Soapstone most often forms at convergent plate boundaries where broad areas of Earth’s crust are subjected to heat and directed pressure. Peridotites, dunites, and serpentinites in this environment can be metamorphosed into soapstone. On a smaller scale, soapstone can form where siliceous dolostones are altered by hot, chemically active fluids in a process known as metasomatism.
Soapstone pencils: Talc is very soft and has a white streak. Since soapstone is made primarily of talc, it will deposit a white powder when it is rubbed against almost any object. This white mark is similar to talcum powder and is easily brushed off without leaving a permanent mark. Soapstone pencils are used by tailors to mark fabric. Soapstone markers are also used by welders. The heat-resistant powder does not burn away and continues to be visible when the workpiece is heated during the welding process.
