C2/c pyroxene from two alkaline sodic suites (Western Ross Embayment-Antarctica): crystal chemical characterization and its petrologic significance

Two C2/c pyroxene suites from sodic alkaline rocks (Basanite-Phonolite: B-Ph and Alkali basalt- Trachyphonolite: AB-T) belonging to the McMurdo Volcanic Group (Mt. Melbourne province and Mr. Erebus, Antarctica) were investigated by single-crystal X-ray diffraction combined with electron probe microanalysis which together provide accurate site occupancy and site configuration parameters. Variations in site volumes distinguish the clinopyroxenes belonging to the more alkaline B-Ph suite from those of the AB-T suite. The former have higher VM2 and, for similar cell volume, lower VMj and higher VT. In these C2/c pyro• the bond lengths of M1 depend on R 3+ content, necessary to balance A1 Iv in the T site. M1 geometric variations are similar for both B-Ph and AB-T suites. However, the M2 site is crucial for variations in polyhedral configurations. The increase in A1 ~v affects the shortest M2-O bond lengths in different ways depending on (Ca+Na) contents in the M2 site. Thus, the clinopyroxenes were distinguished in two different groups not related to the two suites. The first group is characterized by (Ca+Na) higher than 0.90 atoms per formula unit but shows a good positive correlation between the shortest M2-O bond lengths and A1 Iv content, at quite constant (Ca+Na). The second group has (Ca+Na) less than 0.90 atoms per formula unit but shows a poor correlation between the shortest M2-O bond lengths and AI Iv content. In general, shortening of the longest M2-O bond lengths is associated with increase in A1 Iv. The cell and M1 volumes suggest that the clinopyroxenes, including the larger and sometimes resorbed macrocrysts, crystallized at a pressure lower than 5 kbar, fitting the petrography and evolved character of the rocks in question.