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Biotite

This is the mineral Biotite. It has a vitreous, or glassy, luster that is especially visible on its crystal faces. A crystal face is a geometric plane that develops as a crystal grows. Biotite’s crystal faces are flat, sheet-like planes that look glassy under reflected light. The mineral exhibits perfect basal cleavage, meaning that it splits parallel to other cleavage planes. It ranks 2.5 to 3 on Mohs hardness scale, making it a soft and easily scratched mineral. Its specific gravity is 2.7 to 3.1, thus it feels relatively light in your hand. Biotite produces a white streak, and many flakes are often shed when a specimen is drawn across a streak plate. Like many other rock-forming minerals, Biotite belongs to the Silicate chemical family because it contains repeating chains of linked Silicon and Oxygen tetrahedra. These tetrahedra consist of a single, central Silicon atom surrounded by 4 Oxygen atoms. A distinctive characteristic of Biotite and other mica minerals such as Muscovite, Phlogopite, and Lepidolite, is its ability to flake into thin, pliable, and transparent sheets. Biotite is commonly seen as visible crystals embedded within igneous rocks, including Granite, Diorite, and Pegmatite. While Biotite crystals embedded within Granite are often quite small, requiring a hand lens to be seen, those in Pegmatite are large and often measure over a centimeter in diameter. As with other mica minerals, Biotite is often used as a filler and extender in paint. When Biotite is weathered, its chemical structure may be altered to incorporate water, thus transforming it into the mineral Vermiculite. Vermiculite is often incorporated into soil to promote root growth and retain ideal ratios of air, nutrients, and moisture for plants.