A gem-quality purplish-red tourmaline sample of alleged liddicoatitic composition from the Anjanabonoina pegmatite, Madagascar, hasbeen fully characterised using a multi-analytical approach to define its crystal-chemical identity. Single-crystal X-ray diffraction, chem-ical and spectroscopic analysis resulted in the formula: ^X(Na_0.41^□_0.35Ca_0.24)_Σ1.00^Y(Al_1.81Li_1.00

Fe³⁺_0.04Mn³⁺_0.02Mn²⁺_0.12Ti_0.004)_Σ3.00

^ZAl₆[^T(Si_5.60B_0.40)Σ_6.00O₁₈]

(BO₃)₃(OH)₃W[(OH)_0.50F_0.13O_0.37]_Σ1.00, which corresponds to the tourmaline species elbaite having the typical space group R3m and relatively small unit-cell dimensions, a= 15.7935(4) Å, c= 7.0860(2) Å and V= 7.0860(2) ų.Optical absorption spectroscopy showed that the purplish-red colour is caused by minor amounts of Mn³⁺(Mn₂O₃= 0.20 wt.%).Thermal treatment in air up to 750°C strongly intensified the colour of the sample due to the oxidation of all Mn²⁺ to Mn³⁺ (Mn₂O₃ up to 1.21 wt.%). Based on infrared and Raman data, a crystal-chemical model regarding the electrostatic interaction betweenthe X cation and W anion, and involving the Y cations as well, is proposed to explain the absence or rarity of the mineral species ‘liddicoatite’.

Tourmalines from the late-Variscan Arbus pluton (SWSardinia) and its metamorphic aureole were structurally and chemically characterized by single-crystal X-ray diffraction, electron and nuclear microprobe analysis, Mössbauer, infrared and optical absorption spectroscopy, to elucidate their origin and relationships with the magmatic evolution during the pluton cooling stages. The Arbus pluton represents a peculiar shallow magmatic system, characterized by sekaninaite (Fe-cordierite)-bearing peraluminous granitoids, linked via AFC processes to gabbroic mantle-derived magmas. The Fe²⁺-Al-dominant tourmalines occur in: a) pegmatitic layers and pods, as prismatic crystals; b) greisenized rocks and spotted granophyric dikes, as clots or nests of fine-grained crystals in small miaroles locally forming orbicules; c) pegmatitic veins and pods close to the contacts within the metamorphic aureole. Structural formulae indicate that tourmaline in pegmatitic layers is schorl, whereas in greisenized rocks it ranges fromschorl to fluor-schorl. Tourmalines in thermometamorphosed contact aureole are schorl, foitite and Mg-rich oxy-schorl. The main substitution is Na+Fe²⁺↔▢+Al, which relates schorl to foitite. The homovalent substitution (OH)↔F at the O1 crystallographic site relates schorl to fluor-schorl, while the heterovalent substitution Fe²⁺+(OH, F)↔Al+O relates schorl/fluor-schorl to oxy-schorl. Tourmaline crystallization in the Arbus pluton was promoted by volatile (B, F and H₂O) enrichment, low oxygen fugacity and Fe²⁺ activity. The mineralogical evolutive trend is driven by decreasing temperature, as follows: sekaninaite+quartz →schorl+quartz→fluor-schorl+quartz → foitite+quartz. The schorl→foitite evolution represents a distinct trend towards (Al+!) increase and unit-cell volumedecrease. These trends are typical of granitic magmas and consistent with Li-poor granitic melts, as supported by the absence of elbaite and other Li-minerals in the Arbus pluton. Tourmaline-bearing rocks reflect the petrogenetic signi!cance of contribution from a metapelitic crustal component during the evolution of magmas in the middle-upper crust.

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.

Oxy-foitite, □(Fe²⁺Al₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, is a new mineral of the tourmaline supergroup. It occurs in high-grade migmatitic gneisses of pelitic composition at the Cooma metamorphic Complex (New South Wales, Australia), in association with muscovite, K-feldspar and quartz. Crystals are black with a vitreous luster, sub-conchoidal fracture and gray streak. Oxy-foitite has a Mohs hardness of ∼7, and has a calculated density of 3.143 g/cm³. In plane-polarized light, oxy-foitite is pleochroic (O= dark brown and E = pale brown), uniaxial negative. Oxy-foitite belongs to the trigonal crystal system, space group R3m, a = 15.9387(3) Å, c = 7.1507(1)Å and V = 1573.20(6)ų,Z = 3. The crystal structure of oxy-foitite was refined to R1 = 1.48% using 3247 unique reflections from single-crystal X-ray diffraction using MoKα radiation. Crystal-chemical analysis resulted in the empirical structural formula: ^X(□_0.53Na_0.45Ca_0.01K_0.01)_Σ1.00^Y(Al_1.53Fe²⁺_1.16Mg_0.22Mn²⁺_0.05Zn_0.01Ti⁴⁺_0.03)_Σ3.00^Z(Al_5.47Fe³⁺_0.14Mg_0.39)_Σ6.00[(Si_5.89Al_0.11)_Σ6.00O₁₈](BO₃)₃^V(OH)₃^W[O_0.57F_0.04(OH)_0.39]_Σ1.00. Oxy-foitite belongs to the X-site vacant group of the tourmaline-supergroup minerals, and shows chemical relationships with foitite through the substitution ^YAl³⁺+^WO^2-→^YFe²⁺+^W(OH)^1–.

