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.
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 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.
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% 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.
Physical characteristics 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. 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 1,000–1,200 °C (1,830–2,190 °F) 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 use
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 of about half–life size. The Yoruba of Ife also produced a miniature soapstone obelisk with metal studs, named "the staff of Oranmiyan". Soapstone mining in Tabaka, Kenya, occurs in relatively shallow and accessible quarries in the surrounding areas of Sameta, Nyabigege and Bomware. These were at the time open to all to access provided they had the labor resources to do so. This mostly meant the men did the mining as they were custodian to the community land, meaning ancestral lands in Riamosioma, Itumbe, Nyatike etc.
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. The use of soapstone cooking vessels during this period has been attributed to the rock's thermal qualities; compared to clay or metal containers, soapstone retains heat more effectively. Use of soapstone in native American cultures continues 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.
Indigenous peoples of the Arctic have traditionally used soapstone for carvings of both practical objects and art. The qulliq, a type of oil lamp, is carved out of soapstone and used by the Inuit and Dorset peoples. The soapstone oil lamps indicate these people had easy access to oils derived from marine mammals. In the modern period, soapstone is commonly used for carvings in Inuit art. In the United States, locally quarried soapstone was used for gravemarkers in 19th-century northeast Georgia, around Dahlonega, and Cleveland as simple field stone and "slot and tab" tombs. In Canada, soapstone was quarried in the Arctic regions like the western part of the Ungava Bay and the Appalachian Mountain System from Newfoundland.
Asia
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