Pyrophyllite

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Phyllosilicate mineral in the pyrophyllite-talc group

title: "Pyrophyllite" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["aluminium-minerals", "triclinic-minerals", "monoclinic-minerals", "minerals-in-space-group-2", "minerals-in-space-group-15", "luminescent-minerals", "pyrophyllite-talc-group"] description: "Phyllosilicate mineral in the pyrophyllite-talc group" topic_path: "general/aluminium-minerals" source: "https://en.wikipedia.org/wiki/Pyrophyllite" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Phyllosilicate mineral in the pyrophyllite-talc group ::

::data[format=table title="Infobox mineral"]

Field	Value
name	Pyrophyllite
category	Phyllosilicate minerals
group	Pyrophyllite-Talc group
boxbgcolor	#dfce9d
image	Pyrophyllite-118706.jpg
imagesize	260px
caption	Pearly radial cluster of pyrophyllite from Hillsborough District, Orange County, North Carolina (Size: 11 × 7.3 × 6.6 cm)
formula	Al2Si4O10(OH)2
IMAsymbol	Prl
molweight	360.31 g/mol
system	Monoclinic or triclinic
class	Prismatic (2/m)
or pinacoidal ()
symmetry	C2/c or C
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
color	Brown green, brownish yellow, greenish, gray green, gray white
habit	Compact spherulitic aggregates of needlelike radiating crystals; as
cleavage	[001] Perfect
tenacity	Flexible inelastic
mohs	1.5–2
luster	Pearly to dull
refractive	nα=1.534–1.556, nβ=1.586–1.589, nγ=1.596–1.601
opticalprop	Biaxial (−)
2V	53–62
birefringence	δ =0.0450–0.0620
streak	white
gravity	2.65 – 2.9
fusibility	Infusible, exfoliates
diaphaneity	Translucent to opaque
references
::

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.

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.

Deposits containing well-crystallized material are found in:

Manuels, Newfoundland and Labrador, Canada, talc-like bright white appearance, high grade, no impurities; 21 million ton deposit.
Russia – pale green foliated masses, very like talc in appearance, are found at Beresovsk near Yekaterinburg in the Urals.
St. Niklas, Zermatt, Valais, Switzerland
Vaastana, Kristianstad, Sweden
Near Ottrje, Ardennes Mountains, Belgium
Ibitiara, Bahia, Brazil
Nagano Prefecture, Japan
Near Ogilby, Imperial County at Tres Cerritos, Mariposa County, and the Champion mine, White Mountains, Mono County, California, US
Near Quartzsite, La Paz County, Arizona, US
Large deposits at the Deep River region of North Carolina, USA
Graves Mountain, Lincoln County, Georgia, US

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.

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 industrial 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.

References

Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
[http://www.mindat.org/min-3323.html Mindat]
http://webmineral.com/data/Pyrophyllite.shtml Webmineral
[http://rruff.geo.arizona.edu/doclib/hom/pyrophyllite.pdf Handbook of Mineralogy]
Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy,'' 20th ed., p. 430 {{ISBN. 0-471-80580-7
(1981). "Single crystal X-ray refinement of pyrophyllite-1Tc". American Mineralogist.
(1934). "The crystal structures of talc and pyrophyllite". Zeitschrift für Kristallographie.
Nel, LT., H. Jacobs, J.T. Allen and G.R. Bozzoli 1937. ''Wonderstone''. Geological Survey of South Africa Bulletin no. 8.
L. Fang. (2007). "Effect of precompression on pressure-transmitting efficiency of pyrophyllite gaskets". Journal High Pressure Research.

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