The crystal chemistry and the compressibility of silicate-carbonate minerals: Spurrite, galuskinite and tilleyite

Abstract

a b s t r a c tSpurrite Ca 5 (SiO 4 ) 2 (CO 3 ), galuskinite Ca 7 (SiO 4 ) 3 (CO 3 ) and tilleyite Ca 5 (Si 2 O 7 )(CO 3 ) 2 are three representative minerals formed in high-temperature skarns in the silicate-carbonate system. Their crystal chemistry and compressibility have been investigated using first-principles theoretical simulation. These minerals are structurally described as the combination of interwoven layers constituted by Ca polyhedra and Si polyhedra, with the [CO 3 ] triangles being "separators" to depolymerize the SieCa aggregations. With the effect of pressure, the Si polyhedra and the [CO 3 ] groups present rigid behaviors whereas the CaeO bonds undergo considerable compression. Several pressure-induced abnormities in the lattice parameter variations have been identified, revealing the existence of subtle changes in the compression process. Isothermal equations of state parameters are obtained: (2) Å 3 and K 0 0 ¼ 4.0(1) for tilleyite. These compounds have similar K 0 values to calcite CaCO 3 but are much more compressible than larnite b-Ca 2 SiO 4 . Generally for these minerals, the bulk modulus exhibits a negative correlation with the [CO 3 ] proportion. The structural and compressional properties of silicate-carbonate minerals compared with silicates and carbonates are expected to be a guide for further investigations on Si polyhedra and [CO 3 ] coexistent phases. Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). Please cite this article in press as: Gao, J., et al., The crystal chemistry and the compressibility of silicate-carbonate minerals: Spurrite, galuskinite and tilleyite, Geoscience Frontiers (2015), http://dx.

Structural Behavior of Natural Silicate–Carbonate Spurrite Mineral, Ca5(SiO4)2(CO3), under High-Pressure, High-Temperature Conditions

“…A third-order Birch–Murnaghan EOS was used to fit our pressure–volume data, yielding as characteristic parameters a zero-pressure volume V 0 of 977.9(4) Å 3 , a bulk modulus B 0 of 77(1) GPa, and a first-pressure derivative B 0 ′ of 5.9(2). Notably, these experimental results compare relatively well with those obtained from theoretical calculations: V 0 = 1003.31(4) Å 3 , B 0 = 71.1(1) GPa, and B 0 ′ = 5.4(1) . The isothermal compressibility at RT was also determined up to 6.4 GPa in a second experiment using NaCl as the pressure gauge and pressure-transmitting medium.…”

“…With regard to the values of the β angle, its variation can be considered as almost negligible. It decreases at a pace of −0.0261(9)°/GPa up to 15 GPa and then remains nearly stable at 100.86°, to be compared with the value of −0.0216(11)°/GPa obtained from theoretical calculations . A third-order Birch–Murnaghan EOS was used to fit our pressure–volume data, yielding as characteristic parameters a zero-pressure volume V 0 of 977.9(4) Å 3 , a bulk modulus B 0 of 77(1) GPa, and a first-pressure derivative B 0 ′ of 5.9(2).…”

“…The spurrite structural model presents several analogies with that of the larnite β-Ca 2 SiO 4 orthosilicate, because both atomic arrangements are basically composed of [CaO 7 ], [CaO 8 ], and [SiO 4 ] units and can be described as being formed by layers parallel to a certain crystallographic direction. In spurrite, two kinds of layers on (001) can be distinguished . Under room conditions, every O atom of the Si first coordination sphere has three Ca atoms as near neighbors at distances that range from 2.26 to 2.8 Å, except O3, which has four Ca neighbor atoms.…”

“…Our P – V – T data provide the compressibility and thermal expansivity of this mineral. Moreover, a theoretical study of the crystal chemistry and compressibility of three silicate–carbonate minerals has been recently published, their predictions awaiting experimental confirmation.…”

Post-tilleyite, a dense calcium silicate-carbonate phase

“…The least-squares fit of a second-order Birch−Murnaghan equation of state (BM-EoS) 33 to our experimental (theoretical) pressure-volume data of the initial phase give a zero-pressure unitcell volume, V 0 = 1131(1) Å 3 (1119.5(6) Å 3 ) and a bulk modulus, B 0 = 80(2) GPa (75.3(4) GPa). These values reflect a slightly lower compressibility than those reported by Gao and coworkers: V 0 = 1168.90(2) Å 3 , B 0 = 69.7(3) GPa, and B’ 0 = 4.0(1) from DFT calculations using the PBE prescription 34 . Post-tilleyite is only slightly more incompressible with BM-EoS characteristic parameters: V 0 = 1110(4) Å 3 (1088.7(4) Å 3 ) and B 0 = 83(3) GPa (82.1 (3) GPa), according to experimental (theoretical) results.…”