Pyrophyllite
The name Pyrophyllite comes from the Greek words for fire and leaf in allusion to its tendency to exfoliate into fan shapes when heated. The chemical formula of Pyrophyllite is Al2SiO4O10(OH)2 (Aluminum Silicate Hydroxide) its molecular weight is 360.31 gm.
Pyrophyllite is an unusual mineral with regard to its crystal system; it occurs in two varieties, one is monoclinic and the other triclinic. Ordinarily they would be treated as two distinct minerals, but all of their other properties are identical and they are often associated and intergrown. Pyrophyllite has a specific gravity of 2.65 – 2.90 (g/cm3) and the refractive index of 1.534 – 1.601 Biaxial ( – ).
Pyrophyllite is also an unusual mineral in that it is somewhat flexible, but not elastic. It is a member of the Pyrophyllite-Talc Group of minerals that also includes Talc. Pyrophyllite is identical in physical properties to Talc. The two are isomorphous, meaning they share the same monoclinic structure but have different chemistries.
Pyrophyllite is available as interesting and attractive mineral speciments but never as faceted gems. However, it is available rarely as radiating spray inclusions with pearly luster in clear Quartz cabochons (as pictured above). The main source of these beautiful inclusion cabochons is Minas Gerais, Brazil.
Distribution: Some localities for rich or well-crystallized material follow. In Russia, at Krassik, between Pyschminsk and Beresov, Ural Mountains. From St. Niklas, Zermatt, Valais, Switzerland. In Sweden, at Västanå, Kristianstad. From near Ottré, Ardennes Mountains, Belgium. In the USA, found near Ogilby, Imperial County, at Tres Cerritos, Mariposa County, and the Champion mine, White Mountains, Mono County, California; from near Quartzsite, La Paz County, Arizona; at Staley, Randolph County, Glendon and Robbins, Moore County, and Hillsborough, Orange County, North Carolina; in the Brewer mine, Chesterfield County, South Carolina; on Graves Mountain, Lincoln County, Georgia. From Ibitiara, Bahia, Brazil, in large crystals. In a number of mines in Nagano Prefecture, and elsewhere in Japan.
| Category: | Silicate minerals |
| Chemical Formula: | Al2SiO4O10(OH)2 |
| Aluminum Silicate Hydroxide | |
| Molecular Weight: | 360.31 gm |
| Composition: | Aluminum | 14.98 % | Al | 28.30 % | Al2O3 |
| Silicon | 31.18 % | Si | 66.70 % | SiO2 | |
| Hydrogen | 0.56 % | H | 5.00 % | H2O | |
| Oxygen | 53.28 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Monoclinic – Prismatic or Triclinic – Pinacoidal There are two varieties of Pyrophyllite; one is monoclinic and the other triclinic. Ordinarily they would be treated as two distinct minerals, but their other properties are identical and they are often associated and intergrown. |
| Crystal Habit: | As lamellar crystals, to 8 cm; in compact spherulitic aggregates of needlelike radiating crystals; as fine grained foliated laminae, granular, massive. |
| Twinning: | None |
| Cleavage: | Perfect on {001} |
| Fracture: | Flexible |
| Tenacity: | Flexible but inelastic |
| Moh’s Hardness: | 1.0 – 2.0 |
| Density: | 2.65 – 2.90 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | White, pale blue, yellow, apple-green, grayish green, brownish green; colorless in thin section |
| Transparency: | Translucent to opaque |
| Luster: | Pearly to dull |
| Refractive Index: | 1.534 – 1.601 Biaxial ( – ) |
| Birefringence: | 0.062 |
| Dispersion: | Weak; r > v |
| Pleochroism: | n/a |
