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}.
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{{Infobox mineral | name = Annite | category = Phyllosilicate minerals | group = Mica group, trioctahedral mica group, biotite subgroup | image = Annite-pas-130ab.jpg | caption = Annite sample | formula = KFe32+AlSi3O10(OH,F)2 |IMAsymbol=Ann
| strunz = 9.EC.20 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | unit cell = a = 5.39, b = 9.33 c = 10.29 [Å]; β = 100°; Z = 2 | color = Brown to black | habit = Usually in "blocks" of thin sheets or as aggregations of foliated thin scales. Includes large, well formed pseudohexagonal prisms up to 12 cm across and 15 cm long. | twinning = Contact twins with composition surface on {001} and twin axis {310} | cleavage = Perfect in one direction {001} | fracture = Uneven | tenacity = Flexible | mohs = 2.5–3 | luster = Submetallic to vitreous; pearly on cleavage surfaces | opticalprop = Biaxial (−) | refractive = nα = 1.625 – 1.631 nβ = 1.690 nγ = 1.691 – 1.697 | pleochroism = X = brown; Y = Z = dark brown | 2V = Calculated: 12° to 36° | streak = Brownish white | gravity = Approximately 3.3 | density = | diaphaneity = Translucent to transparent | other = Diagnostic: occurrence: in magnesium poor igneous and metamorphic rocks. | references = }}
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