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.

Natural Mg-rich lucchesiite was thermally treated in air and hydrogen atmosphere up to 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 intracrystalline geothermometry based on tourmaline. Overall, the experimental data (structural refinement, Mössbauer, infrared and optical absorption spectroscopy) show that thermal treatment of lucchesiite results in an increase of Fe_tot contents at Z balanced by an increase of Mg and Al at Y . This process is accompanied by a significant deprotonation of the O3 anion site. The Fe order–disorder reaction depends more on temperature, than on redox conditions. During heat treatment in H₂ ,reduction of Fe³⁺ to Fe²⁺ was not observed despite strongly reducing conditions, indicating that the f_O2  conditions do not exclusively control the Fe oxidation state at the present experimental conditions. On the basis of this and previous studies, the intersite order–disorder process induced by thermal treatment indicates that Fe redistribution is an important factor for Fe–Mg–Al-exchange and is significant at temperatures around 800 °C. As a result, Fe–Mg–Al intersite order–disorder is sensitive to temperature variations, whereas geothermometers based solely on Mg–Al order–disorder appear insensitive and involve large uncertainties. The presented findings are important for interpretation of the post-crystallization history of both tourmaline and tourmaline host rocks, and indicate that successful tourmaline geothermometers may be developed by thermal calibration of the Fe-Mg–Al order–disorder reaction.

The scientific value of old and well-preserved collections is priceless. Samples that already have been studied and described can still give very useful information. For instance, minerals with complex solid solutions like amphiboles sometimes show new compositions that are feasible because of crystal-chemistry and charge arrangements, based on the current classification scheme by Hawthorne et al. (2012) for the amphibole supergroup. In the last four years, a fruitful collaboration between the Swedish Museum of Natural History and the Department of Earth Sciences of the University of Milan has allowed the identification of new amphibole species, recognized by CNMNC-IMA. First of all, we identified hjalmarite, [ANaB(NaMn)CMg5TSi8O22W(OH)2], which is related to richterite via the homovalent substitution [B]Ca2+ → [B]Mn2+, and is the second recognized member of the sodium–(magnesium–iron–manganese) subgroup, after ferri-ghoseite. Sjögren (1891) had described a physically similar, MnO-rich sample from Långban, named “astochit”. A related amphibole, although belonging to a different subgroup, that we have formally described is potassic-richterite, [AKB(NaCa)CMg5TSi8O22W(OH)2]. It was found in a sample from the Pajsberg iron and manganese ore mines, which was originally collected by the mineralogist Lars Johan Igelström, probably in the 1850s. The most recent amphibole we have described is ferri-taramite [ANaB(NaCa)C(Mg3Fe3+2)T(Si6Al2)O22W(OH)2], found in a skarn sample from the Jakobsberg manganese mine: it was once examined by Flink (1914), who noted the unusual character of the amphibole and described it as a “strange hornblende”.

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.

Synthetic clinopyroxenes along the CaMgSi₂O₆– CaCoSi₂O₆ join were investigated by a combined chemical-structuralspectroscopic approach. Single crystals were synthesized by flux growth methods, both from Ca-saturated and Ca-deficient starting compositions. Single crystal structure refinements show that the incorporation of Co²⁺ at the octahedrally coordinated cation sites of diopside, increases the unit-cell as well as the M1 and the M2 polyhedral volumes. Spectroscopic investigations (UV–VIS–NIR) of the Ca-rich samples reveal three main optical absorption bands, i.e. ⁴T_1g → ⁴T_2g(F), ⁴T_1g → ⁴A_2g(F) and ⁴T_1g → ⁴T^_1g(P) as expected for Co²⁺ at a six-coordinated site. The bands arising from the ⁴T_1g → ⁴T_2g(F) and the ⁴T_1g → ⁴T_1g(P) electronic transitions, are each split into two components, due to the distortions of the M1 polyhedron from ideal Oh- symmetry. In spectra of both types, a band in the NIR range at ca 5000 cm⁻¹ is caused by the ⁴A_2g → ⁴T_1g(F) electronic transition in Co²⁺ in a cubic field in the M2 site. Furthermore, an additional component to a band system at 14,000 cm⁻¹, due to electronic transitions in Co²⁺ at the M2 site, is recorded in absorption spectra of Ca-deficient samples. No variations in Dq and Racah B parameters for Co²⁺ at the M1 site in response to compositional changes, were demonstrated, suggesting complete relaxation of the M1 polyhedron within the CaMgSi₂O₆– CaCoSi₂O₆ solid solution.