Soapstone

Soapstone (also known as steatite or soaprock) is a talc-schist, which is a type of metamorphic rock. It is composed largely of the magnesium-rich mineral talc. It is produced by dynamothermal metamorphism and metasomatism, which occur in subduction zones, changing rocks by heat and pressure, with influx of fluids but without melting. It has been a carving medium for thousands of years.

The definitions of the terms "steatite" and "soapstone" vary with the field of study. In geology, steatite is a rock that is, to a very large extent, composed of talc. The mining industry defines steatite as a high-purity talc rock that is suitable for the manufacturing of, for example, insulators; the lesser grades of the mineral can be called simply "talc rock". Steatite can be used both in lumps ("block steatite", "lava steatite", "lava grade talc"), and in the ground form. While the geologists logically will use "steatite" to designate both forms, in the industry, "steatite" without additional qualifications typically means the steatite that is either already ground or to be used in the ground form in the future. If the ground steatite is pressed together into blocks, these are called "synthetic block steatite", "artificial block steatite", or "artificial lava talc".

In industrial applications soapstone refers to dimension stone that consists of either amphibole-chlorite-carbonate-talc rock, talc-carbonate rock, or simply talc rock and is sold in the form of sawn slabs. "Ground soapstone" sometimes designates the ground waste product of the slab manufacturing.

Petrologically, soapstone is composed predominantly of talc, with varying amounts of chlorite and amphiboles (typically tremolite, anthophyllite, and cummingtonite, hence its obsolete name, magnesiocummingtonite), and traces of 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 dolomites.

By mass, "pure" steatite is roughly 63% silica, 32% magnesia, and 5% 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 as soapy a feel as soapstone.

Soapstone is relatively soft because of its high proportion of talc, which has a definitional value of 1 on the Mohs hardness scale. Softer grades may feel similar to soap when touched, hence the name. The hardness of soapstone varies due to its varying talc content, from as little as 30% for architectural grades such as those used on countertops, to as much as 80% for carving grades.

Soapstone is easy to carve; it is also durable and heat-resistant and has a high heat storage capacity. It has therefore been used for cooking and heating equipment for thousands of years.