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."
The presence of amazonitic K-rich feldspar is considered an earmark of evolved granitic pegmatites of NYF type. Elements of an explanation of the "NYF pegmatite-amazonite" connection emerged in a ...study of the classic Anjanabonoina "hybrid" granitic pegmatite in Madagascar. The NYF assemblages there and at the Sakavalana and Anjahamiary pegmatites contain magmatic U- and Th-rich accessory phases and an amazonitic microcline perthite in miarolitic cavities. The early miarolitic minerals are overgrown by a Li- and Cs-enriched assemblage, depleted in HFSE, containing pale-gray microcline perthite. The amazonitic microcline perthite at Anjanabonoina has a δ18O value above 15 per mil, and that at Sakavalana, close to 13 per mil, indicative of formation of the NYF-pegmatite-forming magma by the melting of crustal rocks. Locally, the source rocks had become slightly alkaline and enriched in HFSE during periods of metasomatism of the hot granulitic crust accompanying episodes of distension over the course of the protracted Pan-African orogeny. The fertilized crust underwent local anatexis after the culmination of Gondwana re-assembly. The proximity of pyrochlore and other U- and Th-bearing accessory phases in the NYF assemblage provided the ionizing radiation required to interact with H2O trapped in the vacancy next to the Pb ions in the structure. The presence of an amazonitic K-feldspar ultimately signals net additions of Pb, U, Th, and the alkalis to the source prior to anatexis during a period of distension after a major orogenic disturbance. Plumbian K-feldspar also occurs in granitic pegmatite bodies formed by anatexis in the vicinity of a galena-bearing orebody (e.g., at Broken Hill, Australia); in this case, the resulting granitic pegmatite is neither of NYF type nor enriched in rare elements, however.
A chemical and paragenetic study has been performed on Sc silicates and Sc-bearing beryl occurring in Hercynian NYF-miarolitic pink granite and granophyric leucogranite at Baveno and Cuasso al Monte, ...Western Southern-Alps, Italy. In the Baveno and Cuasso al Monte plutons, detailed field work allowed the discovery of a significant number of crystals of bazzite, thortveitite, scandiobabingtonite, cascandite, and jervisite representative of all the known morphological and color varieties of these minerals in the two localities. Other studied samples belong to the historic collections of the Natural History Museum of Milan.
Except for beryl, which crystallizes relatively early in aplitic granophyre, all the other Sc minerals crystallize as late-stage phases in cavities associated with fluorite. Chemical analyses reveal moderate Sc enrichment at the rim of beryl crystals. Bazzite displays a relatively large chemical variation, from .primitive. compositions enriched in Fe
and Al
, to .highly evolved. compositions with values of Sc
up to 17.54 wt%. Scandiobabingtonite shows a perfect inverse correlation between Fe
and Sc concentrations, suggesting complete solid solution between babingtonite and its Sc analogues. A wide variety of compositions have been determined for cascandite, significantly extending the compositional range of this mineral. In particular, the MnO content ranges from 0.37 to 4.87 wt%. The jervisite crystals analyzed in this work have rather homogeneous compositions much closer to the end-member if compared with the holotype analysis reported in literature. Thortveitite shows a wide range of compositions with variation in Sc
, Y
, HREE, and Fe
. Significant Fluctuations of the Sc/Yb ratios are in agreement with similar complex variations in the ratios between REE (e.g., Y/Dy) reported in the literature for crystals of gadolinite-group minerals from the same localities.
