Anthophyllite
Anthophyllite is an orthorhombic 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. The Anthophyllite crystal is characterized by its perfect cleavage along directions 126 degrees and 54 degrees.
| Anthophyllite | |
|---|---|
| General | |
| Category | Inosilicates Amphibole |
| Formula (repeating unit) | ☐Mg2Mg5Si8O22(OH)2 |
| IMA symbol | Ath |
| 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 | |
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