An amphibole quadrilateral, analogous to the well-known pyroxene quadrilateral, is defined by the end member compositions Ca2Mg5Si8O22(OH)2 (tremolite), Ca2Fe5Si8O22(OH)2 (ferrotremolite), Mg7Si8O22(OH)2 (anthophyllite), and Fe7Si8O22(OH)2 (grunerite). Phase relations along the join Mg3.5Fe3.5Si8O22(OH)2-Ca2Mg2.5Fe2.5Si8O22(OH)2, through the center of the quadrilateral, were studied at Pfluid = 2 kbar and at oxygen fugacities defined by the FMQ buffer. Compositions along this join are expressed in mole percent of the calcic end member, actinolite (e.g., Act70). It has not been possible to bracket phase boundaries with reversals because of the reluctance of new phases to nucleate in crystalline starting materials.

Three amphiboles with intermediate Fe : Mg ratios—anthophyllite, cummingtonite, and actinolite—were synthesized. Cummingtonite and actinolite are separated by a solvus that is asymmetric with a very steep cummingtonite limb near Act10 and an actinolite limb that is measurably temperature dependent (Act96±4 at 500°C, Act85±4 at 600°C). The maximum solubility of the Ca-free component in the actinolite is reached at ~600°C. Actinolites with Fe : Mg ≅ 1, if coexisting with cummingtonites, are useful geothermometers and may be used to estimate or put lower limits on, temperatures of metamorphism with a minimum uncertainty of ±35°C.

Actinolite decomposes by the reaction, actinolite → cummingtonite + clinopyroxene + quartz + H2O. The products of this reaction differ from those reported for the decomposition of tremolite( Boyd, 1959) and ferrotremolite( Ernst, 1966).

Anthophyllite is the only amphibole which has been synthesized in Ca-free runs(i.e., Act0). The presence of Ca facilitates nucleation of cummingtonite, and the assemblage anthophyllite + cummingtonite grows at the expense of less stable assemblages between about Act1 and Act7. Cummingtonite alone is present at ~Act10. Some experiments suggest that the absence of cummingtonite in Ca-free runs is only a nucleation problem, but the phase relations between the two Ca-poor amphiboles have not been resolved satisfactorily.

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