ALBERT

Description: Although putatively possessing hexagonal R3m symmetry, reports of optically anomalous tourmaline are common, and recently an occurrence of triclinic tourmaline was reported with dissymmetrization that resulted from non-equivalency of the occupants of the Y sites. We report the atomic arrangement of Ni-bearing dravite from the Berezovskoe gold deposit, Middle Urals, Russia, in the non-conventional triclinic space-group R1 (R = 4.41%) to facilitate comparison with the conventional tourmaline R3m cell. The dissymmetrization occurs as a result of inequalities among both the hexagonally equivalent Y and hexagonally equivalent Z tourmaline sites. The atomic arrangement of this triclinic dravite demonstrates that the atomic arrangement of tourmaline is robust, and is capable of incorporating various substituents by modifying the putative hexagonal structure in lower symmetries, suggesting that further exploration of tourmaline's role in trace-element variation is warranted. Optical studies demonstrate the heterogeneous biaxial character of the crystals. Domains of different optical orientation and 2V correspond directly to trigonal prism |100|, |010| and pedion |001| sectors, indicating optical sectoral zoning. Compositional sectoral and concentric zoning are also observed within the crystals. Spectroscopic studies show the optical absorption spectrum of the Berezovskoe tourmaline has strong absorptions in the 400, 600-700, and 1100 nm regions, in addition to OH features near 1450, 2300, and 2700 nm. We conclude that the color in the E {bot} c polarization comes dominantly from Fe mixed-oxidation-state couples on the Y sites, and from Cr3+. Contributions to the color from the nickel are believed to be minor and will fall in the regions of strong Cr and Fe absorption. The ordered arrangement of cations on the Y and Z sites and the correlation of optical orientation with specific sectors indicate that dissymmetrization occurs during growth by differential incorporation at structurally different atomic sites at the surface of the crystal, which in the bulk are symmetrically equivalent.

Description: Synkinematic tourmaline and quartz growth on slickensides of brittle-ductile normal fault planes was studied by electron-microprobe analysis and X-ray crystallographic methods. The faults show top-to-the-southwest sense of shear and cross-cut mylonitic gneisses of the Cycladic Blueschist Unit on Despotiko Island, located southwest of Antiparos, in the Cyclades, Greece. The black tourmaline crystals, up to 180 {micro}m in length and 50 {micro}m in diameter, form slickenside fibers on these fault planes and therefore have grown during fault slip. They record a chemical variation from X(Na0.66Ca0.20{square} 0.14) Y(Fe2+ 1.17Mg0.92 Al0.78Ti4+ 0.12Mn2+ 0.01) Z(Al5.00Mg1.00) (BO3)3 T(Si5.95Al0.05)O18 V(OH)3 W[(OH)0.88F0.12] to X({square} 0.50Na0.48Ca0.02) Y(Fe2+ 1.75Al1.31 Mn2+ 0.02Ti4+ 0.01) Z(Al5.49Mg0.51) (BO3)3 T(Si5.98Al0.02)O18 V(OH)3 W(OH). The tourmaline compositions belong to the foitite - schorl - dravite series. On the basis of X{square} -XMg, X{square} -Ca, Ca-XMg relationships and significant chemical differences between analogous and antilogous poles, these tourmalines most likely grew at low- to medium-grade metamorphic conditions, corresponding to crystallization temperatures between 300 and 400 {+/-} 50{degrees}C. This is consistent with the low amounts of [4]Al and F in these tourmalines, the observation of almost completely chloritized biotite near the slickensides, and the deformation microstructures of quartz in the fault. Together with structural measurements in the field, these data suggest that the tourmaline slickenside fibers crystallized during a late stage of overall NE-SW-oriented extensional exhumation, bringing the rocks from lower-greenschist facies conditions to the brittle-ductile transition zone.

