Isothermal compression and stability of perovskite-type CaSiO3.

Abstract

Calcium plays an important role for the phase relations and elasticity of the Earth's mantle because of its high abundance in the Earth. Diopside (CaMgSi20~) is a major host mineral of calcium in the upper mantle. Recent experimental studies clarified that diopside decomposes into an assemblage of CaSiO3 plus MgSiO3 at about 17 GPa.1>,2) CaSiO3 crystallizes in a cubic perovskite structure and a solubility of Mg into CaSiO3 is very limited. These experimental results imply that perovskite-type CaSi0 ,3 may be an important constituent mineral in the lower mantle. Its physical and chemical properties are, however, not well known yet. The purpose of the present study is to clarify the stability and equation of state of perovskite-type CaSi0 ,3 under the conditions of the lower mantle through high pressure in situ X-ray diffraction study.The starting material was prepared from a mixture of CaC03 and Si02 which was melted at 1580°C in air and then annealed at 1400°C for 47 hours. The product was examined by X-ray diffraction and was confirmed to be CaSiO3 with the pseudo-wollastonite structure. It was then ground into a fine powder and a small amount (a few percent by weight) of platinum black, which works as an absorber of the YAG laser for heating, was mixed with it. A lever and spring type diamond anvil apparatus3> was used throughout the high pressure experiments. The anvils were beveled, one-quater carat, brilliant-cut diamonds. The culet and central flat size were 0.5 mm and 0.25 mm, respectively, and the bevel angle was 7°. Molybdenum Kc radiation from a high power X-ray source (operated at 55 kV and 160 mA) was collimated by a 60 µm pin-hole collimator and the diffracted X-ray pattern was recorded on a film with a radius of approximately 57.3 mm. Accurate sample to film distance was determined by measuring diffractions from a standard material (e.g. silver) which was stuck on the surface of the diamond anvil. The typical exposure time was 60 to 80 hours. In order to accelerate a possible phase transformation or a decomposition reaction, the sample was heated above 1000°C by a YAG laser after each pressure increment. A thin laser beam (approximately 15 µm in diameter) from the 12 W single mode YAG laser was scanned to heat the whole area of the sample and the total heating duration was about 30 min. After the heating, X-ray measurements were made at room temperature. The pressure was measured using ruby fluorescence scale.4~ The powdered sample was compressed directly, and therefore a large pressure gradient was observed in the sample chamber. Many fine grains of ruby powder were spread over the entire region of the sample and the measured values in the area exposed to the X-ray (70 µm in diameter) were averaged to :k ) Tokyo

The stability and P–V–T equation of state of CaSiO3 perovskite in the Earth's lower mantle

“…. As dscussd above, the compression curve of earlier studies[Tamai and Yagi, 1989;Yagi et al, 1989;Mao et al, 1989; Tartida and Richer, 1989] are essentially the same as that ofWang et al [1996] at large compressions. A significantly different result was obtained for the fitted parameters in this case, especially for q: '7 = 2.06 q-0.05 and q = 2.6 + 0.3.…”

“…Since CaSiO3 perovskite is unstable at ambient P-T conditions, there is no direct measurement of V0 and no elasticity measurements at ambient conditions. DAC measurements have been performed to 130 GPa at 300 K [ Tamai and Yagi, 1989;Yagi et al, 1989;Tarrida and Richer, 1989;Mao et al, 1989]. Fitting these data to (8) yields KT0 = 280 GPa, assuming K•, 0 = 4.…”

On the structure and compressibility of CaSiO3 perovskite

“…Typical lower‐mantle geochemical models contain a few weight percent of CaO [ McDonough and Sun , 1995], some of which is likely to be found in the form of CaSiO 3 perovskite. Early room temperature experiments indicated a cubic perovskite structure with Pm3m symmetry and a high bulk modulus ( K 0 ≥ 275 GPa) [ Mao et al , 1989; Tamai and Yagi , 1989; Tarrida and Richet , 1989; Yagi et al , 1989]. Results of theoretical calculations based on rigid‐ion or first‐principles models were consistent with experimental values of density and bulk modulus, but phonon spectra suggested non‐cubic symmetry for the static structure at high pressures [ Wolf and Jeanloz , 1985; Wolf and Bukowinski , 1987; Hemley et al , 1987; Sherman , 1993].…”

Structure change of Ca1−xSrxTiO3perovskite with composition and pressure

“…The equation of state (EOS) of CaSiO 3 perovskite was determined by diamond anvil cell (DAC) experiments with YAG laser heating up to 134 GPa (Mao et al 1989). The EOS, thermal expansion, and elastic properties of CaSiO 3 have also been determined with DAC or multianvil presses (Yagi et al 1989;Wang et al 1996;Shim et al 2000aShim et al , 2000b.…”