Annite, Microcline, Quartz (Var: Smoky Quartz) Dimensions: 6 cm × 5 cm × 5.5 cm Locality: Poudrette quarry (De-Mix quarry; Demix quarry; Uni-Mix quarry; Carrièr — Photo: Modris Baum · Public domain
Annite
Also known as: Biotite (Fe endmember).
Annite is a phyllosilicate mineral in the mica family. It has a chemical formula of KFe32+AlSi3O10(OH)2. Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite. Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines. There are contact twins with composition surface {001} and twin axis {310}. Annite was first described in 1868 for the first noted occurrence in Cape Ann, Rockport, Essex County, Massachusetts, US. It also occurs on Pikes Peak, El Paso County, Colorado.
Intro adapted from Wikipedia (CC BY-SA).
Annite is an IMA-recognized mineral species in the silicate class, with the chemical formula KFe²⁺₃(AlSi₃O₁₀)(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Mica, Clay structural group.
Key properties: Mohs hardness 2.5–3; specific gravity 3.3; typically brown or black; white streak; Vitreous luster; refractive index 1.667; optically biaxial.
IMA symbol Ann, IMA status: approved, named in 1868.
Found in magnesium-poor igneous and metamorphic rocks.
It is found associated with /Fluorite, zircon (Rockport, Massachusetts, USA).
The name refers to the first-noted occurrence at Cape Ann, Massachusetts, USA.
Collections: Everyday Minerals.