Incommensurately modulated crystal structure of flamite - natural analogue of αH′-Ca2SiO4

Crystal structures of unquenchable high-temperature polymorphs of Ca2SiO4, important in cement chemistry, have eluded single-crystal X-ray analysis. However, the problem may be addressed by studying chemically stabilized Ca2SiO4 polymorphs at ambient temperature. Here an incommensurately modulated crystal structure of flamite [Pnma(0 beta 0)00s, q = 0.2728 (2)b*, a = 6.8588 (2) angstrom, b = 5.4301 (2) angstrom, c = 9.4052 (3) angstrom] is described. It is a mineral analogue of orthorhombic alpha(H)'-Ca2SiO4 (stable between 1160 and 1425 degrees C), naturally stabilized by substitution with phosphorus. The incommensurate modulation results from wave-like displacement of cation sites accompanied by tilting of (Si,P)O-4 tetrahedra and variation of the Na/(Ca + Na + K) ratio along the modulation period. The studied sample from Hatrurim Basin (Negev Desert, Israel) with composition (Ca1.75K0.12Na0.12)(1.99)(Si0.74P0.26)(1.00)O-4 also demonstrates pseudomerohedral cyclic twinning around the a axis, which results from pseudohexagonal topology of the crystal structure and complicates the indexing of X-ray diffraction data.