Crystal chemistry of diopsides from garnet Iherzolites (Cima Lunga/Adula Nappe, Central Alps)

The first crystal chemical data on garnet peridotite clinopyroxene are presented through the study of a suite of diopsides from metamorphic garnet lherzolites from the Cima Lunga/Adula Nappe, Central Alps (CL-Cpx). CL-Cpx have low Al IV occupancies and moderate, though variable, Na and Al VI contents. Small Al IV may be due to fractionation of Al 2 O 3 into coexisting garnet ± amphibole, and involves low T site volumes (2.218-2.226 A 3 ). Contraction of the almost Si-saturated tetrahedron favours expansion of adjacent M1 polyhedron and requires relatively low Al VI . This produces relatively large cell volumes (435.6-438.3 A 3 ), despite the relatively high-pressure origin (garnet stability field)