Pyrophyllite

Hand Specimen identification: Softness, greasy feel, cleavage, and association help identify pyrophyllite, but it cannot be told from other clays without X-ray data. It is easily confused with talc. See Figure 9.

Chemical Composition : Al2Si4O10(OH)2

Hardness : 1 to 2

Specific Gravity2.8

Cleavage/Fracture : perfect basal (001)

Luster/Transparency : pearly/translucent

Color : white

Streak : white

Optical Properties : High birefringence, perfect cleavage, bird’s-eye maple appearance, and lack of color identify pyro-phyllite. Talc and muscovite have smaller 2Vs. Biaxial 1- 2, a = 1.553, b = 1.588, g = 1.600, d = 0.047, 2V = 52° to 62°.

Crystallography : Triclinic, a = 5.16, b = 8.96, c = 9.35, a = 90.03°, b = 100.37°, g = 89.75°, Z = 2; space group P1; point group 1.

Habit : Individual crystals are unknown. Pyrophyllite is usu-ally massive and foliated, sometimes forming platy or radiating masses, such as in Figure 9.

Structure and composition : The three-layered structure consists of individual sheets of Al1O,OH26 octahedra sandwiched between sheets of SiO4 tetrahedra. Fe may replace some of the Al; minor Mg, Ca, Na, or K may also be present.

Occurrence and associations : Pyrophyllite is found in low- and medium-grade metamorphosed shales. Associated minerals include kyanite, feldspar, and quartz.

Varieties :

Related Minerals : Pyrophyllite is isostructural with talc, Mg Si O1 2 3 4 10OH , and structurally similar to minnesotaite,2 1 2Fe3Si4O10 OH 2.

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