magnesiocarpholite

Magnesiocarpholite is a rare magnesium-bearing inosilicate mineral in the carpholite group, with ideal formula MgAl2Si2O6(OH)4. It typically forms very slender, acicular crystals with a silky appearance. Magnesiocarpholite occurs in high-pressure, low-temperature metamorphic rocks and is used by geologists as a marker for subduction-zone conditions in magnesium-rich sedimentary rocks.

== Discovery == Magnesiocarpholite was first recognised as a distinct magnesium-rich member of the carpholite group in the early 1980s. It was first discovered in the Vanoise massif of Savoie, France, which is considered its type locality. In high-pressure laboratory experiments, Christian Chopin, a French mineralogist and metamorphic petrologist, and an emeritus research director at the CNRS working in the geology laboratory of the École Normale Supérieure in Paris, was able to recreate Magnesiocarpholite in a lab as a Mg-rich carpholite under conditions similar to those in deeply buried sedimentary rocks, and showed that this phase had a consistent composition close to MgAl2Si2O6(OH)4. Follow-up work showed that magnesiocarpholite, together with the related mineral magnesiochloritoid, could be used to estimate the pressures and temperatures reached by certain metamorphosed sedimentary rocks.