Description: A nomenclature for tourmaline-supergroup minerals is based on chemical systematics using the generalized tourmaline structural formula: XY3Z6(T6O18)(BO3)3V3W, where the most common ions (or vacancy) at each site are X = Na1+, Ca2+, K1+, and vacancy; Y = Fe2+, Mg2+, Mn2+, Al3+, Li1+, Fe3+, and Cr3+; Z = Al3+, Fe3+, Mg2+, and Cr3+; T = Si4+, Al3+, and B3+; B = B3+; V = OH1- and O2-; and W = OH1-, F1-, and O2-. Most compositional variability occurs at the X, Y, Z, W, and V sites. Tourmaline species are defined in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state is used for the basis of nomenclature. Tourmaline can be divided into several groups and subgroups. The primary groups are based on occupancy of the X site, which yields alkali, calcic, or X-vacant groups. Because each of these groups involves cations (or vacancy) with a different charge, coupled substitutions are required to relate the compositions of the groups. Within each group, there are several subgroups related by heterovalent coupled substitutions. If there is more than one tourmaline species within a subgroup, they are related by homovalent substitutions. Additionally, the following considerations are made. (1) In tourmaline-supergroup minerals dominated by either OH1- or F1- at the W site, the OH1--dominant species is considered the reference root composition for that root name: e.g., dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g., fluor-dravite. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g., "potassium-fluor-dravite."

Description: 〈div data-abstract-type="normal"〉〈p〉Yellowish dravitic tourmaline (dominated by the oxy-dravite component) associated with secondary fluor-dravite/fluor-schorl and dravite/schorl tourmalines was found in a quartz vein cropping out in the eastern part of the Karkonosze Mountains range, SW Poland. The crystal structure of this tourmaline was refined to an 〈span〉R〈/span〉〈span〉1〈/span〉 value of 1.85% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. The tourmaline, a representative of the alkali-tourmaline group, has the structural formula: (Na〈span〉0.75〈/span〉Ca〈span〉0.12〈/span〉□〈span〉0.13〈/span〉)〈span〉Σ1〈/span〉(Mg〈span〉1.93〈/span〉Al〈span〉0.95〈/span〉Ti〈span〉0.06〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20181017132303927-0397:S0026461X18000129:S0026461X18000129_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 V〈span〉0.01〈/span〉)〈span〉Σ3〈/span〉(Al〈span〉5.38〈/span〉Mg〈span〉0.62〈/span〉)〈span〉Σ6〈/span〉B〈span〉3〈/span〉Si〈span〉6〈/span〉O〈span〉27〈/span〉(OH)〈span〉3〈/span〉(O〈span〉0.46〈/span〉OH〈span〉0.33〈/span〉F〈span〉0.21〈/span〉)〈span〉Σ1〈/span〉, and is characterized by an extremely high Mg/(Mg + Fe) ratio of 0.97–0.99, the 〈span〉〈span〉W〈/span〉〈/span〉O〈span〉2–〈/span〉 content that reaches 0.59 apfu resulting in a local predominance of the oxy-dravitic component and Mg–Al disorder on the octahedral 〈span〉Y〈/span〉 and 〈span〉Z〈/span〉 sites of the order of 0.64 apfu. This disordering results in an increasing Z–O〉 distance with ~1.925 Å, and unit-cell parameters 〈span〉a〈/span〉 = 15.916(1) Å and 〈span〉c〈/span〉 = 7.180(1) Å. The tourmaline formed during Variscan prograde metamorphism under the influence of a released (H〈span〉2〈/span〉O,B,F)-bearing fluid. The fluid mobilized the most soluble components of partly altered silicic volcaniclastic material of the Late Cambrian to Early Ordovician bimodal volcanism to become the protolith for adjacent quartzo-feldspathic schists and amphibolites, and propagated them into the surrounding granitic gneisses of the Kowary unit in the eastern metamorphic cover of the Karkonosze granite.〈/p〉〈/div〉

