Talc and 10-Å phase are hydrous phases that are implicated in fluid processes and rheological behaviour in subduction zones. Natural samples of talc show limited compositional variation away from the MgO–SiO 2 –H 2 O (MSH) endmember, with only substitution of Fe 2+ for Mg occurring in significant amounts. In experiments at 2 GPa, talc containing 0.48 apfu Fe 2+ begins to break down in the divariant field talc + anthophyllite +...