The pyroxene structure consists of single chains of tetrahedra extending along the c axis of the unit cell. There are two types of cation positions, designated M1 and M2. The smaller M1 sites lie between the apices of opposing tetrahedra, and are almost regular octahedra. Although Al is commonly present only in small amounts in the pyroxene structure, it can occur in both tetrahedral and octahedral coordination. Granulite-facies pyroxenes, therefore, tend to have tetrahedral Al greater than or equal to octahedral Al, and their Al content can be expressed in terms of the tschermak's molecule end members.
High pressures favour increased coordination number.
Jadeite is the typical Al-bearing pyroxene phase component. Be aware that the result is imprecise and sensitive to the accuracy of the other major constituents. For each of the following examples, recalculate ferric iron and allocate cations to the T, M1 and M2 sites. Clinopyroxene Diopsidic clinopyroxene from the same granulite-facies metabasite as the Opx above. Now check the answer .