Antigorite
Antigorite is a lamellated, monoclinic mineral in the phyllosilicate serpentine subgroup with the ideal chemical formula of (Mg,Fe2+)3Si2O5(OH)4. It is the high-pressure polymorph of serpentine and is commonly found in metamorphosed serpentinites. Antigorite, and its serpentine polymorphs, play an important role in subduction zone dynamics due to their relative weakness and high weight percent of water (up to 13 weight % H2O). It is named after its type locality, the Geisspfad serpentinite, Valle Antigorio in the border region of Italy/Switzerland and is commonly used as a gemstone in jewelry and carvings.
| Antigorite | |
|---|---|
Antigorite crystals in serpentinite from Poland | |
| General | |
| Category | Phyllosilicate Serpentine-Kaolinite group |
| Formula (repeating unit) | (Mg, Fe)3Si2O5OH4 |
| IMA symbol | Atg |
| Strunz classification | 9.ED.15 |
| Crystal system | Monoclinic |
| Identification | |
| Color | Green, yellowish-green, blueish-gray |
| Crystal habit | Massive or platy |
| Cleavage | (001) Perfect |
| Fracture | Brittle |
| Mohs scale hardness | 3.5–4 |
| Luster | Vitreous to greasy |
| Streak | Greenish white |
| Specific gravity | 2.5–2.6 |
| Optical properties | Biaxial (−) |
| Refractive index | 1.53–1.575 |
| Birefringence | δ = 0.005–0.006 |
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