Annite
Phyllosilicate mineral in the mica family / From Wikipedia, the free encyclopedia
Annite is a phyllosilicate mineral in the mica family. It has a chemical formula of KFe32+AlSi3O10(OH)2.[8] Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite.[9] Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines.[6] There are contact twins with composition surface {001} and twin axis {310}.[10]
Quick Facts General, Category ...
Annite | |
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General | |
Category | Phyllosilicates Mica group |
Formula (repeating unit) | KFe32+AlSi3O10(OH,F)2 |
IMA symbol | Ann[1] |
Strunz classification | 9.EC.20 |
Crystal system | Monoclinic |
Crystal class | Prismatic (2/m) (same H-M symbol) |
Space group | C2/m |
Unit cell | a = 5.39, b = 9.33 c = 10.29 [Å]; β = 100°; Z = 2 |
Identification | |
Color | Brown to black |
Crystal 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.[2] |
Twinning | Contact twins with composition surface on {001} and twin axis {310} |
Cleavage | Perfect in one direction {001}[3] |
Fracture | Uneven |
Tenacity | Flexible[4] |
Mohs scale hardness | 2.5–3 |
Luster | Submetallic to vitreous; pearly on cleavage surfaces[5] |
Streak | Brownish white |
Diaphaneity | Translucent to transparent[6] |
Specific gravity | Approximately 3.3 |
Optical properties | Biaxial (−) |
Refractive index | nα = 1.625 – 1.631 nβ = 1.690 nγ = 1.691 – 1.697 |
Pleochroism | X = brown; Y = Z = dark brown |
2V angle | Calculated: 12° to 36° |
Other characteristics | Diagnostic: occurrence: in magnesium poor igneous and metamorphic rocks. |
References | [7][8] |
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Annite was first described in 1868 for the first noted occurrence in Cape Ann, Rockport, Essex County, Massachusetts, US.[7] It also occurs on Pikes Peak, El Paso County, Colorado.[8] It occurs in igneous and metamorphic rocks that are deficient in magnesium and is associated with fluorite and zircon in the type locality.[8]