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 ¼ 43.3726 (18), c ¼ 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 Ca8Àx(Na, K) x (SiO4) 4Àx (PO4) x and that it crystallizes in the space group Pnm21, a ¼ 9.3845 (6), b ¼ 21.7310 (14), c ¼ 6.8346 (4) A, V ¼ 1393.81 (15) A 3, Z ¼ 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, aC 2S, and is characterized by a cyclic triplet of individuals rotated 120 around c, interpreted as symplectite-like transformation ‘‘twinning’’derived from aC 2S. 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.