Blue Tourmaline Research Articles

Chemical Composition and Spectral Variation in Gem-Quality Blue Iron-Bearing Tourmaline from Brazil

This study, conducted a spectroscopic analysis of 10 gem-quality blue tourmaline samples from Minas Gerais, Brazil, focused on detailed variations in their infrared, Raman, and UV-VIS spectra. Conventional gemological tests, electron-probe microanalysis, infrared spectroscopy (mid- and near-infrared), Raman spectroscopy, and UV-visible spectroscopy were used to systematically analyze the chemical composition and spectral characteristics of the samples. The infrared spectra revealed vibrations of [YO6], [TO4], [BO3], [OH], and H2O groups, indicating different bonding profiles, with the [OH] vibrational frequency showing a direct correlation with FeO and MnO content. The Raman spectra primarily reflected the stretching vibrations of metal–oxygen bonds and hydroxyl groups, indicating the complexity of the local environment in the crystal structure. The UV-VIS spectra showed that the broad absorption band around 725 nm was due to intermetallic charge transfer between Fe2+ and Fe3+. This work provides new insights into the local bonding environment within the crystal structure by providing precise spectral data of natural blue tourmaline, and a more accurate classification and evaluation of blue tourmaline through fine spectral change characteristics related to crystal chemistry has important implications for both academic research and the gemstone industry.

Geological and mining heritages in the Seridó UNESCO Global Geopark: Ediacaran to Cambrian mineral deposits revealed by historical mines in Northeast Brazil

The coexistence of mining, geoparks, and conservation is an ongoing discussion for the implementation and development of UNESCO Global Geoparks (UGGps). The Seridó UGGp provides a classic example where mining plays a fundamental role in unveiling geological heritage, showcasing positive relationships between mining and the establishment of sustainable territories based on geoconservation. The aim of this paper is to investigate, within two geosites of the Seridó UGGp—Mina Brejuí and Açude Boqueirão—how geological heritage has been showcased in active mines, correlated with cultural mining heritage, and utilized in geotourism and geoeducation. The paper utilizes adapted geological heritage inventories and valuing techniques, integrating cultural aspects of the mining landscape to provide detailed insights into the complex areas of the geosites. Both geosites have been the sites of regionally significant Ediacaran-Cambrian magmatic-hydrothermal deposits since the early 20th century, yielding metallic and non-metallic raw materials during the World Wars up to the present days. The Mina Brejuí geosite boasts one of the best exposures of W–Mo skarn mineralization in South America within its underground galleries. Açude Boqueirão is a mining district with several types of quarries and mines, and its geoheritage is linked to the initial descriptions of the Borborema regional pegmatitic province with Cambrian Ta–Nb, Li–Be, gemstones, and ornamental deposits, and also for the rare deposit of blue tourmaline gemstone. Particularly, the Boqueirão site requires valorization interventions, as its rich mining heritage is threatened by the expansion of the energy sector and illegal artisanal mining. Active mining operations could present a plethora of opportunities that encompass the geopark's concepts of conservation, tourism, and education. Inventories of mining heritage in geoparks serve as alternative sources for geotouristic routes.

Geochemical characterization and mineral chemistry of blue tourmalines from the pegmatitic body of Bom Jesus Banabuiú-Ceará,northeaster Brazil.

Geochemical characterization and mineral chemistry of blue tourmalines from the pegmatitic body of Bom Jesus Banabuiú-Ceará,northeaster Brazil.

