ALBERT

Description: An experimental study of an Al-rich schorl sample from Cruzeiro mine (Minas Gerais, Brazil) was carried out using electron microprobe analysis, structural refinement and Mössbauer, infrared and optical absorption spectroscopy in order to explore the disordering of Fe 2+ over the Y and Z sites of the tourmaline structure. A structural formula was obtained by merging chemical and structural data. The cation distribution at the two non-equivalent octahedrally coordinated sites ( Y and Z ) was obtained by two different optimization procedures which, minimizing the residuals between observed and calculated data, converged to the formula: X (Na 0.65 0.32 Ca 0.02 K 0.01 ) 1.00 Y ( $${\mathrm{Fe}}_{1.65}^{2+}$$ Al 1.15 $${\mathrm{Fe}}_{0.06}^{3+}$$ $${\mathrm{Mn}}_{0.05}^{2+}$$ Zn 0.05 $${\mathrm{Ti}}_{0.04}^{4+}$$ ) 3.00 Z (Al 5.52 $${\mathrm{Fe}}_{0.30}^{2+}$$ Mg 0.18 ) 6.00 [ T (Si 5.87 Al 0.13 ) 6.00 O 18 ]( B BO 3 ) 3 V (OH) 3 W [(OH) 0.34 F 0.28 O 0.38 ] 1.00 . This result shows a partial disordering of Fe 2+ over the Y and Z sites which explains adequately the mean atomic number observed for the Z site (13.5±0.1). Such a disordering is also in line with the shoulder recorded in the Mössbauer spectrum (fitted by a doublet with isomer shift of 1.00 mm/s and quadrupole splitting of 1.38 mm/s) as well as with the asymmetric bands recorded in the optical absorption spectrum at ~9000 and 14,500 cm –1 (modelled by four Gaussian bands, centred at 7677 and 9418 cm –1 , and 13,154 and 14,994 cm –1 , respectively). The high degree of consistency in the results obtained using the different methods suggests that the controversy over Fe 2+ order can be ascribed to the failure to detect small amounts of Fe 2+ at Z (typically 〈〈10% atoms/site) rather than a steric effect of Fe 2+ on the tourmaline structure.

Description: The crystal structure of hohmannite, $${\mathrm{Fe}}_{2}^{3+}$$ [O(SO 4 ) 2 ]·8H 2 O, was studied by means of single-crystal X-ray diffraction (XRD) and vibrational spectroscopy. The previous structural model was confirmed, though new diffraction data allowed the hydrogen-bond system to be described in greater and more accurate detail. Ab initio calculations were performed in order to determine accurate H positions and to support the experimental model obtained from XRD data. Infrared and Raman spectra are presented for the first time for this compound and comments are made on the basis of the crystal structure and the known literature for sulfate minerals.

Description: Kleberite, ideally Fe 3+ Ti 6 O 11 (OH) 5 , is a new mineral (IMA 2012-023) from Tertiary sands at Königshain, Saxony, northeast Germany. It is also found in heavy mineral sands from the Murray Basin, southeast Australia and at Kalimantan, Indonesia. It occurs as rounded anhedral to euhedral translucent grains, 0.04–0.3 mm across, which are generally red-brown, but grade to orange with decreasing iron content. Associated minerals include ilmenite, pseudorutile, ‘leucoxene’, tourmaline and spinel. The density measured by pycnometry is 3.28 g cm –3 , which is lower than the calculated density of 3.91 g cm –3 , due to intragrain porosity which is not penetrated by the immersion fluid. The intragrain pores, of median diameter 18 nm, are partially filled with impurity phases including kaolinite, diaspore and quartz. Kleberite grains have a uniaxial (–) character, but localized regions are weakly biaxial (–) with 2V close to zero. The mean refractive index, calculated from reflectance measurements, is 2.16(3). The mean empirical formula from electron-microprobe analyses of 15 Königshain kleberite grains is $${\mathrm{Fe}}_{1.01}^{3+}$$ Mg 0.06 Ti 6 O 11.2 (OH) 4.8 [Al 0.59 Si 0.31 P 0.04 O 1.60 ·1.8H 2 O], where the formula element in square brackets represents impurities in the pores. Kleberite forms over a compositional range with [Ti]/[Fe + Ti] atomic ratios from 0.8–0.9. It has monoclinic symmetry, P 2 1 / c , with a = 7.537(1), b = 4.5795(4), c = 9.885(1) Å, β = 131.02(1)°. The six strongest lines in the powder X-ray diffraction (XRD) pattern [listed as d in Å (I)] are as follows: 1.676(100), 2.170(82), 2.466(27), 1.423(22), 3.933(8), 2.764(9). The structure was refined by the Rietveld method on powder XRD data to R p = 6.3, R wp = 8.1, R B = 4.0. Kleberite is isostructural with tivanite; their structural formulae are [ $${\mathrm{Ti}}_{3}^{4+}$$ ][ $${\mathrm{Ti}}_{3}^{4+}$$ Fe 3+ ]O 11 (OH) 5 and [ $${\mathrm{Ti}}_{4}^{4+}$$ ][ $${\mathrm{V}}_{4}^{3+}$$ ]O 12 (OH) 4 , respectively. Kleberite has dominant Ti 4+ in place of V 3+ in the M(2) metal-atom site. The related mineral pseudorutile, [Ti 4 ][(Fe 3+ ,Ti) 4 ](O,OH) 16 , with Fe 3+ 〉 Ti 4+ has dominant Fe 3+ in this site. Kleberite grains from different localities commonly contain residual MgO-rich ferrian ilmenite. The chemical and physical relationships between the ilmenite and coexisting kleberite are used to evaluate different alteration mechanisms involving selective leaching of divalent oxides from ilmenite and pseudomorphic replacement reactions, respectively.

