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 SieCa aggregations. With the
effect of pressure, the Si polyhedra and the [CO3] 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: K0 ¼ 71.1(1) GPa, V0 ¼1003.31(4) Å3 and K0
0 ¼ 5.4(1)
for spurrite; K0 ¼ 75.0(1) GPa, V0 ¼ 1360.30(7) Å3
, K0
0 ¼ 5.4(1) for galuskinite, and K0 ¼ 69.7(3) GPa,
V0 ¼ 1168.90(2) Å3 and K0
0 ¼ 4.0(1) for tilleyite. These compounds have similar K0 values to calcite CaCO3
but are much more compressible than larnite b-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