Sodium amphibole in the post-glaucophane high-pressure domain: The role of eckermannite

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.