Description: In order to clarify details of volatiles incorporation in arrojadites, two samples previously characterized by X-ray diffraction and electron-microprobe (EMP) and LA-ICPMS analysis were investigated by single-crystal FTIR spectroscopy. The present study confirms and makes more quantitative previous results by single-crystal structure refinement about the presence and orientation of three OH – groups (one with partial occupancy) in the arrojadite structure. The FTIR spectra showed the presence of NH 4 + in arrojadite from Yukon, which was confirmed by EMP analysis ( ca . 700 ppm N). Micro FTIR imaging was used to check the homogeneity of OH and NH 4 + across the examined samples, as a prerequisite for the single-crystal polarized study. Based on the hydrogen bonding environment determined by structure refinement, the bands observed in the 3600–3500 cm –1 region can be assigned to the OH1 (at lower frequency) and OH2 (at higher frequency) dipoles. In the spectrum of arrojadite from Yukon several hyperfine components are resolved; these can be assigned to local Mg/ Fe 2+ octahedral configurations at the OH-coordinated octahedra. Both spectra show a very broad absorption extending from ~ 3500 to 2500 cm –1 which is assigned to the OH3 hydroxyl group. The orientation of the O-H dipoles calculated from the FTIR absorbance data are in excellent agreement with those calculated from the refined atomic coordinates, confirming the validity of the latter method also in the case of low-Z elements. Assuming the water content derived from EMP analysis, an integrated molar coefficient (polarized data) I = 63937 l/(mol·cm 2 ) is calculated for the spectroscopic quantification of H 2 O in arrojadite. For N, based on the EMP analysis, we obtain (for unpolarized data in the N-O bending region) i = 11000 ± 2000 l/(mol·cm 2 ) and I = 300 ± 60 l/(mol·cm 2 ) from integrated and linear intensity data, respectively.

Description: The crystal chemistry of wavellite from Zbirov (Czech Republic), ideally Al 3 (PO 4 ) 2 (OH,F) 3 ·5H 2 O, was addressed by means of a multi-methodological approach based on electron microprobe analysis (EMPA) using wave-dispersive spectroscopy, single-crystal X-ray diffraction, powder and single-crystal infrared spectroscopy and Raman spectroscopy. The EMPA data showed the presence of significant F replacing OH in the sample studied. The structure was solved in the Pcmn orthorhombic space group, with the following unit-cell constants: a = 9.6422(7), b = 17.4146(15), c = 7.0094(2) Å, V = 1176.98(10) Å 3 . Phosphorus atoms display tetrahedral (PO 4 ) coordination, while Al cations display octahedral coordination. The mineral framework can be viewed as the repetition of cationic arrays made up of AlO 6 polyhedra, bridged by PO 4 groups and further joined by O–H···O hydrogen bonds. The single-crystal unpolarized Fourier transform infrared (FTIR) spectrum shows combination bands indicating the presence of both OH and H 2 O in the structure. Both FTIR and Raman spectra show a broad absorption extending from 3600 to 2800 cm –1 resulting from the overlapping of several components due to the water molecules and the OH group. The frequencies observed are comparable to those expected on the basis of the Libowitzky relationship for the range of D –H··· A bond systems in the structure.

