Pyrophyllite(redirected from Pencil stone agalmatolite)
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a mineral of the phyllosilicate class, with chemical composition Al2[Si4O10](OH)2; it contains admixtures of MgO, FeO, and Fe2O3. Pyrophyllite crystallizes in the mono-clinic system. It generally occurs in the form of foliate or compact scaly aggregates that are pale yellow, yellowish, or white in color. Pyrophyllite exhibits perfect cleavage. Greasy to the touch, it resembles talc. It has a hardness of 1–1.5 on Mohs’ scale and a density of 2,840 kg/m3.
Pyrophyllite deposits are usually associated with metamorphic rocks, in which the mineral forms packets of pyrophyllite slate or compact agalmatolite aggregates. It sometimes occurs in hydrothermal quartz and ore veins. Pyrophyllite is both fire- and acid-resistant. It is used commercially as a highly aluminiferous raw material for refractory products and as a chemically stable and insulating material.