This is known as "talc carbonation" or "steatization" and produces a suite of rocks known as talc carbonates. Talc is primarily formed by hydration and carbonation by this reaction:

serpentine2 Mg3Si2O5(OH)4 + carbon dioxide3 CO2 → talcMg3Si4O10(OH)2 + magnesite3 MgCO3 + water3 H2OTalc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites by silica-flooding in contact metamorphic aureoles:

dolomite3 CaMg(CO3)2 + silica4 SiO2 + waterH2O → talcMg3Si4O10(OH)2 + calcite3 CaCO3 + carbon dioxide3 CO2Talc can also be formed from magnesium chlorite and quartz in blueschist and eclogite metamorphism by the following metamorphic reaction:

chlorite + quartz → kyanite + talc + waterTalc is also found as a diagenetic mineral in sedimentary rocks where it can form from the transformation of metastable hydrated magnesium-clay precursors such as kerolite, sepiolite, or stevensite that can precipitate from marine and lake water in certain conditions.In this reaction, the ratio of talc and kyanite depends on aluminium content, with more aluminous rocks favoring production of kyanite. This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, and glaucophane within the lower blueschist facies. Such rocks are typically white, friable, and fibrous, and are known as
whiteschist. Talc is a trioctahedral layered mineral; its structure is similar to pyrophyllite, but with magnesium in the octahedral sites of the composite layers. The crystal structure of talc is described as TOT, meaning that it is composed of parallel TOT layers weakly bonded to each other by weak van der Waals forces.
Talc is sometimes used as an adulterant to illegal heroin, to expand volume and weight and thereby increase its street value. With intravenous use, it may lead to pulmonary talcosis, a granulomatous inflammation in the lungs. Sterile talc powder
Sterile talc powder (NDC 63256-200-05) is a sclerosing agent used in the procedure of pleurodesis. This can be helpful as a cancer treatment to prevent pleural effusions (an abnormal collection of fluid in the space between the lungs and the thoracic wall). It is inserted into the space via a chest tube, causing it to close up, so fluid cannot collect there. The product can be sterilized by dry heat, ethylene oxide, or gamma irradiation. Safety
Suspicions have been raised that talc use contributes to certain types of disease, mainly cancers of the ovaries and lungs. According to the IARC, talc containing asbestos is classified as a group 1 agent (carcinogenic to humans), talc use in the perineum is classified as group 2B (possibly carcinogenic to humans), and talc not containing asbestos is classified as group 3 (unclassifiable as to carcinogenicity in humans). Reviews by Cancer Research UK and the American Cancer Society conclude that some studies have found a link, but other studies have not.The studies discuss pulmonary issues, lung cancer, and ovarian cancer.