlorites exhibit the main features of biotites. Well-known representatives of the chlorite group are clinochlore, (Mg,Fe )5Al(Si3Al)Oio(OH)g and its iron-rich variant chamosite. The major Fe doublet in the spectra of chlinichlore and chamosite has the typical values =113 mm/s and A — 2.70 mm/s whereas a minor Fe " doublet gives = 0.39 mm/s and A = 0.67 mm/s [243]. A third doublet is observed at lower temperatures, but has nearly the same hyperfine parameters as the M2 ferrous iron at RT. This doublet might be attributed to substitutions in the brucite layer [251]. The hyperfine parameters of trioctahedral micas obtained in about fifty studies of these silicates are summarized in a review by Dyar [250]. 

In 1934, the first insights into the structure of vermicuUte were obtained by two independent workers using X-ray powder methods. Kazantzev, on the one hand, reported that the unit cell is analogous to that of biotite, but of slightly larger dimensions, with K partly replaced by H and Fe by Mg. The other, Gruner, showed that the structure consists of silicate layers resembling those of mica or talc, with double sheets of water molecules between them. These so-called interlayer water molecules occupy a space very nearly equal to that occupied by a brucite layer in the chlorite structure, with the result that the X-ray diffraction effects obtained from vermiculites and chlorites have certain similarities. 

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