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	Al2Si4O10(OH)2
IMA symbol	Prl[1]
Crystal system	Monoclinic[2] or triclinic[3]
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	[2][3][4][5][6][7]

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.

Occurrence

Pyrophyllite occurs in phyllite and schistose rocks, often associated with kyanite, of which it is an alteration product. It also occurs as hydrothermal deposits. Typical associated minerals include: kyanite, andalusite, topaz, mica and quartz.^[4]

Deposits containing well-crystallized material are found in:^[4]

• Manuels, Newfoundland and Labrador, Canada, talc-like bright white appearance, high grade, no impurities; 21 million ton deposit.
• Russia – pale green foliated masses, very like talc in appearance, are found at Beresovsk near Yekaterinburg in the Urals.
• St. Niklas, Zermatt, Valais, Switzerland
• Vaastana, Kristianstad, Sweden
• Near Ottrje, Ardennes Mountains, Belgium
• Ibitiara, Bahia, Brazil
• Nagano Prefecture, Japan
• Near Ogilby, Imperial County at Tres Cerritos, Mariposa County, and the Champion mine, White Mountains, Mono County, California, US
• Near Quartzsite, La Paz County, Arizona, US
• Large deposits at the Deep River region of North Carolina, USA
• Graves Mountain, Lincoln County, Georgia, US

In South Africa, major deposits of pyrophyllite occur within the Ottosdal region, where it is mined for the production of a variety of manufactured goods, and blocks are quarried and marketed as "Wonderstone" for the carving of sculptures.^[8]

Uses

The compact variety of pyrophyllite is used for slate pencils and tailors' chalk (French chalk), and is carved by the Chinese into small images and ornaments of various kinds. Other soft compact minerals (steatite and pinite) used for these Chinese carvings are included with pyrophyllite under the terms agalmatolite and pagodite.^{[citation needed]}

Pyrophyllite is easily machineable and has excellent thermal stability, so it is added to clay to reduce thermal expansion when firing, but it has many other industrial uses when combined with other compounds, such as in insecticide and for making bricks. Pyrophyllite is also widely used in high-pressure experiments, both as a gasket material and as a pressure-transmitting medium.^[9]

See also

• Talc – Hydrated magnesium phyllosilicate mineral