Biotite leached

Most of the vermiculites listed by Foster apparently were formed by the leaching of K from biotite. Biotite has a negative layer charge near 1.00 per Oi0(OH)2 units. Foster s vermiculites have charges from 1.08 to 0.38 with only five of 25 values... 

The main secondary products are clay minerals, either smectite or kaolinite. These clay minerals are derived mainly and most readily from glass, but feldspars, amphiboles, pyroxenes, and biotite also alter in part to clay minerals. The alteration of volcanic ash to kaolinite involves removal of Ma, Ca, Mg, K and Fe as well as considerable silica. Pure kaolinite corresponds to a mixture of subequal amounts of silica and alumina plus a small amount of water and virtually nothing else—this is thus the product of very intensive leaching. Smectite on the other hand requires some Mg and more silica than kaolinite. It is thus the product of less intensive leaching than that which produces kaolinite. 

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 ... 

Illite Sedimentary rocks a) From muscovite or biotite by leaching of potassium ions b) From montmorillonite by adsorption of potassium ions c) As neoformation from weathering solutions To muscovite/ biotite, to chlorite in marine environments (by addition of magnesium ions) or brines Either residual or neoformation... 

Chlorite group Igneous, metamorphic and sedimentary rocks a) Leaching of biotite, hornblende b) Neoformation after deposition c) Very low- to low-grade metamorphosis To smectites and vermiculite Residual or neoformation in sediments... 

Mg-leaching solution are sufficient to block the alteration of biotite to vermiculite, and Leggett and Moodie [1936] have found similar effects with K and NH4 in a Na-leaching solution. 

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