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 P6 3 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 Ca 8Àx (Na,K) x (SiO 4) 4Àx (PO 4) x and that it crystallizes in the space group Pnm2 1 , 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 Ca 15 &(SiO 4) 6 (PO 4) 2. The structure is a derivative of the hexagonal high-temperature variety of dicalcium-silicate, a-C 2 S, and is characterized by a cyclic triplet of individuals rotated 120 around c, interpreted as symplectite-like transformation ''twinning'' derived from a-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.