Effect of Al On Enstatite Solubility In Cmas Clinopyroxenes .2. Crystal-chemical Considerations

The refinement of the structures of three pyroxenes of composition Di75En10CaTs15, Di65En20CaTs15 and Di70CaTS30 Synthesized in the range T = 1275-1525-degrees-C and P = 14-18 kbar maps out the structural changes occurring in C2/c Di-En-CaTs clinopyroxenes in the CMAS system and completes the existing data along the Di-CaTs join. The most notable features are: 1) the M2 site coordination changes continuously along the Di-En join from 8-fold to 6-fold, with a shift in tetrahedral chains along z. The displacement parameters suggest the coexistence of two structural configurations, clinoenstatite-like and diopside-like; 2) the M2-0 bond lengths become more regular along the Di-CaTs join, as imposed by charge balance requirements. The change in bond lengths is associated with a chain shift along z which is opposite to that observed along the Di-En join. The small values of the displacement parameters suggest an actual change in atomic positions along the join, but with no positional disordering. 3) in Di-En-CaTs ternary clinopyroxenes the opposite chain shifts observed along the two joins compensate. However, displacement parameters are greater than those observed along the join Di-En, suggesting the coexistence of two structural configurations in the average structure. These results suggest that the M2 polyhedron in Al-rich clinopyroxenes is less able to accept extreme Mg at the site than in Al-free clinopyroxenes. This can explain the lower enstatite solubility previously observed in Al-rich clinopyroxenes.