New minerals with modular structure derived from hatrurite from the pyrometamorphic rocks. Part IV: Dargaite, BaCa12(SiO4)(4)(SO4)(2)O-3, from Nahal Darga, Palestinian Autonomy

摘要

Dargaite, ideally BaCa12(SiO4)(4)(SO4)(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 similar to 4.5-5.5. The empirical formula is (Ba0.72K0.24Na0.04)(Sigma 1)(Ca11.95Mg0.04Na0.01)(Sigma 12)([SiO4](0.91) [PO4](0.05)[AlO4](0.03)[Ti4+O4](0.01))(Sigma 4)([SO4](0.84)[PO4](0.14)[CO3](0.02))(Sigma 2)(O2.54F0.46)(Sigma 3). Dargaite is trigonal R (3) over barm, the unit-cell parameters are: a = 7.1874(4) angstrom, c = 41.292(3) angstrom, V = 1847.32(19) angstrom(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 similar to 900 degrees C.