Compressibility trends of the clinopyroxenes, and in-situ high-pressure single-crystal X-ray diffraction study of jadeite

Abstract

The crystal structure of a natural jadeite, NaAlSi 2 O 6 , was studied at room temperature over the pressure range 0-9.17 GPa using single-crystal X-ray diffraction. Unit-cell data were determined at 16 pressures, and intensity data were collected at nine of these pressures. A third-order Birch-Murnaghan equation of state fit to the P-V data yielded V 0 = 402.03(2) Å 3 , K 0 = 136.5(14) GPa, and K 0 ' = 3.4(4). Jadeite exhibits strongly anisotropic compression with unit strain axial ratios of 1.00:1.63:2.10. Silicate chains become more O-rotated with pressure, reducing ∠O3-O3-O3 from 174.7(1)° at ambient pressure to 169.2(6)° at 9.17 GPa and bringing the anions of jadeite closer to a cubic closest-packed arrangement. No evidence of a phase transition was observed over the studied pressure range.In an effort to understand pyroxene compressibilities, selected clinopyroxene bulk moduli were plotted against ambient unit-cell volumes. Two trends were identified and are explained in terms of differences in M2-O3 bonding topologies and the geometric relationship of the bonds with tetrahedral rotation in the silicate chains. Bonds positioned to favor the tetrahedral rotation upon compression are termed "sympathetic," whereas bonds positioned to resist the rotation are termed "antipathetic." Examination of the different pyroxene structures indicates that structures containing antipathetic M2-O3 bonds are less compressible than those with only sympathetic M2-O3 bonds. This behavior has not been previously recognized.

High pressure studies of Eu2+ and Mn2+ doped NaScSi2O6 clinopyroxenes

“…Therefore both structures do not differ signicantly and their changes are reversible upon pressure release. 20 The splitting of the line around 400 cm À1 observed at pressure of 122 kbar and above can be also associated with the change of crystallographic structure, most probably similar to those observed in the other clinopyroxenes, i.e. from C2/c structure to P2 1 /c.…”

“…20, equal to 109.4 GPa. Also values of pressure derivatives of bulk moduli B 0 0 for clinopyroxenes are very oen spurious (sometimes even negative).…”

“…Also values of pressure derivatives of bulk moduli B 0 0 for clinopyroxenes are very oen spurious (sometimes even negative). 20 Lack of good knowledge on the nature of phase transitions and poor data quality are oen blamed for these discrepancies. Therefore a typical value of B 0 0 ¼ 4 has been used in further calculations.…”

High-pressure behavior of space group P2/n omphacite

“…Many recent X-ray diffraction studies have focused on the behavior under high-pressure conditions of clinopyroxene with different compositions (Downs 2003;Origlieri et al 2003; Thompson et al 2005;Bindi et al 2006;McCarty et al 2008;Nestola et al 2005Nestola et al , 2006Nestola et al , 2007Nestola et al , 2008aNestola et al , 2010Gavrilenko et al 2010). This is likely due to the very wide range of geological high-pressure environments in which this mineral is found, from metamorphic rocks to meteorites, and also as inclusions in diamonds (e.g., Nestola et al 2007;Koch-Müller et al 2004).…”

Packing systematics of the silica polymorphs: The role played by O-O nonbonded interactions in the compression of quartz

“…Thompson and Downs 2001a). we have worked extensively on actual and hypothetical packings in pyroxenes (Thompson and Downs 2001a, 2001b, 2004Origlieri et al 2003;Thompson et al 2005;McCarthy et al 2008;nestola et al 2008). However, it is difficult to unambiguously ascribe pressure-induced changes in either packing or polyhedral distortion in many minerals, including pyroxenes, to anion-anion interaction because many of the minerals whose packing arrangements have been described are based on distorted closest-packing of anions.…”