Pyrophyllite

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Pyrophyllite
Pyrophyllite-118706.jpg
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 symmetry Monoclinic / triclinic
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
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
Crystal system Monoclinic 2/m - prismatic[1] or triclinic 1 pinacoidal[2]
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 [1][2][3][4]

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 lustre, 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),[4] 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.

Occurrence[edit]

Radiating fans of golden-brown pyrophyllite needles from Champion Mine, White Mts, Mono County, California (size: 4.0 x 3.0 x 2.3 cm)

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.[3]

Deposits containing well-crystallized material are found in:[3]

It is also reported from South Africa where 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.[citation needed]

Uses[edit]

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. Therefore it is added to clay to reduce thermal expansion when firing but it has many other industry 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.[5]

References[edit]

  1. ^ a b Mindat
  2. ^ a b http://webmineral.com/data/Pyrophyllite.shtml Webmineral
  3. ^ a b c Handbook of Mineralogy
  4. ^ a b Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., p. 430 ISBN 0-471-80580-7
  5. ^ L. Fang et al. (2007). "Effect of precompression on pressure-transmitting efficiency of pyrophyllite gaskets". journal High Pressure Research 27: 367. Bibcode:2007HPR....27..367F. doi:10.1080/08957950701553796. 

Public Domain This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.