Influence of magma composition and oxygen fugacity on the crystal structure of C2/c clinopyroxenes from a basalt-pantellerite suite |
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| Authors: | C Carbonin A Dal Negro S Ganeo E M Piccirillo |
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| Institution: | (1) Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, 35122 Padova, Italy;(2) Istituto di Mineralogia e Petrografia, Università di Trieste, Italy |
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| Abstract: | The crystallochemical variations of clinopyroxene in response to changes in fO
2 and melt composition have been determined for a basalt-pantellerite suite (Boseti Complex, Main Ethiopian Rift) by crystal
structure refinement and microprobe analysis. The pyroxene evolutionary trend has both a “Ca-minimum” and late iron enrichment.
During crystallization from basalts to trachytes, clinopyroxene geometry depends mainly on the relationships between T and
M2 sites; for example, high SiO2 activity in the magma causes high Si occupancy in T site, which in turn requires low Ca occupancy in M2 site in order to
fulfill the local charge balance requirements. In contrast, clinopyroxene crystallized from acid melts is characterized by
high Fe2+ (M1) content and therefore by a very large M1 site. Longer 〈M1-O1〉 and M1-O2 bond lengths require shorter T-O1 and T-O2 bond
lengths and high Si occupancy in T site. It is concluded that the “Ca-minimum” in the clinopyroxene structure is regarded
as the lowest value at which the charge balance requirements are satisfied in a C2/c clinopyroxene structure. |
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