FIELD TRIP GUIDE TO THE SERIDÓ PEGMATITE PROVINCE

The pegmatites of the Seridó Pegmatite Province show a granitic bulk mineralogical composition, composed of K-feldspar, albite, quartz, muscovite and occasionally biotite. Detailed mineralogical studies of the major minerals are scarce. There is, on the other hand, a relatively larger number of published contributions concerning pegmatite accessory minerals. This field trip guide focus on the mainly occurrence locations of these last minerals, which are group here by their most frequently recovered rare metals and gems: (1) Ta-Nb: Tantalite-columbite is the main mineral group, even though other rare varieties may occasionally be recovered as microlite, tapiolite, strueverite, simpsonite, ixiolite, among others. Their grain size varies from the centimeter to decimeter scale. The mineralization is disseminated (less frequently), in pockets, or as aggregates of crystals, usually associated with "replacement bodies" at the contact between Zone III and the quartz core. They also occur sporadically in Zone II and occasionally in homogeneous pegmatites, as crystals in subcentimeter to centimeter scale, frequently disseminated in the quartz-feldspar-muscovite groundmass. In this case, they display lamellar habit and normally lower Ta concentration. (2) Be: Beryl is the main Be-mineral present. Its grain size varies from centimeter to meter, it is normally prismatic and shows diverse colors (white, green, blue, pink, yellow), with green and white prevailing. It occurs in Zones II, III and IV (quartz core), as well as in association with the "replacement bodies". In homogeneous pegmatites, it occurs as centimetric crystals, disseminated in the groundmass. Some pseudoconical forms are also observed, and contain muscovite and quartz inclusions. The gem varieties (aquamarine, morganite, heliodor) are scarce. Beryl and the Ta-Nb minerals, are the main ores exploited in these pegmatites. (3) Li: Li-minerals recovered from the pegmatites are spodumene, lepidolite and amblygonite. The spodumene prevails over amblygonite and lepidolite, although the reserves of Li are still not completely known. Spodumene occurs in individual crystals or aggregates in decimeter to meter dimensions, rarely forming individualized zones. Lepidolite occurs mainly in albitized pegmatites associated with deep blue tourmalines (named Paraíba Tourmaline). Amblygonite occurs in pockets of decimeter to meter width and may be associated with other phosphates such as lithiophilite, triphylite, siclerite, tavoríte, among others. (4) Sn: Cassiterite is the main Sn-ore, although Sn is also present in the crystallographic structure of Ta-Nb minerals. It is found disseminated in centimeter grain size in the "replacement bodies" or in Zone I of heterogeneous pegmatites, and occurs occasionally in some homogeneous pegmatites. Cu (e.g. djurleite), U (e.g. uraninite) and REE (e.g. xenotime) minerals are considered important resources exploted or as potential for further exploration.

Characterization of Blue Tourmaline from Madagascar for Exploring Its Color Origin

In the research, spectroscopic studies of tourmaline for color origin were performed by X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, and X-ray photoelectron spectroscopy. The research work emphasized the analysis of transition metal atoms, including their valence state and coordination number, in order to better understand the effect of transition metal cations on the color origin. The results showed that the blue color of the crystal is caused by the strong absorption at 725 nm in the red spectrum, which generates a symmetrical and wide absorption band. The absorption band at 725 nm is caused by charge transfer between Fe2+ at the Y site and Fe3+ at the Z site. Other ions in the crystal did not generate absorption lines in the visible spectral range. Besides position, its valence state and coordination number were clarified to reveal color variation and the origin of tourmaline. Most importantly, this spectroscopic analysis method makes the coloration mechanism of charge transfer that is difficult to be uncovered to be clearly revealed, which will provide an available material and chemical method to investigate the structure-property relationship for gems as well as reveal the genesis of beautiful colors.

