Average and local structure in P2_1/c clinopyroxenes along the join diopside-enstatite (CaMgSi_2O_6-Mg_2Si_2O_6)

摘要

A single-crystal X-ray refinement has been performed on two synthetic P2_1/c clinopyroxenes of composition Ca_(0.15)Mg_(1.85)Si_2O_6(Di_(15)En_(85)) and Ca_(0.23)Mg_(1.77)Si_2O_6(Di_(23)En_(77)). Splitting of the M2, O2B, O3A and O3B sites was inferred by the presence of significant residuals in the difference Fourier map and by an anomalous size and orientation of the displacement parameters. A refinement performed according to a split-model showed the coexistence of two M2 polyhedral configurations, one clinoenstatite-like, the other like Ca-richer C2/c clinopyroxenes. The two configurations have been interpreted assuming the coexistence of Ca and Mg centred polyhedra, possibly in connection with clustering preliminary to exsolution. The difference between the two configurations increases with increasing Ca content. A TEM investigation of the sample with composition Ca_(0.15)Mg_(1.85)Si_2O_6 showed the lack of exsolution textures and antiphase domains larger than 1 μm. Analysis of the average structural configurations shows that the substitution of Ca for Mg deforms differently Ca-poorer (up to 0.2-0.3 atoms p.f.u) and Ca-richer (between 0.3 and 0.6 Ca atoms p.f.u.) P2_1/c iron-free clinopyroxenes. In Ca-poorer ones only minor changes in the β parameter were found, related to a lack of shift in tetrahedral chains, whereas Ca-richer undergo a significant decrease in the β parameter, possibly due to stronger chain shift.