Soap stone
Soapstone (also known as steatite, or soaprock) is a talc-schist, which is a type of metamorphic rock. It is largely composed of the mineral talc and is thus rich in magnesium. It is produced by dynamothermal metamorphism and metasomatism, which occurs in the zones where tectonic plates are subducted, changing rocks by heat and pressure, with influx of fluids, but without melting. It has been a medium for carving for thousands of years.
Petrologically, soapstone is composed dominantly of talc, with varying amounts of chlorite and amphiboles (typically tremolite, anthophyllite, and cummingtonite, hence its obsolete name, magnesiocummingtonite), and trace to minor iron-chromium oxides. It may be schistose or massive. Soapstone is formed by the metamorphism of ultramafic protoliths (e.g. dunite or serpentinite) and the metasomatism of siliceous dolostones.
By mass, "pure" steatite is approximately 63.37% silica, 31.88% magnesia and 4.74% water. It commonly contains minor quantities of other oxides such as CaO or Al2O3.
Pyrophyllite, a mineral very similar to talc, is sometimes called soapstone in the generic sense since its physical characteristics and industrial uses are similar, and because it is also commonly used as a carving material. However, this mineral typically does not have such a soapy feel as soapstone.
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