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(0β0)00s, q = 0.2728 (2)b∗, a = 6.8588 (2) Å, b = 5.4301 (2) Å, c = 9.4052 (3) Å] is described. It is a mineral analogue of orthorhombic -Ca2SiO4 (stable between 1160 and 1425°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 (Ca 1.75 K 0.12 Na 0.12) 1.99 (Si 0.74 P 0.26) 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.
|Журнал||Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials|
|Состояние||Опубликовано - дек. 2019|