Dumortierite and tourmaline from the Barchi‐Kol diamond‐bearing kyanite gneisses (Kokchetav massif): A Raman spectroscopic study and petrological implications

AbstractTwo minerals of boron, dumortierite and tourmaline, have been studied in a diamondiferous kyanite gneiss (Barchi‐Kol area, Kokchetav massif, Northern Kazakhstan) by Raman spectroscopy and electron microprobe (EMP) analysis. Dumortierite and tourmaline (almost pure schorl end‐member) coexist within the sample, with blue/purple dumortierite inclusions hosted by kyanite porphyroblasts and blue tourmaline crystals occurring as inclusions in garnet. Optically, dumortierite appears almost identical to kyanite host and can be easily overlooked in thin sections during optical and scanning electron microscopic observations. Both boron minerals show a very strong anisotropy, and the most prominent Raman spectra can be obtained if the electric field vector E is parallel to the c axis of the dumortierite and tourmaline crystals. The Raman spectra of the investigated dumortierite are very similar to those of the synthetic dumortierite with a pronounced excess of boron (up to 0.26 [4]B), although the EMP examination revealed only one grain of the natural dumortierite studied here to have a very low boron surplus (up to 0.03 [4]B). Neither Raman spectroscopic study nor EMP analysis detected [4]B in tourmaline. The findings of dumortierite and tourmaline in the diamondiferous gneisses provide important implications for the presence of a boron‐rich fluid or melt during subduction/exhumation processes.

Secondary blue tourmaline after garnet from elbaite-subtype pegmatites; implications for source and behavior of Ca and Mg in fluids

Secondary blue tourmaline after garnet from elbaite-subtype pegmatites; implications for source and behavior of Ca and Mg in fluids

Iron-Bearing Pollucite and Tourmaline Replacement of Garnet In the Garnet Line In the Mt. Mica and Havey Pegmatites, Western Maine

The garnet line is a planar fabric occurring within the footwall wall zone just beneath the core zone in many of the western Maine pegmatites. This study focuses on the mineralogical, textural, and chemical characteristics of the garnet line within the Mt. Mica pegmatite, Oxford Co., Maine and a similar, but less pronounced, garnet line feature at the Havey pegmatite, Androscoggin Co., Maine. Within garnet lines of both pegmatites there is evidence of multiple stages of crystallization. The first stage resulted in formation of the garnet layer, which hosts an abundance of high field strength element (HFSE)-bearing micro-inclusions (uraninite, zircon, and columbite group and microlite supergroup species) and rare earth element (REE)-bearing phosphate minerals [monazite-(Ce) and xenotime-(Y)]. Field relations suggest that this is a large-scale oscillatory nucleation feature that is similar to the line rock in San Diego Co., California pegmatites described by [Webber et al . (1997)][1]. The second stage of crystallization is inferred to be post-magmatic, related to a late-stage hydrothermal fluid which metasomatically altered the garnets and produced black and blue tourmaline rims. These late-stage fluids are inferred to have originated and migrated from adjacent highly evolved miarolitic cavities. The fluids are also responsible for presence of Cs-rich micas and Fe-rich pollucite in the fractured portions of garnets in the garnet line. [1]: #ref-29

The diversity of B-isotope variations in tourmaline from rare-element pegmatites in the Borborema Province of Brazil