Description: Copper- and Mn-bearing elbaitic tourmaline (“Paraíba tourmaline”) sometimes contains significant amounts of Pb and Bi. Their position in the tourmaline crystal structure was studied with correlation analysis and bond valence calculations. Correlations between the F content and the X-site charge allow predicting the X-site occupancy. Three sets of tourmaline analyses were studied: (1) Pb-rich tourmalines from the Minh Tien pegmatite, Vietnam; (2) Cu-, Pb- and Bi-bearing tourmalines from the Mulungu mine, Brazil; (3) Cu- and Bi-bearing tourmalines from the Alto dos Quintos mine, Brazil. Two correlations were plotted: (1) the charge by considering only Na1+, Ca2+ and K1+; (2) the charge by adding Pb2+ and Bi3+ to the X-site charge. When plotting correlations for the Minh Tien tourmalines, the correlation significantly improves by adding Pb2+ to the X site. For the Alto dos Quintos tourmalines, only a slight increase of the correlation coefficient is observed, while such a correlation for tourmalines from Mulungu interestingly shows a slight decrease of the correlation coefficient. Bond valence calculations revealed that Bi3+ and Pb2+ can indeed occupy the X site via BiLi(NaAl)−1, PbLi(NaCu)−1 and possibly PbCu(NaAl)−1 substitutions as seen in the investigated tourmaline samples. At the Y site, Pb4+ can be substituted via PbLi(AlCu)−1, and PbVO(AlVOH)−1, while Bi5+ does not have any stable arrangement in Cu-bearing fluor-elbaite. The occurrence of Pb4+ at the Y site could be one explanation for the results of the correlations of the Mulungu tourmalines. Another explanation could be that during the tourmaline crystallization some additional Bi and Pb came into the pegmatitic system and hence disturbed the correlation between the average X-site charge and the F content. Further plots of such correlations in “Paraíba tourmaline” samples might also help to distinguish between the worldwide localities of these rare and sought-after tourmalines.

Description: In the Eastern Alps (Central Europe), about two dozen spodumene pegmatite bodies extend heterogeneously over the Austroalpine Unit. They are spatially associated with leucogranites and simple pegmatites formed during the Permian extensional event. The pegmatites and leucogranites were overprinted during the Alpine orogeny with different intensities, leading to various structural and chemical changes to the magmatic features. The magmatic assemblage consists of K-feldspar, quartz, plagioclase, muscovite, garnet, and tourmaline, with additional spodumene in the pegmatites. Beryl is rarely present and other accessory phases such as apatite, cassiterite, or Nb-Ta phases are exceptional. Mineral compositions indicate that the magmatic garnet is rich in Mn and Fe and has Ca-rich and Mn-poor Alpine metamorphic overgrowths. Sm-Nd ages of carefully separated crystals of magmatic garnet from the leucogranites and spodumene pegmatites fall in the range of 245–280 Ma. These Permian-Early Triassic ages are in good agreement with previously published results from associated simple pegmatites. Tourmalines from spodumene pegmatites belong to the elbaite–schorl series, with an unusual but significant dravite component. The occurrence of tourmaline and the absence of biotite may be due to the low Ti content (〈0.06 wt.% TiO2) in the melt of both rock types. Trace-element compositions (Rb, Li, Cs, Tl, or Ba) of magmatic muscovite show that the leucogranites and spodumene pegmatites belong to the same fractionation trend, with more fractionated signatures in the spodumene pegmatites. Whole-rock geochemical compositions indicate that both lithologies have a common peraluminous granitic geochemistry, similar minor- and trace-element distribution patterns, and share a fractionation trend of trace elements including Be, Li, Rb, Cs, Sn, Ge, Ba, and REE. Positive and negative Eu anomalies in analyzed leucogranites and spodumene pegmatites, respectively, are interpreted to be due to the extraction of Eu in plagioclase crystallizing in leucogranite during the melt evolution. Values of ε(t)Nd (approximately –8) and initial 87Sr/86Sr ratios (0.7098–0.7353) indicate a crustal origin for the primary melts. Using the Permian metamorphic conditions of the country rock as a proxy, the spodumene pegmatites intruded at more than 10 km depth at somewhat higher structural levels than the leucogranites. This set of field and petrographic observations as well as geochemical and geochronological data suggest that the leucogranites and spodumene pegmatites of the Austroalpine Unit are contemporaneous and cogenetic, whereby the spodumene pegmatites show a higher degree of chemical evolution and concentration of rare metals.