Abstract
An amphibole close to eckermannite in composition, ideally Na3Mg4AlSi8O22(OH)2, was encountered in experiments on a bulk composition close to that of glaucophane at 6.2 GPa, ~550–650 °C. The synthetic amphibole has an average composition corresponding to ANa0.96B(Na1.80Mg0.20)C(Mg4Al)T(Si7.85Al0.15) O22(OH)2. This composition is displaced from that of end-member eckermannite by exchange vectors +0.15 BMgTAlBNa–1TSi–1 and +0.05 A□BMgANa–1BNa–1 (□ = vacant site). In terms of end-members, it corresponds to 80% eckermannite + 15% Mg-katophorite, Na(NaMg)(Mg4Al)(Si7Al)O22(OH)2, + 5% Mg-winchite, (NaMg)(Mg4Al)Si8O22(OH)2, and as such is essentially binary. The absence of a glaucophane component implies that the stability of sodium amphibole at very high pressures (>4 GPa) involves eckermannitic rather than glaucophanic compositions. The stabilization of the eckermannite-pyrope tie line allows this highly Na-rich amphibole to occur even in bulk compositions that are not particularly Na-rich. In blueschist facies metabasites, it is possible that eckermannite forms by the reaction 9 jadeite + 7 talc → 3 eckermannite + 3 pyrope + 13 coesite + 4 H2O, above the stability limit of glaucophane that is defined by the reaction glaucophane → 2 jadeite + talc.
Acknowledgments
This work was supported by a Natural Environment Research Council Ph.D. studentship to Harriet Howe. We thank Heath Bagshaw, Jonathan Fellowes, and John Waters for technical assistance and advice in the analytical labs in Manchester. Reviewers Frank Hawthorne and David Jenkins are thanked for their helpful comments on the manuscript.
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