| The equation of states and phase transitions of the clinopyrozenes (i.e., diopsides and hedenbergites) and FeF{dollar}sb2{dollar}, a structural analog of SiO{dollar}sb2{dollar} (stishovite), have been studied to {dollar}sim{dollar}40 GPa and {dollar}sim{dollar}1200{dollar}spcirc{dollar}C, in a diamond anvil cell using X-ray diffraction technique. The experimental results are highlighted as follows:; The isothermal Bulk moduli (K{dollar}sb{lcub}rm T{rcub}){dollar} of FeF{dollar}sb2{dollar}(I) and FeF{dollar}sb2{dollar}(II) at room temperature were determined to be 112(5) GPa and 133(5) GPa, respectively. The K{dollar}sb{lcub}rm T{rcub}{dollar} of both phases decrease with increasing temperature. Thermal expansivities ({dollar}alphasb{lcub}rm v{rcub}{dollar}, deg{dollar}sp{lcub}-1{rcub}{dollar} {dollar}times{dollar} 10{dollar}sp5{dollar}) at 1 atmosphere are determined to be 4.11(7) and 5.173(3) for FeF{dollar}sb2{dollar}(I) and for FeF{dollar}sb2{dollar}(II), respectively and both of these values decrease with increasing pressure. The pressure dependence of {dollar}alphasb{lcub}rm v{rcub}{dollar} in FeF{dollar}sb2{dollar}(II) is approx. 50% less than that in FeF{dollar}sb2{dollar}(I).; The univariant phase boundary of FeF{dollar}sb2{dollar}(I) and FeF{dollar}sb2{dollar}(II) shows a positive slope of 293 deg/GPa. The phase transition hysteresis is found to be independent of temperature. The mechanism of phase transition appears to be a double-stage type involving first the displacive rearrangement of cations and then diffusion of anions.; The natural diopside transforms into cubic perovskite (cpv) + {dollar}gamma{dollar}-spinel (sp) + stishovite (st) at {dollar}sim{dollar}17 GPa, then to cpv + ilmenite (il) at {dollar}sim{dollar}19 GPa, and then to cpv + orthorhombic perovskite (opv) at {dollar}sim{dollar}23 GPa. However, the diopsidic glass becomes crystalline at {dollar}sim{dollar}8 GPa, and then transforms directly to cpv at {dollar}sim{dollar}18 GPa.; The natural hedenbergite (Ca(Fe,Mn)Si{dollar}sb2{dollar}O{dollar}sb6{dollar}) transforms into a Ca-rich garnet (gt) + sp + st at {dollar}sim{dollar}14 GPa, then to gt + wustite (w) + st at {dollar}sim{dollar}18 GPa, and then to cpv + w + st at {dollar}sim{dollar}24 GPa. The synthetic hedenbergite (CaFeSi{dollar}sb2{dollar}O{dollar}sb6{dollar}), however, transforms into cpv + sp + st at {dollar}sim{dollar}16 GPa, and then to cpv + w + st at {dollar}sim{dollar}20 GPa.; These experimental results indicate that in a calcium-rich mantle, the Ca-rich garnet would play a major role at the transition region. |
