Anthophyllite
| Anthophyllite | |
|---|---|
 |
|
| General | |
| Category |
Inosilicates Amphibole |
|
Formula (repeating unit) |
☐Mg2Mg5Si8O22(OH)2 |
| Strunz classification | 9.DE.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pnma |
| Unit cell | a = 18.5, b = 17.9 c = 5.28 [Å]; Z = 4 |
| Identification | |
| Color | Gray to green, brown, and beige |
| Crystal habit | Rarely as distinct crystals. Commonly lamellar or fibrous. |
| Cleavage | Perfect on {210}, imperfect on {010}, {100} |
| Tenacity | Brittle; elastic when fibrous |
| Mohs scale hardness | 5.5 - 6 |
| Luster | Vitreous, pearly on cleavage |
| Streak | White to gray |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.85 - 3.2 |
| Optical properties | Biaxial (+) |
| Refractive index | nα=1.598 - 1.674, nβ=1.605 - 1.685, nγ=1.615 - 1.697; indices increase with Fe content |
| Birefringence | δ = 0.017 - 0.023 |
| 2V angle | 57° - 90° |
| Diagnostic features | Characterized by clove brown color, but unless in crystals, difficult to distinguish from other amphiboles without optical and/or X-ray tests |
| References | |
Anthophyllite is an amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are classed as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral.
Anthophylite is the product of metamorphism of magnesium-rich rocks, especially ultrabasic igneous rocks and impure dolomitic shales. It also forms as a retrograde product rimming relict orthopyroxenes and olivine, and as an accessory mineral in cordierite-bearing gneisses and schists. Anthophyllite also occurs as a retrograde metamorphic mineral derived from ultramafic rocks along with serpentinite. Geographically, it occurs in Pennsylvania, southwestern New Hampshire, central Massachusetts, Franklin, North Carolina, and in the Gravelly Range and Tobacco Root Mountains of southwest Montana.
Anthophyllite is formed by the breakdown of talc in ultramafic rocks in the presence of water and carbon dioxide as a prograde metamorphic reaction. The partial pressure of carbon dioxide (XCO2) in aqueous solution favors production of anthophyllite. Higher partial pressures of CO2 reduces the temperature of the anthophyllite-in isograd.
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