Pyrophyllite

Pyrophyllite – composed of aluminum silicate hydroxide and expressed as Al2Si4O10(OH)2 in scientific terms – belongs to the Montmorillonite group of minerals. It bears a very close resemblance with talc in terms of its physical and optical properties but chemically it differs in the composition. Talc contains magnesia whereas phyrophyllite contains alumina. It is practically difficult to find out the difference without a chemical test for aluminum. Silica and Alumina are its major constituents and ferrous oxide, calcium oxide, magnesia and titanium dioxide are some of its major impurities.

The physical structure of pyrophyllite can be expressed as a stacked construction of silicate layers and gibbsite layers. Two different layers of gibbsite sandwich a layer of pyrophyllite between them. However, there is hardly any bonding among these layers. It ultimately results in an enhanced softness and perfect cleavage formation of pyrophyllite.

Pyrophyllite is usually found in colors like white, gray, yellow, pale green, blue, and at times it can even be colorless. Its luster can be greasy to dull, but most commonly there’s a gleaming on the cleavage surface. The transparency of its crystals may vary from being translucent to opaque. The cleavages are formed parallel to their surfaces, and for this reason they are flexible but inelastic. Pyrophyllite gives a typical greasy feel when touched.