A new natural high-pressure (Na, Ca)-hexaluminosilicate [(CaxNa1-x)Al3+xSi3-xO11] in shocked Martian meteorites

A (Ca,Na)-hexaluminosilicate, whose Ca end member was previously synthesized in numerous high-pressure experiments, has been identified by Raman spectroscopy in heavily shocked Martian meteorites. This mineral has a structural formula close to (CaxNa1-x)Al3+xSi3-xO11 and is similar to the calcium aluminum silicate phase previously synthesized in high-pressure experiments performed on anorthite and rocks of basaltic composition. This new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with SiO2-stishovite. The first setting, encountered in Zagami, consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition. The second setting contains the (Na, Ca)-hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures. The mineralogical association (Na,Ca)-hexaluminosilicate+stishovite was observed in shock melt pockets, which have distinct bulk compositions in seven Martian shergottites. This new mineral represents, after majorite, the second natural occurrence of a silicate mineral with silicon in both four and six coordination. The assemblage stishovite+(Na,Ca)-hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites. The (Na, Ca)-hexaluminosilicate mineral is a potential carrier of Al and Na during subduction of oceanic crust in the lower mantle of the Earth. (C) 2003 Elsevier B.V. All rights reserved.