Average structure and M2 site configurations in C2/c clinopyroxenes along the Di-En join

Abstract

In order to clarify the structural configurations observed in Di_ss in the Ca-rich region of the Di-En join (in which TEM observations show neither exsolution microstructures nor evidence of spinodal decomposition) single crystals large enough for X-ray diffraction analyses, with composition (Ca_0.66Mg_0.34)MgSi₂O₆, have been equilibrated close to the solvus atT=1350° C for 317 h, and quenched at room temperature. The refinement in C2/c space group shows that in the M2 site Ca and Mg are fully ‘ordered’ in two split positions (M2_occ: 0.66 Ca; M2′_occ: 0.34 Mg). Since the average structure shows a relevant elongation of anisotropic thermal ellipsoids of the O2 and O3 oxygen atoms, the refinement has been carried out according to a split model for O2 and O3 atoms: Ca appears 8-coordinated (as in diopside) and Mg shows a sixfold coordination similar to that of high-pigeonite. This coordination for Mg is significantly different from the fourfold coordination (Zn-like in Zn-cpx) proposed previously and it is a more probable coordination for Mg from a crystalchemical point of view. The same results were obtained refining a Di₈₀En₂₀ cpx, equilibrated atT=1230° C, according to the same O-split model. The data support the coexistence of a Di-like configuration for Ca and of a highPig-like configuration for Mg away from the solvus also. AtT very near toT _solidus the different configurations, observed at room temperature in the quenched samples, should converge and Ca and Mg should retain a single disordered configuration in the M2 site.