Pyrophyllite

Pyrophyllite
Pearly radial cluster of pyrophyllite from Hillsborough District, Orange County, North Carolina (Size: 11 × 7.3 × 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]

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.

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.

  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. ^ a b Mindat
  3. ^ a b 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.