Goldschmidtite, (K,REE,Sr)(Nb,Cr)O3: A new perovskite supergroup mineral found in diamond from Koffiefontein, South Africa
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Abstract Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O3. A single grain of goldschmidtite with a maximum dimension of ∼100 μm was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ∼170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis (n = 11, WDS, wt%) is: Nb2O5 44.82, TiO2 0.44, ThO2 0.10, Al2O3 0.35, Cr2O3 7.07, La2O3 11.85, Ce2O3 6.18, Fe2O3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K2O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K0.50La0.15Sr0.13Ba0.09Ce0.08)Σ0.95(Nb0.70Cr0.19Fe0.05Al0.01Mg0.04Ti0.01)Σ1.00O3. Goldschmidtite is cubic, space group Pm3m, with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å3, Z = 1, resulting in a calculated density of 5.32(3) g/cm3. Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO3. Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm–1 as well as a pronounced peak at 815 cm–1, which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888–1947), who formalized perovskite crystal chemistry and identified KNbO3 as a perovskite-structured compound.Keywords:
Chromite
Geothermobarometry
Pleochroism
Diopside
Sanidine
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Abstract Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O3. A single grain of goldschmidtite with a maximum dimension of ∼100 μm was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ∼170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis (n = 11, WDS, wt%) is: Nb2O5 44.82, TiO2 0.44, ThO2 0.10, Al2O3 0.35, Cr2O3 7.07, La2O3 11.85, Ce2O3 6.18, Fe2O3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K2O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K0.50La0.15Sr0.13Ba0.09Ce0.08)Σ0.95(Nb0.70Cr0.19Fe0.05Al0.01Mg0.04Ti0.01)Σ1.00O3. Goldschmidtite is cubic, space group Pm3m, with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å3, Z = 1, resulting in a calculated density of 5.32(3) g/cm3. Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO3. Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm–1 as well as a pronounced peak at 815 cm–1, which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888–1947), who formalized perovskite crystal chemistry and identified KNbO3 as a perovskite-structured compound.
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A new heterophyllosilicate mineral schullerite was found in the Lohley basalt quarry in the Eifel volcanic region, Germany, as a member of the late mineral assemblage comprising nepheline, leucite, augite, phlogopite, magnetite, titanite, fresnoite, barytolamprophyllite, fluorapatite, perovskite, and pyrochlore. Flattened brown crystals of schullerite up to 0.5 × 1 × 2 mm in size and their aggregates occur in miarolic cav� ities of alkali basalt. The mineral is brittle, with a Mohs hardness 3-4 and perfect cleavage parallel to (001). Dcalc = 3.974 g/cm 3 . Its IR spectrum is individual and does not contain bands of OH - , or H2O. Schul� lerite is biaxial (-), α = 1.756(3), β = 1.773(4), γ = 1.780(4), 2Vmeas = 40(20)°. Dispersion is weak, r Y > Z, brown to dark brown. Chemical composition (electron microprobe, mean of fivepoint analyses, Fe 2+ /Fe 3+ ratio determined by the Xray emission spectroscopic data, wt %): 3.55 Na 2 O, 0.55 K2O, 3.89 MgO, 2.62 CaO, 1.99 ArO, 28.09 BaO, 3.43 FeO, 8.89 Fe2O3, 1.33 Al2O3, 11.17 TiO2, 2.45 Nb2O5, 26.12 SiO2, 2.12 F, -0.89 -O=F2, 98.98 in total. The empirical formula is (Ba1.68Sr0.18K0.11Na1.05Ca0.43Mn0.47Mg0.88
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Miessiite, a new mineral species of ideal composition Pd11Te2Se2, is cubic, space group Fd 3 m (#227), a 12.448(2) A, V 1929.0 (4) A3, Z = 8. The strongest six lines of the X-ray powder-diffraction pattern [ d in A(I)( hkl )] are: 2.395(80)(511,333), 2.197(100)(440), 1.875(25)(622), 1.555(25)(800), 1.305(25)(931) and 1.271(30)(844). The mineral was discovered in a platinum-group-mineral (PGM) placer sample taken from the Miessijoki river, Lemmenjoki area, Inari Commune, Finnish Lapland, Finland. Associated PGM and other heavy minerals include stillwaterite, isomertieite, mertieite-II, cooperite, braggite, kotulskite, vincentite, tantalite, thorianite, pyrite, magnetite, chromite, and isoferroplatinum with inclusions of laurite and Os–Ir–Ru alloy. Miessiite is black with a metallic luster. The fragments are malleable and silvery grey in color. The average micro-indentation hardness VHN100 is 362 ( n = 4) and corresponds to a Mohs hardness of 2–2½. Under reflected plane-polarized light, the mineral is isotropic with a light grey color in comparison with the associated PGM grains. Measured values of reflectance are tabulated, obtained in air and in oil for a single grain. The reflectance values in air for the standard COM wavelengths are: 48.88 (470 nm), 51.63 (546 nm), 53.91 (589 nm) and 56.82% (650 nm). D (calc) is 10.94 g/cm3 (for the empirical formula and unit-cell parameter refined from powder data). The electron-microprobe analyses yielded (wt.%) Pd 75.17, Se 9.61, Te 17.06, total of 101.84 wt.%, which corresponds to Pd11.02 Te2.09 Se1.90, based 15 atoms. The mineral is named after the locality, the Miessi river (in Saami language, Miessijohka ).
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Microscope study and electron microprobe analysis of lunar rocks and soil show that ilmenite, troilite, and native iron are accompanied by trace amounts of ulvöspinel, titanochromite (new mineral name), an unidentified Ti-Fe oxide, and a complex Zr-Y silicate. The assemblage requires a strongly reducing environment. Textures and modal proportions show that the rocks present are not a differentiation series. The restricted nature of the opaque mineral assemblage suggests a narrow range of composition for the materials from which the parent liquids of the rocks were generated. Textural variety mnust reflect differences in cooling rates, probably related to depths of formation.
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