Abstract

The new mineral gazeevite, BaCa6(SiO4)2(SO4)2O (Rforumlam, a = 7.1540(1), c = 25.1242(5) Å, V = 1113.58(3) Å3, 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 AB6(TO4)2(TO4)2W, occurs together with the structurally related minerals of the nabimusaite series, 3:1 type AB12(TO4)4(TO4)2W3, where A = Ba, K, Sr…; B = Ca, Na…; T = Si, P, V5+, S6+, Al…; W = O2–, F. Single antiperovskite layers {[WB6](TO4)2} in the structure type of gazeevite–zadovite and triple {[W3B12](TO4)4} layers in arctite–nabimusaite are intercalated with single A(TO4) layers. These minerals with an interrupted antiperovskite structure are characterized by a modular layered structure derived from hatrurite, Ca3(SiO4)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), ε = 1.636(2) (λ = 589 nm) and nonpleochroic; Mohs’ hardness is ∼4.5, VHN50 = 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(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 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.

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