What is Pyrophyllite?
Pyrophyllite is a soft aluminium silicate clay mineral with pearly cleavage, used in ceramics and valued by collectors for foliated aggregates.
Mineralogy and crystal structure
Reference formula: Al2Si4O10(OH)2. Pyrophyllite is a layered triclinic silicate and a close structural relative of talc. Its softness and water-poor layers distinguish it from swelling clays. Pyrophyllite is classified with silicates and has a triclinic crystal system. Typical hardness is 1–2 on the Mohs scale and specific gravity is approximately 2.6–2.9.
Colour, lustre and habit
Typical colours include white, grey, green, yellow, brown. The usual massive, foliated, radiating habit and pearly, waxy, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Pyrophyllite forms in or is associated with hydrothermal alteration, metamorphosed aluminous rocks and advanced argillic zones. Important documented localities include China, Japan, Brazil, Korea, the United States and South Africa.
Soft layers in an altered rock
Pyrophyllite translates hydrothermal alteration into a tactile mineral: soft pearly sheets formed where hot fluids removed alkalis and concentrated aluminium.
Collecting and care
Avoid waterlogging, dust and abrasion; keep specimens dry and do not use powdered material without appropriate controls.
