High‐pressure Raman spectra of tuite, γ‐Ca₃(PO₄)₂

Abstract

The Raman spectra of a synthetic tuite, γ -Ca 3 (PO 4 ) 2 , have been measured up to 23.6 GPa using a diamond-anvil cell at room temperature. The data were collected during compression and decompression. No pressure-induced phase transformation was observed in this pressure range. The behaviors of the phosphate modes and the external modes under high pressures were analyzed. The Raman wavenumbers of all modes increased linearly and continuously with pressure. The pressure coefficients of the phosphate modes range from 0.0034 to 0.0049 GPa −1 for ν 3 , 0.0044 GPa −1 for ν 1 , from 0.0020 to 0.0033 GPa −1 for ν 4 , and 0.0037 GPa −1 for ν 2 , whereas the pressure coefficients of the external modes range from 0.0120 to 0.0138 GPa −1 . The corresponding Grüneisen parameters are 0.341-0.491, 0.441, 0.200-0.331 and 0.371 for the phosphate modes ν 3 , ν 1 , ν 4 and ν 2 , respectively, and from 1.20 to 1.38 for the external modes.

Compressibility of carbonophosphate bradleyite Na3Mg(CO3)(PO4) by X-ray diffraction and Raman spectroscopy

“…Santillán and Williams (2004) studied the asymmetric stretching vibration and bending vibrations of [CO 3 ] 2− units in siderite FeCO 3 and rhodochrosite MnCO 3 , and also determined the approximate values (0.1-0.4). The average γ value of the internal modes of [PO 4 ] 3− groups in tuite has been obtained to be 0.36 (Zhai et al 2010), while in fluorapatite to be 0.44 (Comodi et al 2001a). These low γ values are compatible with the incompressibility of both anion groups.…”

“…At ambient conditions, Raman spectrum shows fifteen peaks in the range 150-1,200 cm −1 (Fig. 2), where two strongest symmetrically stretching vibration (A g ) peaks (970.8 and 1,078.7 cm −1 ) are, respectively, assigned to the [PO 4 ] 3− and [CO 3 ] 2− groups according to the results in previous reports Williams et al 1993;Santillán and Williams 2004;Zhai et al 2010). At high pressure, most Raman peaks become obscure due to the strong background from diamond anvils.…”

“…Most reports have demonstrated that the Raman modes of [PO 4 ] 3− and [CO 3 ] 2− groups, respectively, in phosphates or carbonates would shift to higher frequencies with pressure, and this is also observed in bradleyite. The pressure dependence of [PO 4 ] 3− mode shifts in bradleyite is calculated to be ~3.54(6) × 10 −3 cm −1 /GPa below ~16 GPa, which is a little lower than 4.1 × 10 −3 in tuite γ-Ca 3 (PO 4 ) 2 (Zhai et al 2010), and 4.8 × 10 −3 in fluorapatite Ca 5 (PO 4 ) 3 F (Comodi et al 2001a). The symmetrical stretching vibration is structurally related to the P-O bond compressibility; therefore, the lower value indicates that the P-O bond in bradleyite is more incompressible than that in tuite/fluorapatitie.…”

A comparison of the Ca3(PO4)2 and CaSiO3 systems, with a new structure refinement of tuite synthesized at 15 GPa and 1300 C

“…Its crystal structure was determined by Sugiyama and Tokonami (1987) using the sample made from hydroxylapatite at 12 GPa and 2300 °C. Recently, Zhai et al (2009Zhai et al ( , 2010Zhai et al ( , 2011 have also obtained tuite through conversion of β-Ca 3 (PO 4 ) 2 at 7 GPa and 1200 °C to measure its high-pressure and high-temperature properties with Raman spectroscopy and powder X-ray diffraction. Natural tuite was originally described by Chen et al (1995) as a high-pressure polymorph of Ca-phosphate, Ca 5 (PO 4 ) 3 Cl, in shock-induced melt veins of the Sixiangkou L6 chondrite, which contains 3.72 wt% Cl, as compared to 4.84 wt% Cl in the other Ca-phosphate, chlorapatite.…”

“…Among them, the α′-, α-, and β-phases are stable in the different temperature ranges, from 1470 to 1756, 1120 to1470, and room temperature to 1120 °C, respectively (Nurse et al 1959;Fix et al 1969;Famery et al 1994;Knowles et al 1999;Yashima and Sakai 2003), whereas the γ-phase, also known as tuite, is a high-pressure form and stable at least in the upper mantle conditions (Murayama et al 1986;Xie et al 2003). The Ca 3 (PO 4 ) 2 system has been a subject of numerous investigations because α-and β-phases are important bioceramic materials in surgery and dentistry (e.g., Famery et al 1994 and references therein) and tuite is a potential host for rareearth elements (REE) and large lithophile elements (LLE) under the Earth's mantle conditions (Murayama et al 1986;Sugiyama and Tokonami 1987;Xie et al 2003;Zhai et al 2010Zhai et al , 2011.…”