Annite
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 | |
|---|---|
Annite sample | |
| General | |
| Category | Phyllosilicates Mica group |
| Formula (repeating unit) | KFe32+AlSi3O10(OH,F)2 |
| IMA symbol | Ann |
| 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. |
| Twinning | Contact twins with composition surface on {001} and twin axis {310} |
| Cleavage | Perfect in one direction {001} |
| Fracture | Uneven |
| Tenacity | Flexible |
| Mohs scale hardness | 2.5–3 |
| Luster | Submetallic to vitreous; pearly on cleavage surfaces |
| Streak | Brownish white |
| Diaphaneity | Translucent to transparent |
| 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 | |
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. It occurs in igneous and metamorphic rocks that are deficient in magnesium and is associated with fluorite and zircon in the type locality.
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