Titanian chondrodite and clinohumite in marbles from the �tztal crystalline basement

Abstract

Marble lenses within the polymetamorphic Otztal crystalline complex frequently contain pockets of clinohumite and chondrodite and their titanium-rich varieties. Chondrodites contain up to 9.1 wt% TiO2 and clinohumites about half that amount. The maximum amounts of Ti in both minerals do not exeed values of 0.4 in the formula. XTi and XF are correlated by a slope of −1, indicating the efficiency of the TiO2Mg−1F−2 substitution at constant OH (Evans andTrommsdorff, 1983). Clinohumites sometimes contain lamellae which have nonstoichiometric cation ratios reflecting a composition between clinohumite and olivine. In many cases Ti-rich clinohumites and chondrodites coexist with Mg-ilmenite. Ti-rich clinohumites and chondrodites often break down to a symplectite of Ti-poor clinohumite (or olivine) and Mg-ilmenite. Parts of the marbles, which are barren of Mg-ilmenite contain F-rich clinohumites. The latter do not show any signs of breakdown features. We conclude therefore that fluorine stabilizes the humite group minerals to a wide range of metamorphic conditions. Ti-rich humite-group minerals are not stable till amphibolite facies conditions, and below F-rich humite-group minerals and olivine are the more stable phases in a H2O-rich, SiO2-poor carbonate system.