Tourmaline chemical and B-isotope compositions have been determined from 4 rare-element pegmatites and 1 barren pegmatite in the Borborema Pegmatite Province of NE Brazil, and also from a leucogranite considered to be parental to the pegmatites. The rare-element pegmatites are of the LCT (lithium–cesium–tantalum) class and they display a classic zoning structure with tourmaline present in all units but the quartz core. Tourmaline major-element composition varies systematically from schorl–dravite in the border and wall zones to varicolored elbaite in the inner zones, consistent with increasing differentiation of a pegmatite magma. Previous studies supposed a continuous variation in Li contents from schorl–dravite to elbaite but the SIMS data instead show a major gap between 0.07wt.% and 0.72wt.% Li2O, followed by a continuous variation to the maximum value of 1.8wt.%. The barren pegmatite and leucogranite contain schorl–dravite only.The full range of δ11B values in tourmaline from the Borborema pegmatites is from −20.2 to +1.6‰ (n=62), which comes close to the total known diversity of pegmatite tourmalines worldwide and is much broader than the range of the associated leucogranite (−15.1 to −13.4‰, n=3). However, more than 80% of the pegmatite tourmalines define a “main range” between −17 and −9‰ δ11B. The average value of the latter (−13.5‰, s.d.=2.3‰, n=50) is identical within error with the leucogranite, supporting suggestions of a genetic relationship. The B-isotope compositions of barren and mineralized pegmatites overlap within the main range. We find no correlation between chemical composition and B-isotope ratios. Famous for the Borboreman province are gem-quality crystals of blue “Paraíba type” elbaite. The δ11B values of blue tourmaline from the Capoeira pegmatite also fall within the main range (−16.4 to −11.9‰, n=5).Four of the 5 pegmatites studied are heterogeneous in δ11B, with typical variations of 6‰. However, there are no consistent trends in these examples from outer to inner units or from core to rim of zoned crystals that would fit any one scenario of melt evolution. Instead, the factors controlling B-isotope evolution, e.g., crystallization, fluid exsolution or mixing with external B reservoirs, appear to vary locally and can be specific to the individual pegmatite or internal zone within it. For example, the Carrascão pegmatite shows a shift from higher to lower δ11B values in tourmaline from the outer to inner zone (−14.9 to −9.1‰ vs. −20.2 to −13.9‰, resp.), and there is a parallel trend of lighter rims in zoned elbaite from the inner intemediate zone. These trends are consistent with 11B removal by tourmaline crystallization and/or fluid exsolution. An opposite trend in isotopic variation was found in zoned tourmaline from the wall zone of the Boqueirão pegmatite, whose core compositions are in the main range (−15.1 to −14.9‰) and whose rims are much heavier (−5.5 to −1.7‰). Similarly high δ11B values, about 10‰ beyond the main range, were found in wall-zone tourmaline in the barren Fazenda Turmalina pegmatite (−6.0 to +1.6‰). The patchy distribution of heavy B-isotopic compositions, found only in outer zones of pegmatites, and the strong isotopic contrast with the main range suggest the possibility of mixing with heavy B from the country rocks, a potential source being layers of marble and calc-silicate rocks.

Aluminous and alkali-deficient tourmaline from the Singhbhum Shear Zone, East Indian shield: Insight for polyphase boron infiltration during regional metamorphism

In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing veins of variable thickness (tens of micrometers to 1 m thick) are hosted in kyanite-quartzite and kyanite-mica schist. The veins have been classified into three types, which are, from oldest to youngest, generation I (tourmaline), II (dumortierite + tourmaline), and III (tourmaline) veins. Alkali- and Mg-rich tourmaline [ X Mg = Mg/(Mg + Fe) = 0.68 ± 0.09; X = Na, Ca, K, □ (vacancy) = 0.40 ± 0.12] is the sole borosilicate in generation I veins, which have been folded in response to regional deformation. Generation II veins were emplaced along shear bands (1 mm to 1 m thick) developed parallel to the axial planes of these folds. Long axes of fibrous dumortierite and prismatic tourmaline of generation II veins are oriented along the shear bands and have been bent around lenticular remnants of host kyanite-quartzite. Generation III veins have a dendritic pattern, crosscut generation II veins and show aggregates of fibrous to acicular tourmaline. Prismatic tourmaline in generation II veins is optically zoned with a green tourmaline core that is variably replaced and rimmed by blue tourmaline. Fibrous to acicular tourmaline in generation III veins is comprised up of blue tourmaline with compositions similar to the rim composition of prismatic tourmaline in generation II veins. Green and blue tourmaline is aluminous (Al total >7 apfu) and alkali-deficient (X = 0.71 ± 0.08). High Y Al content, high X, low X Mg (0.19 ± 0.10), and excess cation charge indicate tourmaline in generation II veins is rich in an “oxy-foitite” component. Foitite-rich tourmaline in generation III veins has tetrahedral Al and a slightly lower Mg-content and X than those of generation II veins. Optical zoning in prismatic tourmaline corresponds to an abrupt compositional change with paragenetically older green tourmaline having higher Al and X Mg , but lower alkali content in the X-site than the blue tourmaline rim. The compositional variation in green and blue tourmaline can be explained by a combination of coupled substitutions represented by AlO[R(OH)] −1 and Al(NaR) −1 , where R = (Fe 2+ + Mg). Pseudosections in the system Na 2 O-K 2 O-Al 2 O 3 -SiO 2 -H 2 O constructed from bulk chemical compositions of the studied rocks and the P-T slopes of two isochors computed from brine-rich inclusions trapped in quartz grains indicate that borosilicate formation in generation II and III veins occurred within 4.1 ± 0.5 kbar and 377 ± 21 °C. The mineral assemblages and textures suggest that the borosilicate-bearing veins formed from infiltration-driven alteration of host kyanite-quartzite and kyanite-mica schist along structurally controlled conduits by more than one batch of chemically distinct boron-rich aqueous fluids.

