New minerals with a modular structure derived from hatrurite from the pyrometamorphic rocks. Part III. Gazeevite, BaCa₆(SiO₄)₂(SO₄)₂O, from Israel and the Palestine Autonomy, South Levant, and from South Ossetia, Greater Caucasus

Abstract The new mineral gazeevite, BaCa 6 (SiO 4 ) 2 (SO 4 ) 2 O ( R 3 m , a = 7.1540(1), c = 25.1242(5) Å, V = 1113.58(3) Å 3 , Z = 3), was found in an altered xenolith in rhyodacites ofthe Shadil-Khokh volcano, Southern Ossetia and at three localities in larnite pyrometamorphic rocks of the Hatrurim Complex; Nahal Darga and Jabel Harmun, Judean Mountains, Palestinian Autonomy, and Har Parsa, Negev Desert, Israel. Larnite, fluorellestadite–fluorapatite, srebrodolskite–brownmilleriteand mayenite-supergroup minerals are the main minerals commonly associated with gazeevite. Gazeevite is isostructural with zadovite and aradite; the 1:1 type AB 6 (TO 4 ) 2 ( TO 4 ) 2 W , occurs together with the structurallyrelated minerals of the nabimusaite series, 3:1 type AB 12 ( TO 4 ) 4 ( TO 4 ) 2 W 3 , where A = Ba, K, Sr...; B = Ca, Na...; T = Si, P, V 5+ , S 6+ , Al...; W = O 2– ,F – . Single antiperovskite layers {[ WB 6 ]( TO 4 ) 2 } in the structure type of gazeevite–zadovite and triple {[ W 3 B 12 ]( TO 4 ) 4 } layers in arctite–nabimusaite areintercalated with single A ( TO 4 ) layers. These minerals with an interrupted antiperovskite structure are characterized by a modular layered structure derived from hatrurite, Ca 3 (SiO 4 )O. Gazeevite is colourless, transparent, with a white streakand vitreous lustre. Gazeevite is brittle, shows pronounced parting and imperfect cleavage on {001}; it is uniaxial (–), ω = 1.640(3), ε = 1.636(2) (λ = 589 nm) and nonpleochroic; Mohs' hardness is ∼4.5, VHN50 = 417 kg mm –2 . The calculateddensity is = 3.39 g cm –3 . The main lines of the calculated powder X-ray diffraction pattern are as follows ( d (Å)/ I / hkl ): 3.58/100/110, 3.07/91/021, 2.76/47/116, 1.789/73/220, 3.29/60/113, 2.78/36/024, 2.12/25/125, 2.21/21/208. Raman spectra of gazeeviteare compared with spectra of other minerals. The formation of gazeevite and minerals of the nabimusaite–dargaite series is connected with high-temperature alteration of an early assemblage of clinker minerals affected by later fluids generated by volcanic activity or combustion processes.