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

  • Harriet Howe , Alison R. Pawley EMAIL logo and Mark D. Welch
Published/Copyright: May 28, 2018
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American Mineralogist
From the journal American Mineralogist Volume 103 Issue 6

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.

Keywords: Glaucophane; eckermannite; high pressure; blueschist; subduction

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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Received: 2017-11-20
Accepted: 2018-2-28
Published Online: 2018-5-28
Published in Print: 2018-6-26

© 2018 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2018-6399
Keywords for this article
Glaucophane; eckermannite; high pressure; blueschist; subduction

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  8. Crystal chemistry of martian minerals from Bradbury Landing through Naukluft Plateau, Gale crater, Mars
  10. Petrogenesis of martian sulfides in the Chassigny meteorite
  12. Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)
  14. Snapshots of primitive arc magma evolution recorded by clinopyroxene textural and compositional variations: The case of hybrid crystal-rich enclaves from Capo Marargiu Volcanic District (Sardinia, Italy)
  16. Three-dimensional distribution of primary melt inclusions in garnets by X-ray microtomography
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  20. Determination of Al/Si order in sillimanite by high angular resolution electron channeling X-ray spectroscopy, and implications for determining peak temperatures of sillimanite
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