annite

Annite is a mica mineral in the biotite group, representing the iron-rich endmember of the series. It is a dark, sheet silicate mineral that forms in both igneous and metamorphic rocks and is commonly found in granitic and granodioritic contexts as well as in schists and gneisses.

The ideal chemical formula for annite is KFe3AlSi3O10(OH)2. In natural samples, iron in the octahedral sheet is the dominant cation, with aluminum occupying part of the octahedral sites and magnesium often substituting in varying amounts. Annite is the iron-rich counterpart to phlogopite, which is richer in magnesium.

Physically, annite is part of the monoclinic mica family and exhibits the characteristic perfect basal cleavage of micas, forming platy crystals. It has a dark brown to black color with a vitreous to sub-vitreous luster and a pale brown streak. The hardness is about 2.5 to 3 on the Mohs scale, and its specific gravity ranges roughly from 3.2 to 3.5. Annite is typically opaque in hand specimens but can appear translucent in very thin sheets and often shows pleochroism, appearing different shades of brown in polarized light.

Occurrence and significance: annite occurs in a variety of rock types, especially where iron-rich biotite forms during metamorphism or magmatic differentiation. It is commonly used in studies of metamorphic petrology as an indicator of the iron–manganese–magnesium composition of rocks and to assess the conditions under which the rock formed. It is not a primary ore mineral but is of interest to mineral collectors and researchers studying biotite chemistry.