New minerals with modular structure derived from hatrurite from the pyrometamorphic rocks. Part IV: Dargaite, BaCa₁₂(SiO₄)₄(SO₄)₂O₃, from Nahal Darga, Palestinian Autonomy

Abstract

Abstract Dargaite, ideally BaCa 12 (SiO 4 ) 4 (SO 4 ) 2 O 3 , is an additional member of the arctite group belonging to minerals with a modular intercalated antiperovskite structure derived from hatrurite. The holotype specimen was found at a small outcrop of larnite pseudoconglomerates in the Judean Mts, West Bank, Palestinian Autonomy. Larnite, fluorellestadite–fluorapatite, brownmillerite, fluormayenite–fluorkyuygenite and ye'elimite are the main minerals of the holotype specimen; ternesite, shulamitite and periclase are noted rarely. Dargaite, nabimusaite and gazeevite occur in linear zones with higher porosity within larnite rocks. Pores are filled with ettringite and Ca-hydrosilicates, less commonly with gibbsite, brucite, baryte, katoite and calciolangbeinite. Dargaite is colourless, transparent with a white streak and has a vitreous lustre. It exhibits pronounced parting and imperfect cleavage along (001). Mohs’ hardness is ~4.5–5.5. The empirical formula is (Ba 0.72 K 0.24 Na 0.04 ) Σ1 (Ca 11.95 Mg 0.04 Na 0.01 ) Σ12 ([SiO 4 ] 0.91 [PO 4 ] 0.05 [AlO 4 ] 0.03 [Ti 4+ O 4 ] 0.01 ) Σ4 ([SO 4 ] 0.84 [PO 4 ] 0.14 [CO 3 ] 0.02 ) Σ2 (O 2.54 F 0.46 ) Σ3 . Dargaite is trigonal R $\overline 3 $ m , the unit-cell parameters are: a = 7.1874(4) Å, c = 41.292(3) Å, V = 1847.32(19) Å 3 and Z = 3. The crystal structure of dargaite was refined from X-ray single-crystal data to R 1 = 3.79%. The calculated density is 3.235 g cm –3 . The following main Raman bands are distinguished on the holotype dargaite (cm –1 ): 122, 263, 323, 464, 523, 563, 641 and 644, 829 and 869, 947, 991 and 1116. The formation conditions of dargaite are linked to the local occurrence of pyrometamorphic by-products (gases, fluids and melts) transforming earlier mineral associations at ~900°C.