High-pressure behavior of space group P2/n omphacite

Abstract

A single-crystal X-ray diffraction (XRD) study, using a diamond-anvil cell at high pressure and room temperature, was performed on a crystal from a natural space group P2/n omphacite sample with composition very close to Jd 55 Di 45 and with a high degree of order in cation distribution. Unit-cell parameters were determined at 13 different pressures up to about 7.5 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) fitted to the P-V data yielded V 0 = 421.43(4) Å 3 , K T0 = 122(1) GPa, and K′ = 5.1(3). The K T0 value for this sample lies between the data obtained for the two end-members jadeite and diopside, and describes a slight positive curvature trend.During the same experiment, intensity data were collected and crystal structures were refined at 5 pressures up to 7.3 GPa. Both M1 and M2 polyhedra volumes showed a slight but significant change in slope at about 4 GPa. This behavior can likely be explained in terms of tilt angle variation of TA and TB tetrahedral, which also showed a change in slope with pressure, rather than in terms of bond length compression anomaly.

New thermoelastic parameters of natural C2/c omphacite

“…Concerning the bulk modulus data, we have compared the values relative to sample Jd 55 Di 45 (C2/c sample N3, this work and P2/n sample N4 by Pandolfo et al 2012) with those of jadeite (Nestola et al 2006), diopside (Gavrilenko et al 2010) and two vacancy-bearing omphacites by McCormick et al (1989) (see Fig. 7).…”

“…6 data points); (c) a calibration of experimental pressure done using the ruby fluorescence instead of the quartz EoS (which was used in the other highpressure experiments); and (d) the impossibility to refine the first pressure derivative. Using the data by Gavrilenko et al (2010), Pandolfo et al (2012) and Nestola et al (2006) plotted in Fig. 7, we obtained the following weighted linear equation, which expresses the evolution of K T0 as a function of Jd molar %:…”

“…Neutral versus ionized scattering factors were refined for all sites that are not involved in chemical substitutions (O and Si) (Hawthorne et al 1995). When the refinement reached convergence, full-matrix least squares were carried out using the data from the electron microprobe analysis (with 1r error) as chemical constraints to obtain the site partitioning using the same procedure already described in Pandolfo et al (2012). Observed degrees of order calculated following Carpenter et al (1990) for these two natural crystals are reported in Table 2.…”

“…Two further studies have been Electronic supplementary material The online version of this article (doi:10.1007/s00269-012-0484-1) contains supplementary material, which is available to authorized users. performed on ordered P2/n omphacite by Pavese et al (2001) by means of powder diffraction and by (Pandolfo et al 2012) by single-crystal X-ray diffraction. Concerning the thermal expansion data, to our knowledge there is only one study on a P2/n sample (Pavese et al 2000).…”

High Pressure Single Crystal Diffraction at PX^2

“…Though pressure media such as argon, silicone oil or methanol-ethanol-water solution were used in previous single crystal diffraction experiments that did not exceed 10 GPa [21][22][23] , these pressure media become significantly nonhydrostatic between 5-10 GPa…”