Abstract: A suite of orthopyroxenes from spinel Iherzolite xenoliths associated with basanites occurring in the Victorian (Australia) post-Pliocene ‘Newer Volcanics’ province was investigated by means of a crystal chemical methodology which provides accurate site occupancy and site configuration parameters.

The M1 configuration is essentially constrained by Al^VI rather than Fe²⁺. In addition, Fe³⁺, Cr³⁺ and Ti⁴⁺ are confined to M1 (Molin, 1989) and Al^IV to TB. M2 is controlled by Fe M2 2+ ⇌ Mg M2 , constrained by (Fe²⁺ + Ca)_M2 > 0.14 atoms per formula unit (p.f.u.). Cation substitution in TB and M2 constrains the sum of the volumes of the respective polyhedra V_TB+V_M2 to remain essentially constant. Therefore, M2 favours the retention of the large Fe²⁺ up to melting-point, causing non-ideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe²⁺ carrier during partial mantle melting.