Geologic Features
Talc is hydrated magnesium silicate, as is also serpentine, a mineral with which talc is closely associated. Both are common alteration products of magnesian silicate minerals such as olivine, pyroxene, and amphibole. Talc is also derived from the recrystallization of magnesian carbonates.
Talc deposits consist of lenses and bands in metamorphic limestones, schists, and gneisses of ancient age. The talc itself is usually schistose like the wall rocks, and is largely a product of mechanical mashing. In some cases, also, talc results from the alteration of igneous rocks without mashing—as in the case of the large talc and soapstone deposits of Virginia, which are the result of rather complete alteration of basic igneous rocks such as peridotites and pyroxenites.
Talc is known to result from the weathering of magnesian silicates under surface conditions, but the common occurrence of the principal deposits, in highly crystalline rocks which have undergone extensive deep-seated metamorphism, is an indication that processes other than weathering have been effective. It has been suggested that hot ascending solutions have been responsible for the work, but without much proof. A more plausible explanation for many deposits is that the talc results from the dynamic metamorphism or shearing of impure magnesian carbonates (as in highly magnesian limestones), the process resulting in elimination of the carbon dioxide and recrystallization of the residue. Certain talc deposits, such as those of Ontario, show clearly traces of the original bedding planes of limestone crossing the cleavage of the talc, and the rock bears all the evidence of having formed in the same manner as a common slate. Talc and slate are almost the only mineral products which owe their value principally to dynamic metamorphism.