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

The crystal structure of Ca0.8Mg1.2Si2O6 clinopyroxene (Di80En20) at Τ=-130°, 25°, 400° and 700°C

“…As observed in the work by Tribaudino et al (1989), thermal ellipsoid anisotropies in the Di 80 En 2 o clinopyroxene at room temperature (Bruno et al, 1982) are mostly due to positional displacement of the atoms in the structure. This displacement is particularly evident in the M2 site along the diad axis and forces a M2-M2' split refinement.…”

“…As observed by Tribaudino et al (1989), a split-model for the surrounding oxygens also (O-split model) enables a 6-fold coordination to be assigned to M2'; this is more likely for Mg (bond lengths in Tribaudino et al, 1989, Tables 5 and 8). In Di 80 En 20 clinopyroxene, the M2'-0 bond lengths seem to increase with temperature, but the experimental uncertainty is very high.…”

“…Substitution of a larger cation for a smaller one in the M2 site makes it more nearly 8-coordinated in the average structures, as can be seen in the C2/c region of the ClFs-Hd (Ohashi, Burnham, Finger, 1975) and ClEn-Di joins (Tribaudino et al, 1989). By contrast in Di 8 oEn 20 clinopyroxene (as in diopside and in other chain silicates) an increase in temperature leads to a more nearly 6-coordinated M2 site.…”

“…Tribaudino, Benna, Bruno (1989) have since refined the average structures of Di 80 En 2 o and Di 66 En 34 clinopyroxenes according to a split model also for oxygen atoms (O-split model), in which Mg in M2' is 6-coordinated. They also observed that in the large M2 cavity in C2/c pyroxenes Ca atoms assume a Di-like configuration (8-coordinated) and Mg atoms, as suggested by Rossi et al (1987), assume a high Pig-like configuration (6-coordinated) (Brown, Prewitt, Papike, Sueno, 1972).…”

Raman spectroscopy of CaCoSi2O6–Co2Si2O6 clinopyroxenes

“…Detailed data on crystal structure and microstructures for pyroxenes are seldom available: an example is the series diopside-enstatite (Di-En, CaMgSi 2 O 6 -Mg 2 Si 2 O 6 ) of Ca-Mg pyroxenes, where structural and microstructural data on several synthetic samples are available (Tribaudino et al 1989(Tribaudino et al , 2005 and quantum mechanical calculations were performed on the end-member diopside (Prencipe et al 2012). In Di-En series, Ca and Mg exchange in the M2 site, whereas Mg and Si occupy the M1 and T sites.…”