Experimentally determined compositions of protopyroxene, orthopyroxene and pigeonite (including metastable pigeonite) with liquids in the system CaO-MgO-Al203-Si02
Biggar, Gordon M.
European Journal of Mineralogy Volume 1 Number 2 (1989), p. 181 - 192
published: May 3, 1989
manuscript accepted: Oct 7, 1988
manuscript received: Apr 9, 1988
ArtNo. ESP147050102004, Price: 29.00 €
Abstract Microprobe analyses of liquids and pyroxenes from repetitive and consecutive experiments of up to 27 days duration resolve many of the previous ambiguities about pyroxene stability in the system CaO-MgOAl2O3- SiO2. Protopyroxene, orthopyroxene, pigeonite and diopside are stable down to about 1,320 °C where pigeonite ceases to be stable. Below 1,320 °C, protopyroxene, orthopyroxene and diopside are stable. Pigeonite frequently occurred in first heat experiments below 1,320 °C, as has been reported in the literature, but in the present experiments it was eventually eliminated by repeated subsequent heating and is now regarded as metastable. The compositional ranges of protopyroxene (< 0.9 %.wt CaO, < 2 %.wt A12O3), orthopyroxene (2.0-3.0 %.wt CaO, < 1.1 %.wt A12O3) and pigeonite (3.2-5.3 %.wt CaO, < 0.35 %.wt Al2O3) were found to vary with temperature, and orthopyroxene was more aluminous when it co-existed with forsterite than when it co-existed with tridymite.