Shock-Induced Transformations in the System NaAISiO_4 -SiO_2 : a new Interpretation

Abstract

New internally consistent interpretations of the phases represented by the high pressure phase shock wave data for an albite-rich rock, Jadeite, and nepheline in the system NaAlSiO_4-SiO_2, are obtained using the results of static high pressure investigations, and the recent discovery of the hollandite phase in a shocked meteorite. We conclude that nepheline transforms directly to the calcium ferrite structure, whereas albite transforms possibly to the hollandite structure. Shock Hugoniots for the other plagioclase and alkali feldspars also indicate that these transform to hollandite structures. The pressure-volume data at high pressure could alternatively represent the compression of an amorphous phase. Moreover, the shock Hugoniot data are expected to reflect the properties of the melt above shock stresses of 60–80 GPa. The third order Birch-Murnaghan equation of state parameters are: K_(os) = 275±38 GPa and K′_(os) = 1.6±1.5 for the calcium ferrite type NaAlSiO_4, K_(os) = 186±33 GPA and K′_(os) = 2.6±1.7 for the albite-rich hollandite, K_(os) = 236±45 GPa and K′_(os) = 2.3±2.0 for the orthoclase-rich hollandite, and K_(os) = 190 to 210 GPa and K′_(os) ≈ 2.2 for the anorthite-rich hollandite.

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