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 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.
Terminology
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 will define steatite as a high-purity talc rock that is suitable for 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 the one to be utilized 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", "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 sawed slabs. "Ground soapstone" sometimes designates the ground waste product of the slab manufacturing.
Petrology
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 63.37% 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.
Physical characteristics
Soapstone is relatively soft because of its high talc content, talc having a definitional value of 1 on the Mohs hardness scale. Softer grades may feel similar to soap when touched, hence the name. No fixed hardness is given for soapstone because the amount of talc it contains varies widely, 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, heat-resistant and has a high heat storage capacity. It has therefore been used for cooking and heating equipment for thousands of years.
Soapstone is often used as an insulator for housing and electrical components, due to its durability and electrical characteristics and because it can be pressed into complex shapes before firing. Soapstone undergoes transformations when heated to temperatures of into enstatite and cristobalite; on the Mohs scale, this corresponds to an increase in hardness to 5.5–6.5. The resulting material, harder than glass, is sometimes called "lava".
Historical usage
Africa
Ancient Egyptian scarab signets and amulets were most commonly made from glazed steatite. The Yoruba people of West Nigeria used soapstone for several statues, most notably at Esie, where archaeologists have uncovered hundreds of male and female statues about half of life size. The Yoruba of Ife also produced a miniature soapstone obelisk with metal studs called "the staff of Oranmiyan".
Americas
Native Americans have used soapstone since the Late Archaic Period. During the Archaic archaeological period (8000-1000 BC), bowls, cooking slabs, and other objects were made from soapstone. Use of soapstone in native American cultures continue to the modern day. Later, other cultures carved soapstone smoking pipes, a practice that continues today. The soapstone's low heat conduction allows for prolonged smoking without the pipe heating up uncomfortably.