Two different genetic models are discussed to explain the precipitation of Sc silicates as late stage phases in cavities. (1) during the latest stages of magma crystallization, HFSE and Sc were extracted from the silicate liquid and partitioned into fluids due to the complexing effect of F. Indeed, in view of the NYF geochemistry of the granite and the significant abundance of the associated F-bearing minerals, Fluorides (but not other complexing agents such as carbonates and phosphates) played the major role in concentrating HFSE and Sc. In cavities, such elements resulted in a series of rare accessory phases when F was extracted from Fluids because of the precipitation of zinnwaldite and Fluorite. (2) HFSE, Sc, and Y+REE were mainly incorporated by gadolinite-(Y) and siderophyllite crystallizing from residual magma. Many of the accessory phases crystallized in cavities because of the aggressive effect of subcritical hydrous, F-rich Fluids on the previously formed gadolinite-(Y) (liberating REE, Y, B, Be, Fe, Ca), siderophyllite (liberating Fe, Ti, and possibly Nb-Ta, Sc, etc.), and feldspars (liberating Ca, P, Cs, Ba). This second model is consistent with the widespread hydrothermal alteration of the Baveno and Cuasso al Monte granites and granophyres
Chemical compositions and mineral assemblages of tourmalines from the elbaite-subtype Manjaka pegmatite in the Sahatany Valley, central Madagascar, and its exocontact were examined using EPMA and ...LA-ICP-MS. Several textural, compositional and paragenetic types of tourmalines were recognized in the individual pegmatite units and in the zones located towards the host rock in the order: wall pegmatite unit -> border pegmatite unit -> contact zone -> recrystallization zone; the latter zone evidently originated after the host Mg-rich calc-silicate rock (Di + Tr+Qz>Pl + Kfs>Phl + Dol > Cal). Zoned prismatic crystals from the border unit evolved from the core Tur la (dravite < Fe-rich elbaite), Tur lb (Mn,Fe-rich to Mn-rich fluor-elbaite > elbaite), to the crystal rims Tur II (elbaite > darrellhenryite) via the substitutions: (1) 2:-R2+ = >LiAl, (2) !Mn"F = !Fe2+"'OH, (3) !Li05"'OH = !Al05"'O, and (4) !Li7'Al"'OH3 = !Al7'Si"'O3. The contact zone, -2 mm thick, contains abundant Tur FV (Li,Fe,Al-enriched dravite > oxy-dravite), and the recrystallization zone, ~1.5 cm thick, common Tur V (fluor-uvite > dravite > fluor-dravite, magnesio-lucchesiite, uvite) via the substitutions: (1) 2!R2+ = !LiAl and (5) !R2+"'OH =! A1"'O, (6) 'Na"'OH = vCa"'O and (7) 'Na' Al = vCa!R2+. The chemical compositions of the individual types of tourmalines suggest that the mobility of elements between calc-silicate rock and pegmatite was low. Only weak influx of Mg, V and Cr into pegmatite and Li and Al from the pegmatite to the host rock - recrystallization zone, respectively, were observed. The existence of thin B-rich contact zone with dominant Tur FV suggests low influx of B into the host rock in early magmatic stage. High contents of F in Tur V from the recrystallization zone were very likely triggered by influx of B,F-enriched residual pegmatite fluids. Very high «(B,O3), but low «(H,O) and «(F) in the pegmatite melt constrained very low degree of external contamination of the pegmatite. Compositional evolution in tourmalines from Manjaka was compared with the pegmatites and their exocontact at Stoffhtitte, Koralpe, Austria, and Blizna I, Moldanubian Zone, Czech Republic.
Context.
The Atacama Large Millimeter/submillimeter Array (ALMA) has been in operation since 2011, but it has not yet been populated with the full suite of its planned frequency bands. In particular, ...ALMA Band 2 (67−90 GHz) is the final band in the original ALMA band definition to be approved for production.
Aims.
We aim to produce a wideband, tuneable, sideband-separating receiver with 28 GHz of instantaneous bandwidth per polarisation operating in the sky frequency range of 67−116 GHz. Our design anticipates new ALMA requirements following the recommendations of the 2030 ALMA Development Roadmap.
Methods.
The cryogenic cartridge is designed to be compatible with the ALMA Band 2 cartridge slot, where the coldest components – the feedhorns, orthomode transducers, and cryogenic low noise amplifiers – operate at a temperature of 15 K. We use multiple simulation methods and tools to optimise our designs for both the passive optics and the active components. The cryogenic cartridge is interfaced with a room-temperature (warm) cartridge hosting the local oscillator and the downconverter module. This warm cartridge is largely based on GaAs semiconductor technology and is optimised to match the cryogenic receiver bandwidth with the required instantaneous local oscillator frequency tuning range.
Results.
Our collaboration has resulted in the design, fabrication, and testing of multiple technical solutions for each of the receiver components, producing a state-of-the-art receiver covering the full ALMA Band 2 and 3 atmospheric window. The receiver is suitable for deployment on ALMA in the coming years and it is capable of dual-polarisation, sideband-separating observations in intermediate frequency bands spanning 4−18 GHz for a total of 28 GHz on-sky bandwidth per polarisation channel.
Conclusions.
We conclude that the 67−116 GHz wideband implementation for ALMA Band 2 is now feasible and that this receiver provides a compelling instrumental upgrade for ALMA that will enhance observational capabilities and scientific reach.
PDF
