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Chemical and structural variability in cubic spinel oxides
Full text linkThe empirical relations between cubic spinel oxides of different compositions were investigated using data from 349 refined crystal structures. The results show that the spinel structure is able to tolerate many constituents (at least 36) by enlarging and decreasing the tetrahedra and octahedra. This is reflected in a large variation in tetrahedral and octahedral bond distances. The oxygen positional parameter (u) may be regarded as a measure of the distortion of the spinel structure from cubic close packing or of the angular distortion of the octahedron. The distortion can best be explained in terms of ionic potential (IP), which merges the size and charge properties of an ion. Sterically induced distortion depends on ion size, whereas electrostatically induced distortion is caused by cation–cation repulsion across faces of tetrahedra and shared edges of octahedra. The strong correlations between the u parameter and the IP at the T and M sites are consistent with the main role played by the both charge and size. Large distortions (u ≫ 0.27) result in oxygen–oxygen distances of the octahedron shorter than 2.50 Å, which would lead to structural instability because of increased non-bonded repulsion forces between the oxygen atoms
Tests of a scintillating-fibre detector using position-sensitive photomultiplier readout
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Crystal chemical characterisation of red Beryl by ‘standardless’ laser-induced breakdown spectroscopy and single-crystal refinement by X-Ray diffraction. An example of validation of an innovative method for the chemical analysis of minerals
No full textLaser-induced breakdown spectroscopy (LIBS) is a valuable technique for performing qualitative and quantitative chemical determinations of all elements in one shot, including low atomic number elements such as Li and Be. This technique does not require any sample preparation to reveal the atomic species, even when present in trace amounts (< 0.01% m/m). In this study, for the first time, we provide an accurate mineral formula for a Cs-rich red beryl by combining crystallographic data obtained using the traditional single-crystal X-ray diffraction technique and quantitative chemical data obtained with an innovative ‘standardless’ method: Calibration-free-LIBS (CF-LIBS). In particular, a new LIBS prototype coupled with a petrographic microscope (CF-μLIBS) was used to analyse chemically homogeneous areas of about 10 μm spot size, causing minimal damage to the mineral. The results showed that calibration-free quantitative analysis is suitable for the quantification of major and minor low and high atomic number elements in beryl. The accuracy of quantification of low atomic number elements by CF-μLIBS led to the empirical formula: [12](Cs0.006Na0.019K0.017Ca0.019)Σ0.061[4](Be2.989Li0.011)Σ3.000[6](Ti0.053Mn0.051Mg0.007Al1.890)Σ2.000[4](Be0.116Fe0.024Si5.860)Σ6.000 O18. This formula is consistent with the crystal-structure refinement data and demonstrates the validity of CF-μLIBS for chemical analyses of minerals containing low atomic number elements
A first report on anion vacancies in a defect MgAl2O4 natural spinel
No full textThe chemical and structural features of a natural spinel sensu stricto (s.s.) sample were
studied by a multi-analytical approach, including electron microprobe analysis (EMP), Fourier
transform infrared spectroscopy (FTIR), and single crystal X-ray diffraction structural
refinement (SREF). The sample, coming from impure dolomitic marbles of Pegu (Myanmar),
has an anomalous chemistry with an Mg-content exceeding that of the ideal formula. In
addition, a chemical zoning along a line-scan of EMP analyses was observed, with Mg and
Al amounts showing opposite trends. The comparatively high and low concentrations,
respectively, of divalent and trivalent cations lead to a deficit of positive charges. Thus, the
requirement of neutrality of global charges for crystal structures appears to be violated, in this
case. The possible reasons accounting for the anomalous chemistry are discussed. Based on
the combined EMP, FTIR and SREF results, it is concluded that anion vacancies can
adequately compensate for the observed deficit of positive charges. Thus, the analysed sample
is a defect spinel. This is the first report of anion vacancies in a natural spinel s.s. With
reference to the ideal formula MgAl2O4, the formation of anion vacancies, coupled to an
excess of Mg and a deficiency of Al, may be described by the substitution mechanism
2Mg2++V□→2Al3++O2–, where V□ represents an oxygen vacancy
Nomenclature and classification of the spinel supergroup
Full text linkA new, IMA-approved classification scheme for the spinel-supergroup minerals is here reported. To belong to the spinel supergroup, a mineral must meet two criteria: (i) the ratio of cation to anion sites must be equal to 3:4, typically represented by the general formula AB2X4 where A and B represent cations (including vacancy) and X represents anions; (ii) its structure must comprise a heteropolyhedral framework of four-fold coordination polyhedra (TX4) isolated from each other and sharing corners with the neighboring six-fold coordination polyhedra (MX6), which, in turn, share six of their twelve X-X edges with nearest-neighbor MX6. Regardless of space group, the X anions form a cubic close-packing and each X anion is bonded to three M-cations and one T-cation. The fifty-six minerals of the spinel supergroup are divided into three groups on the basis of dominant X anion: O2– (oxyspinel), S2– (thiospinel), and Se2– (selenospinel). Each group is composed of subgroups identified according to the dominant valence and then the dominant constituent (or heterovalent pair of constituents) represented by the letter B in the formula AB2X4. The oxyspinel group (33 species) can be divided into the spinel subgroup 2-3 (A2+B3+2O4) and the ulvöspinel subgroup 4-2 (A4+B2+2O4) , the thiospinel group (20 species) into the carrollite subgroup 1-3.5 (A1+B3.5+2S4) and the linnaeite subgroup 2-3 (A2+B3+2S4) , finally, the selenospinel group (3 species) into the bornhardtite subgroup 2-3 (A2+B3+2Se4) and the potential “tyrrellite subgroup” ( A1+B3.5+2S4 , currently composed by only one species). Once the subgroup is established based on the valence of B, then the mineral species is identified by the combination of the dominant A- and B-cations. Moreover, the present nomenclature redefines the ideal formulae of titanomaghemite, cuprorhodsite, malanite, maghemite, filipstadite, tegengrenite, rhodostannite, toyohaite and xingzhongite as well as discredits “iwakiite”, “hydrohetaerolite” and “ferrorhodsite”
Mangani-pargasite, NaCa2(Mg4Mn3+)(Si6Al2)O22(OH)2, a new mineral species of the amphibole supergroup
Full text linkMangani-pargasite, ideally NaCa2(Mg4Mn3+)(Si6Al2)O22(OH)2, is a new mineral species of the calcium amphibole subgroup of the amphibole supergroup. The type specimen was found on the mine dump of the Långban Fe-Mn-(Ba-As-Pb-Sb) deposit in Värmland, Sweden. Crystal chemical analyses resulted in the empirical chemical formula: A(Na0.90Pb0.07K0.03)Σ1.00B(Ca1.93Mn2+0.07)Σ2.00C(Mg4.25Mn3+0.39Al0.26 Fe3+0.10)Σ5.00T(Si6.35Al1.65)Σ8.00O22W(OH)2. In order to complete the description of this newly approved (IMA 2018-151) mineral we report here additional data to those published in papers by Jonsson and Hålenius (2010) and Hålenius and Bosi (2012). Mangani-pargasite is biaxial positive, with a=1.635(5), b=1.645(5), g=1.660(5) and the measured optic angle 2V is 85(5)°. The dispersion is weak (r>v), and the optic orientation is: Y||b; Z^c=25(3)°. Mangani-pargasite is red to brownish red with weak pleochroism; X=pale reddish brown, Y=pale reddish brown and Z=pale brownish red; X≈Y>Z. The unit-cell parameters are a=9.9448(5), b=18.0171(9), c=5.2829(3) Å, b=105.445(3)°, V=912.39(9) Å3, Z=2, space group C2/m. The ten strongest reflections in the X-ray powder diffraction pattern [d-values in Å, I, (h k l)] are: 8.420, 29, (110); 3.368, 17, (131), 3.279, 49, (240); 3.141, 100, (310); 2.817, 44, (33 0); 2.698, 21, (151); 2.389, 18, (350); 1.904, 29, (510); 1.650, 22, (461) and 1.448, 46, (661)
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