mica

[Lat. mica a crumb; Fr. & Sp. mica. Not related to Lat. mico =to shine, to glimmer.] Min.: A name originally given to the shining, scaly constituent of many rocks and earths. The great diversity of chemical composition and other characters led to its division into several species, which were supposed to have distinctive crystal lographic and chemical characters. The word is now used to designate a group of minerals having certain characters in common, the most imp or tant of which is the eminently perfect basal cleav. age, which affords very thin, tough, and shining lamine. The species hitherto distinguished are, Phlogopite, Lepidolite and Cryophyllite (regarded are, as orthorhombic); Biotite (hexagonal); Lepidomelane (hexagonal?); Astrophyllite and Muscovite (orthorhombic, but with monoclinic habit). Tschermak, who has recently optically investigated this difficult group of minerals, refers them all to the monoclinic system, his examinations showing that the axis of elasticity is inclined a few degrees to the normal to the plane of cleavage. Bauer confirms these results. Tschermak divides the micas into two groups-those which are characterized by hav ing the optic-axial plane perpendicular to the plane of symmetry, which includes Anomite, Lepidolite, Muscovite, Paragonite, and Margarite; and those which have the optic-axial plane parallel to the 2701 mi-ca-fi-lite, s. [MICA PHI LITE.)