Pyrophyllite

Pyrophyllite
Pearly radial cluster of pyrophyllite from Hillsboro District, Orange County, North Carolina (Size: 11 x 7.3 x 6.6 cm)
General
Category	Silicate mineral
Formula (repeating unit)	Al2Si4O10(OH)2
Crystal system	Monoclinic or triclinic
Crystal class	Prismatic (2/m) or pinacoidal (1)
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

Compact spherulitic aggregates of needlelike radiating crystals; as

Pyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al₂Si₄O₁₀(OH)₂. It occurs in two more or less distinct varieties, namely, as crystalline folia and as compact masses; distinct crystals are not known.

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 before the blowpipe, exfoliates and swells up to many times its original volume, hence the name pyrophyllite, from the Greek pyros (fire) and phyllos (a leaf), given by R. Hermann in 1829. The color of both varieties is white, pale green, greyish or yellowish; they are very soft (hardness of 1 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.

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.

Deposits containing well-crystallized material are found in:

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.

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.