The recently accepted new mineral flamite IMA No. 2013–122, (Ca,Na,K)2(Si,P)O4, found in the pyrometamorphic rocks of the Hatrurim Formation, Israel, was reported to crystallize in the hexagonal space group P63 with unit-cell parameters a = 43.3726(18), c = 6.8270(4) Å. A careful re-examination of single crystals from the type locality and holotype material, however, shows that flamite is better described by the formula Ca8−x(Na,K)x(SiO4)4−x(PO4)x and that it crystallizes in the space group Pnm21, a = 9.3845(6), b = 21.7310(14), c = 6.8346(4) Å, V = 1393.81(15) Å3, Z=4 and is isostructural with the known synthetic P-doped clinker phase Ca15□(SiO4)6(PO4)2. The structure is a derivative of the hexagonal high-temperature variety of dicalcium-silicate, α-C2S, and is characterized by a cyclic triplet of individuals rotated 120° around c, interpreted as symplectite-like transformation “twinning” derived from α-C2S. 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, Ca7(SiO4)2(PO4)2, and the high-temperature polymorphs of C2S.

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