pyrophyllite

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A hydrated aluminum silicate with composition Al₂Si₄O₁₀(OH)₂. The mineral is commonly white, grayish, greenish, or brownish, with a pearly to waxy appearance and greasy feel. It occurs as compact masses, as radiating aggregates (see illustration), and as foliated masses. Pyrophyllite belongs to the layer silicate (phyllosilicate) group of minerals. The mineral is soft (hardness 1–1½ on the Mohs scale) and has easy cleavage parallel to the structural layers. The mineral is highly stable to acids.

Specimen of pyrophylllte. (Pennsylvania State University)

Pyrophyllite is used principally for refractory materials and in other ceramic applications. The main sources for pyrophyllite in the United States are in North Carolina. An unusual form from the Transvaal is called African wonderstone. See also Silicate minerals.

Environment

A mineral of metamorphosed rocks.

Sometimes in radiating bundles of small micaceous crystals attached to quartz crystals or embedded in a quartzite rock. Also occurs in compact, fine-grained, soapstonelike refractive masses.

White, silvery, pale green, or stained black and brown. Luster pearly to greasy; hardness 1-2; specific gravity 2.8-2.9; cleavage perfect micaceous. Flexible flakes; translucent to opaque.

Alkaline aluminum silicate (28.3% Al ₂ O ₃ ; 66.7% SiO ₂ ; 5.0% H ₂ O).

Micaceous or radiating clusters (quartz overgrowths) writhe, exfoliate, and glow whitely when heated on charcoal, without fusing (whence the name pyrophyllite: pyro , "fire," and phyl , "leaf"). Compact material also whitens and gives blue color on moistening with drop of cobalt nitrate solution and heating (aluminum test).

Before heating, more silvery and lighter in color than vermiculite (a hydrated micaceous silicate). Heated material is very white. Talc becomes violet rather than blue from cobalt nitrate. The fine-grained micas are harder.

The best U.S. specimens are the coarse crystalline masses on quartz crystals from Lincoln Co., Georgia; almost identical specimens are found in Quartzsite, Arizona, and near Copiapo, Chile. Radiating masses are found in Montgomery Co., North Carolina, in the Chesterfield district of South Carolina, and at Indian Gulch, California.

The "agalmatolite" of Chinese "soapstone" carvings is a fine-grained pyrophyllite. In its massive state, it is valued as a carrier for insecticide dusts. The compression chambers for the tetrahedral anvil press, used in early diamond synthesis, were cut from it.

• Mineral compounds

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
Chemical formula	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
Molar 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

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]

• Russia - pale green foliated masses, very like talc in appearance, are found at Beresovsk near Yekaterinburg in the Urals.
• St. Niklas, Zermatt, Valais, Switzerland
• at Vaastana, Kristianstad, Sweden
• near Ottrje, Ardennes Mountains, Belgium
• Ibitiara, Bahia, Brazil
• Nagano Prefecture, Japan
• Manuels, Newfoundland and Labrador, Canada
• USA
  • It is found near Ogilby, Imperial County at Tres Cerritos, Mariposa County, and the Champion mine, White Mountains, Mono County, California
  • near Quartzsite, La Paz County, Arizona
  • the large deposits at the Deep River region of North Carolina
  • Graves Mountain, Lincoln County, Georgia

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

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

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. doi:10.1080/08957950701553796. 

• Mineral galleries
• USGS

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

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