Description: The crystal chemistry of sekaninaite from Dolní Bory, Czech Republic, was characterized by a multi-method approach. Particular emphasis was put on the characterization of the channel constituents (i.e. H 2 O and CO 2 ). Electron microprobe analysis shows the sample to be close to the Fe endmember [ X Fe = Fe/(Fe+Mg) = 94%) with significant Mn (1.48 wt.%) present; laser ablation mass-spectrometry showed the presence of 0.42 wt.% Li 2 O. H 2 O and CO 2 contents (1.48 and 0.17 wt.%, respectively) were determined via secondary-ion mass-spectrometry. Sample homogeneity was checked by Fourier-transform infrared (FTIR) imaging using a microscope equipped with a focal plane array detector. Single-crystal FTIR spectroscopy confirmed the presence of two types of H 2 O groups in different orientations (with prevalence of the type II orientation), and that CO 2 is oriented preferentially normal to the crystallographic c axis. Using the Beer-Lambert relation, integrated molar coefficients, i , were calculated for both types of H 2 O ( i H 2 O [I] = 6000±2000; i H 2 O [II] = 13000±1000) and for CO 2 ( i CO 2 = 2000±1000).

Description: The new mineral pharmazincite, KZnAsO 4 , was found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is closely associated with shchurovskyite, dmisokolovite, bradaczekite, arsmirandite, tilasite, johillerite, tenorite, hematite, aphthitalite and As-bearing orthoclase. Pharmazincite occurs as prismatic to acicular crystals up to 1 mm long and up to 0.03 mm thick typically combined in near-parallel, radial or chaotic intergrowths, open-work aggregates or crusts up to 2 mm across. Pharmazincite is colourless to white, transparent, with a vitreous lustre. It is brittle, with a stepped fracture and a perfect cleavage parallel to [001]. D calc is 4.75 g cm –3 . Pharmazincite is optically uniaxial (–), = 1.649(2), = 1.642(2). The Raman spectrum is reported. The chemical composition (wt.%, electron-microprobe data) is: K 2 O 18.98, CaO 0.14, MgO 1.20, CuO 4.41, ZnO 27.58, Fe 2 O 3 0.15, P 2 O 5 0.50, As 2 O 5 46.67, total 99.63. The empirical formula, calculated based on 4 O apfu, is: (K 0.97 Ca 0.01 ) 0.98 (Zn 0.82 Cu 0.13 Mg 0.07 ) 1.03 (As 0.98 P 0.02 ) 1.00 O 4 . The strongest reflections of the powder X-ray diffraction pattern [ d ,Å( I )( hkl )] are: 6.36(28)(111), 4.64(45)(220), 4.35(48)(002), 3.260(36)(411), 3.179(100)(222), 2.770(26)(113), 2.676(77)(600), 2.278(15)(602) and 1.710(15)(713, 115). Pharmazincite is hexagonal, a = 18.501(4), c = 8.7114(9) Å, V = 2582.4(8) Å 3 and Z = 24 (single-crystal XRD data). Its space group is P 6 3 , by analogy with synthetic KZnAsO 4 that has a crystal structure based upon a tetrahedral tridymite-type {ZnAsO 4 } – framework. It is isostructural with megakalsilite KAlSiO 4 . The new mineral is named for its chemical constituents.

Description: This paper presents a microbeam (electron microprobe, Raman spectroscopic and X-ray microdiffraction) study of cancrinite-group minerals of relevance to alkaline igneous rocks. A solid solution is known to exist between cancrinite and vishnevite with the principal substitutions being by and Ca for Na. In the present study, several intermediate members of the cancrinite–vishnevite series from a syenitic intrusion at Cinder Lake (Manitoba, Canada), were used to examine how chemical variations in this series affect their spectroscopic and structural characteristics. The Cinder Lake samples deviate from the ideal cancrinite–vishnevite binary owing to the presence of cation vacancies. The only substituent elements detectable by electron microprobe are K, Sr and Fe (0.03–0.70, 0–0.85 and 0–0.45 wt.% respective oxides). The following Raman bands are present in the spectra of these minerals: ~631 cm –1 and ~984–986 cm –1 [ vibration modes]; ~720–774 cm –1 and ~1045–1060 cm –1 [ vibration modes]; and ~3540 cm –1 and 3591 cm –1 [H 2 O vibration modes]. Our study shows a clear relationship between the chemical composition and Raman characteristics of intermediate members of the cancrinite–vishnevite series, especially with regard to stretching modes of the and anions. From cancrinite-poor (Ccn 6.5 ) to cancrinite-dominant (Ccn 91.3 ) compositions, the vibration modes disappear from the Raman spectrum, giving way to modes. X-ray microdiffraction results show a decrease in unit-cell parameters towards cancrinite-dominant compositions: a = 12.664 (1) Å, c = 5.173(1) Å for vishnevite (Ccn 22 ); a = 12.613 (1) Å, c = 5.132(1) Å for cancrinite (Ccn 71 ). Our results demonstrate that Raman spectroscopy and X-ray microdiffraction are effective for in situ identification of microscopic grains of cancrinite–vishnevite where other methods (e.g. infrared spectroscopy) are inapplicable. The petrogenetic implications of cancrinite–vishnevite relations for tracing early- to late-stage evolution of alkaline magmas are discussed.