New minerals with a modular structure derived from hatrurite from the pyrometamorphic Hatrurim Complex. Part II. Zadovite, BaCa6[(SiO4)(PO4)](PO4)(2)F and aradite, BaCa6[(SiO4)(VO4)](VO4)(2)F, from paralavas of the Hatrurim Basin, Negev Desert, Israel

摘要

Zadovite, BaCa6[(SiO4)(PO4)](PO4)(2)F (R (3) over bar m, a = 7.0966(1) angstrom, c = 25.7284(3), V = 1122.13(3) angstrom(3), Z = 3) and aradite, BaCa6[(SiO4)(VO4)] (VO4)(2)F (R (3) over bar m, a = 7.1300(1), c = 26.2033(9) angstrom, V = 1153.63(6) angstrom(3), Z = 3) are two new mineral species of a novel modular structure type related closely to the structure of nabimusaite, KCa12(SiO4)(4)(SO4)(2)O2F. Both minerals occur in paralavas enclosed in pyrometamorphic rocks of the Hatrurim Complex, Negev desert, Israel. Zadovite and aradite are colourless, transparent with a white streak, have a vitreous lustre and an uneven fracture. Both minerals are uniaxial (-) with refractive indices (589 nm) omega = 1.711(2), epsilon = 1.708(2) (zadovite) and omega = 1.784(3), epsilon = 1.780(3) (aradite). The zadovite structure type comprises two tetrahedral sites, which may host a broad compositional range of atoms such as Si, P, V and S. Results of electron microprobe analyses show a correlation between excess Si4+ and S6+ contents, suggesting the substitution scheme 2(P,V)(5+) = Si4+ + S6+ at the tetrahedral sites. This points to the possibility of new minerals isostructural with zadovite with end-member formulae BaCa6(SiO4)(2)[(VO4)(SO4)]F, BaCa6(SiO4)(1.5)[(SO4)(0.5)](PO4)(2)]F and BaCa6[(SiO4)(1.5)(SO4)(0.5)](VO4)(2)F. The Raman spectra of aradite and zadovite reflect the varying PO4 (e.g. change of band intensity at similar to 1031 cm(-1)) and VO4 contents (e.g. change of band intensity at similar to 835 cm(-1)). The presence of SO4 leads to an additional Raman band at similar to 997 cm(-1). The structure of zadovite-series minerals belonging to the nabimusaite group is characterized by a 1:1 alternation of antiperovskite-like {[FCa6](TO4)(2)}(4+) modules and Ba(TO4)(2-)(4) modules.