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 x 7.3 x 6.6 cm)
General
CategorySilicate minerals
Formula
(repeating unit)		Al2Si4O10(OH)2
IMA symbolPrl[1]
Crystal systemMonoclinic[2] or triclinic[3]
Crystal classPrismatic (2/m)
or pinacoidal (1)
Space groupC2/c or C1
Unit cella = 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 mass360.31 g/mol
ColorBrown green, brownish yellow, greenish, gray green, gray white
Crystal habitCompact spherulitic aggregates of needlelike radiating crystals; as fine grained foliated laminae, granular, massive
Cleavage[001] Perfect
TenacityFlexible inelastic
Mohs scale hardness1.5-2
LusterPearly to dull
Streakwhite
DiaphaneityTranslucent to opaque
Specific gravity2.65 - 2.9
Optical propertiesBiaxial (-)
Refractive indexnα=1.534-1.556, nβ=1.586-1.589, nγ=1.596-1.601
Birefringenceδ =0.0450-0.0620
2V angle53-62
FusibilityInfusible, 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]

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.

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 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.[9]

See also
  • Talc – Hydrated magnesium phyllosilicate mineral

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Mindat
  3. http://webmineral.com/data/Pyrophyllite.shtml Webmineral
  4. Handbook of Mineralogy
  5. Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., p. 430 ISBN 0-471-80580-7
  6. Lee, J.H.; Guggenheim, S. (1981). "Single crystal X-ray refinement of pyrophyllite-1Tc". American Mineralogist. 66: 350–357. Retrieved 30 December 2020.
  7. Gruner, J.W. (1934). "The crystal structures of talc and pyrophyllite". Zeitschrift für Kristallographie. 88 (1–6): 412–419. doi:10.1524/zkri.1934.88.1.412. S2CID 101680970.
  8. Nel, LT., H. Jacobs, J.T. Allen and G.R. Bozzoli 1937. Wonderstone. Geological Survey of South Africa Bulletin no. 8.
  9. L. Fang; et al. (2007). "Effect of precompression on pressure-transmitting efficiency of pyrophyllite gaskets". Journal High Pressure Research. 27 (3): 367. Bibcode:2007HPR....27..367F. doi:10.1080/08957950701553796. S2CID 97317483.

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This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Pyrophyllite". Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. pp. 695–696.

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