Nataliakulikite, Ca4Ti2(Fe3+,Fe2+)(Si,Fe3+,Al)O11, a New Perovskite-Supergroup Mineral from Hatrurim Basin, Negev Desert, Israel

Nataliakulikite, Ca4Ti2(Fe3+,Fe2+)(Si,Fe3+,Al)O-11, is a mineral intermediate between perovskite CaTiO3 and brownmillerite Ca-2(Fe,Al)(2)O-5. It was discovered as a minor mineral in a high-temperature pyrometamorphic larnite-gehlenite rock at the Nahal Morag Canyon of the Hatrurim Basin, Israel. Nataliakulikite is associated with larnite, flamite, gehlenite, magnesioferrite, Fe3+-rich perovskite, fluorapatite, barite, Hashemite, and retrograde phases (afwillite, hillebrandite, portlandite, calcite, ettringite, hydrogarnet, and other hydrated Ca-silicates). The mineral forms brown subhedral or prismatic grains (up to 20 mu m) and their intergrowths (up to 50 mu m). Its empirical formula (n = 47) is (Ca3.992Sr0.014U0.004)(Ti1.933Zr0.030Nb0.002) (Fe0.6103+Fe0.4052+Cr0.005Mn0.005)(Si0.447Fe0.3373+Al0.216)O-11 and shows Si predominance in tetrahedral site. The unit-cell parameters (HRTEM data) and space group are: a = 5.254, b = 30.302, c = 5.488 angstrom, V = 873.7 angstrom(3), Pnma, Z = 4. These dimensions and Electron backscatter diffraction (EBSD) data strongly support the structural identity between nataliakulikite and synthetic Ca4Ti2Fe23+O11 (2CaTiO(3).Ca2Fe23+O5), an intermediate compound in the system CaTiO3-Ca2Fe23+O5. In general, this mineral is a Si-Fe2+-rich natural analog of synthetic Ca4Ti2Fe23+O11. The X-ray powder diffraction data (CuK alpha -radiation), calculated from unit-cell dimensions, show the strongest lines {d [angstrom], (I-calc)} at: 2.681(100), 1.898(30), 2.627(26), 2.744(23), 1.894(22), 15.151(19), 1.572(14), 3.795(8). The calculated density is 4.006 g/cm(3). The crystal structure of nataliakulikite has not been refined because of small sizes of grains. The Raman spectrum shows strong bands at 128, 223, 274, 562, and 790 cm(-1). Nataliakulikite from the Hatrurim Basin crystallized under the conditions of combustion metamorphism at high temperatures (1160-1200 degrees C) and low pressures (HT-region of the spurrite-merwinite facies).