Almandine-rich garnet from near Collobrières, southern France, and its petrological significance
Woodland, Alan B.; Droop, Giles; O’neill, Hugh St. C.
European Journal of Mineralogy Volume 7 Number 1 (1995), p. 187 - 194
published: Feb 8, 1995
manuscript accepted: Jul 25, 1994
manuscript received: Apr 27, 1994
ArtNo. ESP147050701016, Price: 29.00 €
Abstract Garnets from metamorphosed ironstones from near Collobrières, southern France, are some of the most almandine-rich garnets yet reported. They occur in a quartz-free assemblage, containing fayalite, grunerite, magnetite, and apatite. Peak metamorphic conditions are estimated to be 630 ± 60°C and 5 ± 2 kbar. Six garnets were characterised by electron microprobe, Mossbauer spectrometry, and X-ray diffraction. On an Fe3+-free basis, five of the garnets contain ≈ 95 mol % almandine and one contains > 97 mol % almandine. Mossbauer spectra indicate a uniform Fe3+/ΣFe = 0.03. This exceeds the amount that can be accounted for as an andradite component and a skiagite (Fe32+ Fe23+ Si3O12) component must be considered. The use of garnet molar volumes to estimate Fe contents shows some promise, but general application awaits further determination of endmember component molar volumes and an accounting for non-ideal volumes of mixing. The occurrence of fayalite indicates relatively reduced metamorphic conditions, below FMQ. The presence of Fe3+ in our garnets under such conditions suggests similar or larger amounts of Fe3+ should be present in aluminous garnets in medium and high grade metamorphic rocks.