Amphibole-plagioclase equilibria: An empirical model for the relation albite+tremolite=edenite+4 quartz |
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| Authors: | Frank S Spear |
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| Institution: | (1) Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, 02139 Cambridge, Massachusetts, USA |
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| Abstract: | Equilibria between plagioclase, calcic amphibole and quartz can be described, in part, by the relation among mineral components: NaAlSi3O8+ Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2; this relation governs the partitioning of Na between plagioclase and the A-site of coexisting amphibole. Data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade. The ideal portion of the equilibrium constant (K
id = X
Na, A/X, A · X
Ab) derived from natural samples is sensitive to bulk composition, inasmuch as both plagioclase and amphibole are highly non-ideal. Samples from a single outcrop have values ranging from 0.5 (X
Ab=0.74) to 4.1 (X
Ab=0.10). The continuous reaction, NaAlSi3O8+Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2, proceeds to the right with increasing grade of metamorphism and for a given bulk composition, K
id increases with increasing temperature. Two related discontinuous reactions, actinolite+albite=hornblende+oligoclase+quartz and actinolite+oligoclase=hornblende+anorthite+quartz, also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of Na between plagioclase and amphibole A-site. A Schreinemakers' net is presented that is consistent with natural occurrences. The results of this study should aid in the delineation of metamorphic facies within amphibolites. |
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