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

• We give a systematic investigation on the crystal chemistry and the compressibility of silicate-carbonate minerals.
• Pressure-induced discontinuities in the compressional evolutions are identified.
• The structure and compressibility among silicate-carbonate minerals, carbonates and silicates are discussed.
• Increasing the [CO3]2− proportion will decrease the bulk modulus.

Spurrite Ca5(SiO4)2(CO3), galuskinite Ca7(SiO4)3(CO3) and tilleyite Ca5(Si2O7)(CO3)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 [CO3] triangles being “separators” to depolymerize the Si–Ca aggregations. With the effect of pressure, the Si polyhedra and the [CO3] groups present rigid behaviors whereas the Ca–O 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: K0 = 71.1(1) GPa, V0 = 1003.31(4) Å3 and K0′ = 5.4(1) for spurrite; K0 = 75.0(1) GPa, V0 = 1360.30(7) Å3, K0′ = 5.4(1) for galuskinite, and K0 = 69.7(3) GPa, V0 = 1168.90(2) Å3 and K0′ = 4.0(1) for tilleyite. These compounds have similar K0 values to calcite CaCO3 but are much more compressible than larnite β-Ca2SiO4. Generally for these minerals, the bulk modulus exhibits a negative correlation with the [CO3] 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 [CO3] coexistent phases.

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