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

Abstract

Abstract Zadovite, BaCa 6 [(SiO 4 )(PO 4 )](PO 4 ) 2 F ( R 3 m , a = 7.0966(1) Å, c = 25.7284(3), V = 1122.13(3) Å 3 , Z = 3) and aradite, BaCa 6 [(SiO 4 )(VO 4 )] (VO 4 ) 2 F ( R 3 m , a = 7.1300(1), c = 26.2033(9) Å, V = 1153.63(6) Å 3 , Z = 3) are two new mineral species of a novel modular structure type related closely to the structure of nabimusaite, KCa 12 (SiO 4 ) 4 (SO 4 ) 2 O 2 F. 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) ω = 1.711(2), ε = 1.708(2) (zadovite) and ω = 1.784(3), ε = 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 Si 4+ and S 6+ contents, suggesting the substitution scheme 2(P,V) 5+ = Si 4+ + S 6+ at the tetrahedral sites. This points to the possibility of new minerals isostructural with zadovite with end-member formulae BaCa 6 (SiO 4 ) 2 [(PO 4 )(SO 4 )]F, BaCa 6 (SiO 4 ) 2 [(VO 4 )(SO 4 )]F, BaCa 6 [(SiO 4 ) 1.5 (SO 4 ) 0.5 ](PO 4 ) 2 F and BaCa 6 [(SiO 4 ) 1.5 (SO 4 ) 0.5 ](VO 4 ) 2 F. The Raman spectra of aradite and zadovite reflect the varying PO 4 (e.g. change of band intensity at ∼1031 cm –1 ) and VO 4 contents (e.g. change of band intensity at ∼835 cm –1 ). The presence of SO4 leads to an additional Raman band at ∼997 cm –1 . The structure of zadovite-series minerals belonging to the nabimusaite group is characterized by a 1:1 alternation of antiperovskite-like {[FCa 6 ]( T O 4 ) 2 } 4+ modules and Ba(TO 4 ) 2– 4 modules.