Biotite is one of the most abundant members of the group of minerals known as micas. In fact, the mineral, which varies somewhat in color though it is usually quite dark, is sometimes alternatively referred to as black mica. While frequently found in igneous and metamorphic rocks, biotite’s tendency to easily alter when subjected to chemical weathering makes the mineral a rare occurrence in sedimentary rocks. Interestingly, in a stage of partial alteration, biotite lightens in color and has an appearance similar to metallic flakes, which are occasionally misidentified by the casual observer as flecks of gold.
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