Original paper

Fe-Mg solid solution of sursassite, (Fe,Mg)4(Mg,Fe,Al)2Al4[Si6O21/(OH)7]

Wunder, Bernd; Gottschalk, Matthias

Abstract

Sursassites (Fe,Mg)4(Mg,Al,Fe)2Al4[Si6O21/(OH)7] have been synthesized along the Mg-Fe join at 4.0 - 5.0 GPa, 600 - 650 °C and at fO2 defined by the iron-wuestite buffer using gels as starting materials. (Fe,Mg)-sursassites have been investigated by scanning electron microscopy, X-ray powder diffraction, and electron microprobe. Pure Fe-sursassite of the composition (Fe)4(Fe0.55Al1.45)Al4[Si6O21.45/(OH)6.55] formed as an almost single-phase product at 4.0 GPa, 650 °C. The cell-dimensions of Fe-sursassite are a = 8.627(2) Å, b = 5.736(1) Å, c = 9.704(3) Å, β = 108.73(2)°, V = 454.8(2) Å3. We assume that in comparison to Mg-sursassite, Fe-bearing sursassites might be stable towards lower pressures, indicating that Fe-bearing sursassite is probably more important as a host of water than the Mg-endmember within subducting slabs. This would provide a mechanism for the transport of water into the mantle for metapelitic and metabasaltic compositions. The assemblage Fe-chloritoid + ferrosilite + Fe-staurolite formed at 3.5 GPa, 600 °C from the Mg-free gel. Although not verified by equilibrium experiments, the stability of Fe-sursassite might be limited by this assemblage towards lower pressures.

Keywords

fe-mg solid solutionsursassitehigh-pressure metamorphismsubduction zone