The crystal structure of flamite and its relation to Ca2 SiO4 polymorphs and nagelschmidtite

Abstract

The recently accepted new mineral flamite IMA No. 2013–122, (Ca,Na,K) 2 (Si,P)O 4 , found in the pyrometamorphic rocks of the Hatrurim Formation, Israel, was reported to crystallize in the hexagonal space group P 6 3 with unit-cell parameters a = 43.3726(18), c = 6.8270(4) A. A careful re-examination of single crystals from the type locality and holotype material, however, shows that flamite is better described by the formula Ca 8−x (Na,K) x (SiO 4 ) 4−x (PO 4 ) x and that it crystallizes in the space group Pnm 2 1 , a = 9.3845(6), b = 21.7310(14), c = 6.8346(4) A, V = 1393.81(15) A3, Z =4 and is isostructural with the known synthetic P-doped clinker phase Ca 15 □(SiO 4 ) 6 (PO 4 ) 2 . The structure is a derivative of the hexagonal high-temperature variety of dicalcium-silicate, α-C 2 S, and is characterized by a cyclic triplet of individuals rotated 120° around c , interpreted as symplectite-like transformation “twinning” derived from α-C 2 S. The composition of intergrown flamite lamellae is chemically variable concerning the degree of Na, K and P substitution. We present the structure of flamite and discuss its close relationship to nagelschmidtite, Ca 7 (SiO 4 ) 2 (PO 4 ) 2 , and the high-temperature polymorphs of C 2 S.