The complex chemical composition of biotite is doubtless the cause of a variety of alteration products when it has been altered by hydrothermal solutions at moderate temperatures and pressures. As a rule biotite is unstable under hydrothermal conditions and alters to numerous minerals that seem to depend primarily on the composition of the solution at any given point. Common alteration products are green biotite, chlorite, muscovite, sericite, hydromica, kaolinite, and other clay minerals, calcite, epidote-zoisite, leucoxene, rutile, pyrite and other sulfides, and less commonly several other minerals.A complex of several minerals that have replaced biotite within a single thin section is a common occurrence indicating that equilibrium is not normally attained during hydrothermal alteration. The structure of biotite exerts a strong control on the alteration as shown by the occurrence of two or three minerals in bands parallel to the cleavage of the biotite.The original composition of biotite has a strong influence on the alteration products but minerals such as calcite retain only oxygen of the original elements.

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