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

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.