X-ray Diffraction Study of Ferric Calcium Silicate Perovskite to 62 GPa
Abstract
The effect of ferric iron in calcium silicate perovskite (Ca-pv) has not been studied, although significant solubility was documented (Wang and Yagi,1998). We have synthesized ferric Ca-pv from natural Ca3Fe2Si3O12 (andradite) starting material in the laser-heated diamond cell and measured X-ray diffraction patterns at 19-62 GPa. Samples were loaded with pressure medium (either Ar or NaCl) and heated to 2000 K before each measurement. Au and Pt were used as internal pressure standards during separate runs. Major diffraction lines observed after laser heating are consistent with those of perovskite structure. We found a mixture of ferric Ca-pv and andradite exists after laser-heating at 21 GPa. Cold compression of andradite to 40 GPa followed by laser-heating resulted in complete transformation to ferric Ca-pv. EBS imagery and EPMA analysis show no evidence of phase separation to hematite and Ca-pv. Thus our results indicate the solubility of ferric iron in Ca-pv is at least 25 mol%, which is accommodated by substitution into both the Ca- and Si- sites. Near 30 GPa a new peak appears, which can not be explained by rigid octahedral tilting in perovskite nor Fe2O3. Splitting of the 200 line becomes distinct at 40 GPa, which is identical to the tetragonal distortion reported for pure Ca-pv. We found that the volume of ferric Ca-pv is identical to pure Ca-pv reported by Mao et al (1989) and Shim et al (2002) within the uncertainty at 42-58 GPa, when the Pt scale is used. We also found that the pressure calculated from the Pt scale (Holmes et al., 1989) is systematically higher than that from the gold scale (Shim et al., 2002 and Heinz et al., 1989). Based on the fact that the 300 K EOS for Au was determined using a combination of static compression and shock-wave data, we believe that Au is a better pressure standard to use than Pt. Because the EOS for Ca-pv is based on the Pt scale, our results suggest the bulk modulus of Ca-pv may be 9-24% lower than previously suggested.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFMMR23A0049L
- Keywords:
-
- 3620 Mineral and crystal chemistry (1042);
- 3630 Experimental mineralogy and petrology;
- 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction