The crystal structure of svabite, ideally Ca₅(AsO₄)₃F, was studied using a specimen from the Jakobsberg mine, Värmland, Sweden, by means of single-crystal X‑ray diffraction data. The structure was refined to R₁ = 0.032 on the basis of 928 unique reflections with F_o > 4s(F_o) in the P6₃/m space group, with unit-cell parameters a = 9.7268(5), c = 6.9820(4) Å, V = 572.07(5) Å3. The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%, average of 10 spot analyses): SO₃ 0.49, P₂O₅ 0.21, V₂O₅ 0.04, As₂O₅ 51.21, SiO₂ 0.19, CaO 39.31, MnO 0.48, SrO 0.03, PbO 5.19, Na₂O 0.13, F 2.12, Cl 0.08, H₂O_calc 0.33, O (≡ F+Cl) –0.91, total 98.90. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca_4.66Pb_0.16Mn_0.04Na_0.03)_Σ4.89(As_2.96S_0.04Si_0.02P_0.02)_Σ3.04O₁₂[F_0.74(OH)_0.24Cl_0.01]. Svabite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra running along c are connected through TO₄ tetrahedra with channels hosting M2 cations and X anions. The crystal structure of synthetic Ca₅(AsO₄)₃F was previously reported as triclinic. On the contrary, the present refinement of the crystal structure of svabite shows no deviations from the hexagonal symmetry. An accurate knowledge of the atomic arrangement of this apatite-remediation mineral represents an improvement in our understanding of minerals able to sequester and stabilize heavy metals such as arsenic in polluted areas.

Eight spinel single-crystal samples belonging to the spinel sensu stricto-magnesiocoulsonite series (MgAl₂O₄-MgV₂O₄) were synthesized and crystal-chemically characterized by X‑ray diffraction, electron microprobe and optical absorption spectroscopy. Site populations show that the tetrahedrally coordinated site (T) is populated by Mg and minor Al for the spinel sensu stricto compositions, and only by Mg for the magnesiocoulsonite compositions, while the octahedrally coordinated site (M) is populated by Al, V³⁺, minor Mg, and very minor amounts of V⁴⁺. The latter occurs in appreciable amounts in the Al-free magnesium vanadate spinel, T(Mg)M(Mg_0.26V³⁺_1.48V⁴⁺_0.26)O₄, showing the presence of the inverse spinel VMg₂O₄. The studied samples are characterized by substitution of Al³⁺ for V³⁺ and (Mg²⁺+V⁴⁺) for 2V³⁺ described in the system MgAl₂O₄-MgV₂O₄-VMg₂O₄.

The present data in conjunction with data from the literature provide a basis for quantitative analyses of two solid-solution series MgAl₂O₄-MgV₂³⁺O₄ and MgV₂³⁺O₄-V⁴⁺Mg₂O₄. Unit-cell parameter increases with increasing V³⁺ along the series MgAl₂O₄-MgV₂O₄ (8.085–8.432 Å), but only slightly increases with increasing V³⁺ along the series VMg₂O₄-MgV₂O₄ (8.386–8.432 Å). Although a solid solution could be expected between the MgAl₂O₄ and VMg₂O₄ end-members, no evidence was found. Amounts of V⁴⁺ are nearly insignificant in all synthetic Al-bearing vanadate spinels, but are appreciable in Al-free vanadate spinel.

An interesting observation of the present study is that despite the observed complete solid-solution along the MgAl₂O₄-MgV₂O₄ and MgV₂O₄-VMg₂O₄ series, the spinel structure seems to be unable to stabilize V⁴⁺ in any intermediate members on the MgAl₂O₄-Mg₂VO₄ join even at high oxygen fugacities. This behavior indicates that the accommodation of specific V-valences can be strongly influenced by crystal-structural constraints, and any evaluation of oxygen fugacities during mineral formation based exclusively on V cation valence distributions in spinel should be treated with caution. The present study underlines that the V valency distribution in spinels is not exclusively reflecting oxygen fugacities, but also depends on activities and solubilities of all chemical components in the crystallization environment.

Iron-bearing oxy-dravite was thermally treated in air and hydrogen atmosphere at 800 °C to study potential changes in Fe, Mg and Al ordering over the octahedrally coordinated Y and Z sites and to explore possible applications to intersite geothermometry based on tourmaline. Overall, the experimental data (structural refinement, Mössbauer, infrared and optical absorption spectroscopy) show that heating Fe-bearing tourmalines results in disordering of Fe over Y and Z balanced by ordering of Mg at Y, whereas Al does not change appreciably. The Fe disorder depends on temperature, but less on redox conditions. The degree of Fe³⁺–Fe²⁺ reduction is limited despite strongly reducing conditions, indicating that the f_O2 conditions do not exclusively control the Fe oxidation state at the present experimental conditions. Untreated and treated samples have similar short- and long-range crystal structures, which are explained by stable Al-extended clusters around the O1 and O3 sites. In contrast to the stable Al clusters that preclude any temperature-dependent Mg–Al order– disorder, there occurs Mg diffusion linked to temperaturedependent exchange with Fe. Ferric iron mainly resides around O²⁻ at O1 rather than (OH)⁻, but its intersite disorder induced by thermal treatment indicates that Fe redistribution is the driving force for Mg–Fe exchange and that its diffusion rates are significant at these temperatures. With increasing temperature, Fe progressively disorders over Y and Z, whereas Mg orders at Y according to the order–disorder reaction: ^YFe + ^ZMg → ^ZFe + ^YMg. The presented findings are important for interpretation of the post-crystallization history of both tourmaline and tourmaline host rocks and imply that successful tourmaline geothermometers may be developed by thermal calibration of the Mg– Fe order–disorder reaction, whereas any thermometers based on Mg–Al disorder will be insensitive and involve large uncertainties.

Measurement of oxygen isotope ratios in common silicate minerals such as olivine, pyroxene, feldspar, garnet, and quartz is increasingly performed by Secondary Ion Mass Spectrometry (SIMS). However, certain mineral groups exhibit solid solution series, and the large compositional spectrum of these mineral phases will result in matrix effects during SIMS analysis. These matrix effects must be corrected through repeated analysis of compositionally similar standards to ensure accurate results. In order to widen the current applicability of SIMS to solid solution mineral groups in common igneous rocks, we performed SIMS homogeneity tests on new augite (NRM-AG-1) and enstatite (NRM-EN-2) reference materials sourced from Stromboli, Italy and Webster, North Carolina, respectively. Aliquots of the standard minerals were analysed by laser fluorination (LF) to establish their δ¹⁸O values. Repeated SIMS measurements were then performed on randomly oriented fragments of the same pyroxene crystals, which yielded a range in δ¹⁸O less than ± 0.42 and ± 0.58‰ (2σ) for NRM-AG-1 and NRM-EN-2, respectively. Homogeneity tests verified that NRM-AG-1 and NRM-EN-2 do not show any crystallographic orientation bias and that they are sufficiently homogeneous on the 20 μm scale to be used as routine mineral standards for SIMS δ¹⁸O analysis. We subsequently tested our new standard materials on recently erupted pyroxene crystals from Merapi volcano, Indonesia. The δ¹⁸O values for Merapi pyroxene obtained by SIMS (n = 204) agree within error with the LF-derived δ¹⁸O values for Merapi pyroxene but differ from bulk mineral and whole-rock data obtained by conventional fluorination. The bulk samples are offset to higher δ¹⁸O values as a result of incorporation of mineral and glass inclusions that in part reflects crustal contamination processes. The Merapi pyroxene SIMS data, in turn, display a frequency peak at 5.8‰, which allows us to estimate the δ¹⁸O value of the primary mafic magma at Merapi to ~ 6.1‰ when assuming closed system differentiation.

Six natural, blue colored spinel crystals were studied chemically by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) techniques and optically by UV–VIS–NIR–MIR spectroscopy in the range 30,000–2,000 cm−1  to investigate the causes of their blue color hues. The positions of the absorption bands vary only marginally with the principal composition of the samples (gahnite vs. spinel s.s .). Although blue colors in spinels are frequently the result of various electronic processes in Fe cations, we demonstrate by comparison with synthetic Co-bearing samplesthat Co acts as an important chromophore also in natural spinels. Already at concentration levels of a few ppm (e.g.,>10 ppm), cobalt gives rise to absorption bands at ~18,000, 17,000 and 16,000 cm−1  that result in distinct blue coloration. In spinels with insignificant Co contents, different shades of paler blue (from purplish to greenish blue) colors are caused by electronic transitions in TFe2+, MFe2+, MFe3+ and Fe2+–Fe3+  cation pairs.

The crystal chemistry of allactite from Långban, Värmland (Sweden) was investigated by single-crystal X-ray and neutron diffraction, optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and electron microprobe analysis by wavelength-dispersive spectroscopy (EPMA-WDS). The optical spectra indicate the presence of Mn in valence state 2+ only. Assuming 16 O atoms per formula unit, arsenic as As5+ and the (OH) content calculated by charge balance, the resulting formula based on the EPMA-WDS data is (Mn_6.73²+Ca_0.13Mg_0.12Zn_0.02)_Σ7.00(As⁵⁺)_2.00O₁₆H₈, very close to the ideal composition Mn₇(AsO₄)₂(OH)₈. In the unpolarized FTIR spectrum of allactite, fundamental (OH)-stretching bands are observed at 3236, 3288, 3387, 3446, 3484, 3562 and 3570 cm^–1, suggesting that a number of OH environments, with different hydrogen bond strengths, occur in the structure. The neutron structure refinement shows that four independent H sites occur in allactite with full site occupancy, all as members of hydroxyl groups. The complex hydrogen-bonding scheme in the allactite structure is now well defined, with at least nine hydrogen bonds energetically favourable with mono-, bi- and trifurcated configurations.

Gatedalite, Zr(Mn²⁺₂Mn³⁺₄)SiO₁₂, is a new mineral of the braunite group. It is found in hausmannite impregnated skarn together with jacobsite, Mn-bearing calcite, tephroite, Mn-bearing phlogopite,långbanite, pinakiolite and oxyplumboroméite at the Långban Mn-Fe oxide deposit, Värmland, central Sweden. The mineral occurs as very rare, small ≤60 µm), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm³. It is opaque and weakly anisotropic with reflectivity in air varying between 17.1 and 20.8% in the visible spectral range. Gatedalite is tetragonal, space group I41/acd, with the unit-cell parameters a = 9.4668(6) Å , c = 18.8701(14) Å , V = 1691.1(2) ų and Z = 8. The crystal structure was refined to an R1 index of 5.09% using 1339 unique reflections collected with MoKa X-ray radiation. The five strongest powder X-ray diffraction lines [d in Å, (I), (hkl)] are: 2.730(100)(224), 2.367(12)(040), 1.6735(12)(440), 1.6707(29)(048) and 1.4267(16)(264). Gatedalite is a member of the braunite group (general formula AB₆SiO₁₂). It is related to braunite (Mn²⁺Mn³⁺₆SiO₁₂) through the net cation exchange (Zr⁴⁺+Mn²⁺)➝2Mn³⁺, which results from the substitutions Zr⁴⁺ ➝ Mn²⁺ at the 8-fold coordinated site (A in the general formula) coupled with a 2Mⁿ²⁺ ➝ 2Mn³⁺ substitution at the 6-fold coordinated sites (B in the general formula).