High pressure-high temperature stability and crystal chemical properties of clinopyroxenes with 6-coordinated silicon

Pyroxenes are amongst the most important geological materials of the terrestrial planets. In the Earth’s crust and upper mantle, pyroxenes are stable in a wide range of P-T conditions and bulk compositions in metamorphic as well as magmatic environments. It has long been assumed that clinopyroxenes contain silicon in tetrahedral coordination only. This assumption was disproved by the high pressure synthesis (15 GPa/1600°C) of a clinopyroxene Na(Mg0.5Si0.5)Si2O6 (NaPx) that contains both 4- and 6-coordinated silicon, a phase that was later also found as an inclusion in diamond. The discovery that the clinopyroxene structure can accomodate 6-coordinated silicon is of fundamental importance for an understanding of silicate crystal chemistry at high P and T . This is because a change in coordination of silicon from fourfold to sixfold has significant effects on material properties such as density, elastic parameters and, consequently, on the P-(T) stability fields of these phases. Based on a successful reconnaissance study, this project proposes a systematic investigation of the stability fields and crystal- and thermochemical properties of clino-pyroxenes with 6-coordinated silicon synthesized in simplified chemical systems involving NaPx and solid-solutions of NaPx with diopside, enstatite, jadeite, kosmochlor and Kcpx in a P-T range approximately 5-20 GPa and 1400-1600°C using a multianvil device.

Leitung: Konzett, Jürgen

Finanziert durch: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

Laufzeit: 2005 - 2007

Kooperationspartner: Bayrisches Geoinstitut/ Universität Bayreuth; University of Arizona/Tucson