Pyrophyllite
Pyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al2Si4O10(OH)2. It occurs in two forms (habits): crystalline folia and compact masses; distinct crystals are not known.
| Pyrophyllite | |
|---|---|
Pearly radial cluster of pyrophyllite from Hillsborough District, Orange County, North Carolina (Size: 11 × 7.3 × 6.6 cm) | |
| General | |
| Category | Silicate minerals |
| Formula (repeating unit) | Al2Si4O10(OH)2 |
| IMA symbol | Prl |
| Crystal system | Monoclinic or triclinic |
| Crystal class | Prismatic (2/m) or pinacoidal (1) |
| Space group | C2/c or C1 |
| Unit cell | a = 5.16 Å, b = 8.966(3) Å, c = 9.347(6) Å; α = 91.18°, β = 100.46°, γ = 89.64°; Z = 2 |
| Identification | |
| Formula mass | 360.31 g/mol |
| Color | Brown green, brownish yellow, greenish, gray green, gray white |
| Crystal habit | Compact spherulitic aggregates of needlelike radiating crystals; as fine grained foliated laminae, granular, massive |
| Cleavage | [001] Perfect |
| Tenacity | Flexible inelastic |
| Mohs scale hardness | 1.5–2 |
| Luster | Pearly to dull |
| Streak | white |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 2.65 – 2.9 |
| Optical properties | Biaxial (−) |
| Refractive index | nα=1.534–1.556, nβ=1.586–1.589, nγ=1.596–1.601 |
| Birefringence | δ =0.0450–0.0620 |
| 2V angle | 53–62 |
| Fusibility | Infusible, exfoliates |
| References | |
The folia have a pronounced pearly luster, owing to the presence of a perfect cleavage parallel to their surfaces: they are flexible but not elastic, and are usually arranged radially in fan-like or spherical groups. This variety, when heated, exfoliates and swells up to many times its original volume. The color of both varieties is white, pale green, greyish or yellowish; they are very soft (hardness of 1.0 to 1.5) and are greasy to the touch. The specific gravity is 2.65–2.85. The two varieties are thus very similar to talc.
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