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Incommensurately modulated crystal structure of flamite – natural analogue of ‐Ca 2 SiO 4
Author(s) -
Rashchenko Sergey V.,
Seryotkin Yurii V.,
Sokol Ella V.,
Kokh Svetla.
Publication year - 2019
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520619013623
Subject(s) - orthorhombic crystal system , crystallography , crystal structure , crystal twinning , natural bond orbital , crystal (programming language) , crystal chemistry , single crystal , chemistry , materials science , molecule , microstructure , organic chemistry , computer science , programming language
Crystal structures of unquenchable high‐temperature polymorphs of Ca 2 SiO 4 , important in cement chemistry, have eluded single‐crystal X‐ray analysis. However, the problem may be addressed by studying chemically stabilized Ca 2 SiO 4 polymorphs at ambient temperature. Here an incommensurately modulated crystal structure of flamite [ Pnma (0β0)00 s , 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 ‐Ca 2 SiO 4 (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.