Titanian hydroxyl-clinohumite: Formation and breakdown in antigorite rocks (Malenco,Italy)

Petrographic evidence is presented for the breakdown of titanian hydroxyl-clinohumite to olivine+magnesian ilmenite (or geikielite)±magnetite in the outermost zone of the Bergell aureole in the Malenco Serpentinite, Prov. Sondrio, Italy. The breakdown coincides in the field with the isograd reaction: antigorite+diopside=olivine+tremolite+H₂O. It is therefore concluded that this variety of clinohumite is unstable above approximately 520° C at a pressure of 3 kbars. Elsewhere in the Malenco Serpentinite, titanian hydroxyl-clinohumite may be found to have reacted with CO₂ to produce antigorite, magnesian ilmenite and magnesite. Titanian hydroxyl-chondrodite was detected in one sample. Under crustal pressures, the stability field of F-free clinohumite is entirely contained inside that of antigorite. The stable occurrence of titanian clinohumite in high-grade metamorphic ultrabasic rocks may be attributed to the substitution of F for OH.