A Ca- and Al-rich silicate phase synthesized from glass and yoshiokaite, a Ca- and Al-rich silicate mineral found on the Moon, have compositions which can be described as Ca8-0.5x0.5x, Al16-xSixO32 (2.8 < x < 6.0 for lunar samples). Powder diffraction patterns for these materials are very similar and suggest the same structure. Refinement (R = 0.051) of single-crystal intensity data for synthetic Ca5.32.7, Al10.7Si5.3O32 (P3, a = 9.927, c = 8.220 Å, z = 1, Dcalc = 2.74 g/cm3) gives a structure which is a stuffed derivative of high tridymite with layers having the topology of nepheline. In contrast to nepheline, adjacent layers along c show opposite distortions. Ca occupies the Na site of nepheline, whereas the K site of nepheline is vacant. The observed space group is P3c1, but this is an artifact of twinning; the true space group is P3 (pseudo P3c1) with intimate submicroscopic twinning (twin axis [110]) producing the higher average symmetry. No Si,Al ordering is present, based on the near-equal T-O average distances.

The structure is distinct from that of nepheline and kalsilite, and a complex structural series could exist in Na, K, Ca mixed compositions. The contrasting layer stacking in nepheline and yoshiokaite suggests a structural reason for the very limited substitution of Ca in nepheline. For compositions where sufficient Ca exists to require entry into the K site of nepheline, it is quite likely that the CaAl2O4 monoclinic structure is stabilized.

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