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

The new mineral gazeevite, BaCa 6 (SiO 4 ) 2 (SO 4 ) 2 O ( R m , a  = 7.1540(1), c  = 25.1242(5) A, V  = 1113.58(3) A 3 , Z  = 3), was found in an altered xenolith in rhyodacites of the 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–brownmillerite and mayenite-supergroup minerals are the main minerals commonly associated with gazeevite. Gazeevite is isostructural with zadovite and aradite; the 1:1 type AB 6 ( T O 4 ) 2 ( T O 4 ) 2 W , occurs together with the structurally related minerals of the nabimusaite series, 3:1 type AB 12 ( T O 4 ) 4 ( T O 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 ]( T O 4 ) 2 } in the structure type of gazeevite–zadovite and triple {[ W 3 B 12 ]( T O 4 ) 4 } layers in arctite–nabimusaite are intercalated with single A ( T O 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 streak and vitreous lustre. Gazeevite is brittle, shows pronounced parting and imperfect cleavage on {001}; it is uniaxial (–), ω = 1.640(3), e = 1.636(2) (λ = 589 nm) and nonpleochroic; Mohs’ hardness is ∼4.5, VHN 50  = 417 kg mm –2 . The calculated density is = 3.39 g cm –3 . The main lines of the calculated powder X-ray diffraction pattern are as follows ( d (A)/ 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 gazeevite are 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.