Biotite mica is a black, platy mineral which forms in sheets, it belongs to the mica group of minerals which also includes muscovite, phlogopite and lepidolite micas. Being a common phyllosilicate mineral, it is also found in many rocks, particularly of igneous and metamorphic origin, such as mica schist, gneisses and granites. The crystal system of mica in general is monoclinic and like other mica minerals, biotite has a perfect basal cleavage and consists of flexible sheets or lamellae, which easily flake off, with tabular to prismatic crystals. For this reason, mica is used in the oil drilling process, particularly in the drilling fluid, where it acts to prevent loss of circulation and as a sealant. When micas are found in their crystal sheet form, it is often referred to as "books" as it resembles their many pages. These mineral samples of biotite mica come from Baluchistan, Pakistan and are supplied in labelled card trays and are available in two sizes.