Properties of pyroxenes 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/1600C) 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 syste-matic 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 1400-1600C using a multianvil device. Methods to be applied involve single crystal/powder diffraction with X-ray and synchrotron sources under ambient and high pressure conditions (diamond anvil cell), electron microprobe analysis, laser Raman spectro-scopy, and low-temperature calorimetry to constrain the P-T stability of NaPx and to assess the crystal- and thermochemical properties, and the elastic behavior of NaPx and its solid solutions. The results of this study will provide new insights into the high pressure-high temperature crystal chemistry and phase relations of one of the most important groups of rock- forming silicates and will allow to assess the potential stability field of pyroxenes with 6-coordinated silicon in the Earth`s upper mantle and transition zone. Since (clino)pyroxenes are not only amongst the most important terrestrial silicates but also part of almost all meteorite classes, we believe that this study will represent a significant contribution to our understanding of the inner structure of the terrestrial planets in general.

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/1600C) 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 syste-matic 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 1400-1600C using a multianvil device. Methods to be applied involve single crystal/powder diffraction with X-ray and synchrotron sources under ambient and high pressure conditions (diamond anvil cell), electron microprobe analysis, laser Raman spectro-scopy, and low-temperature calorimetry to constrain the P-T stability of NaPx and to assess the crystal- and thermochemical properties, and the elastic behavior of NaPx and its solid solutions. The results of this study will provide new insights into the high pressure-high temperature crystal chemistry and phase relations of one of the most important groups of rock- forming silicates and will allow to assess the potential stability field of pyroxenes with 6-coordinated silicon in the Earth`s upper mantle and transition zone. Since (clino)pyroxenes are not only amongst the most important terrestrial silicates but also part of almost all meteorite classes, we believe that this study will represent a significant contribution to our understanding of the inner structure of the terrestrial planets in general.