Skeletal tourmaline, undercooling, and crystallization history of the Stone Mountain granite, Georgia, U.S.A.

The Stone Mountain pluton, in Georgia, is composed of tourmaline-bearing biotite–muscovite granite. The pluton was intruded at midcrustal depths and is cut by late-stage dikes. Tourmaline occurs as large skeletal crystals in the granite in the same areas as late-stage aplite and pegmatite–aplite dikes. Skeletal tourmaline also occurs in the aplite zone of pegmatite–aplite dikes. Pegmatites and thin feldspar – quartz – tourmaline veins contain euhedral tourmaline. Isolated skeletal crystals of tourmaline in the granite and tourmaline-bearing dikes are surrounded by a fine-grained leucocratic, Fe-depleted halo of feldspar and quartz. Electron-microprobe analyses show that all of the tourmaline consists of schorl. Skeletal tourmaline shows an early brown to late blue pleochroism. Most euhedral tourmaline is uniformly brown, but a few grains from pegmatite have a blue core and a brown rim. Brown tourmaline is high in Ti (>0.1 apfu ), whereas the blue tourmaline is lower in Ti ( apfu ). There is little compositional variation among the different textures of the tourmaline. These textures indicate an initial crystallization at undercoolings of 40–100°C. A boron-enriched fluid reacted with the existing granite to form skeletal tourmaline with a Fe-depleted halo. In the pegmatite–aplite dikes, the B-enriched fluid (melt) segregated along the outer margins of the dikes and crystallized skeletal tourmaline from an undercooled melt. Removal of B from the melt raised the solidus temperature and caused the aplite that encloses the skeletal tourmaline to form. Crystallization of the remaining melt at lower undercoolings produced euhedral tourmaline of the pegmatite and veins. The development of skeletal crystals of tourmaline at Stone Mountain indicates that undercooling is an important process in some midcrustal granite systems.

Gemologic Characterization and Mineral Chemistry of Blue Tourmalines from the Peg-Matitic Body of Bom Jesus, Banabuiú - Ceará, Northeast Brazil

Gemologic Characterization and Mineral Chemistry of Blue Tourmalines from the Peg-Matitic Body of Bom Jesus, Banabuiú - Ceará, Northeast Brazil

Origin of quartz cores in tourmaline from Roche Rock, SW England

AbstractThe nature and mode of origin of quartz-cored tourmalines (QCT) are studied from hydrothermal quartz veins within massive quartz-tourmaline (MQT) rocks at Roche, SW England. The QCT are annular, have blue maximum absorption colour and occur together with tourmalines with brown cores rimmed by blue tourmaline. Where the quartz core is not continuous throughout the length of the QCT crystals, the tourmaline core has brown maximum absorption colour, similar to tourmalines without quartz cores. Both the blue and brown tourmalines are schorl, but are compositionally distinct showing different Fe/(Mg + Ti) ratios and Ca concentrations. Fluid inclusion data indicate quartz precipitation from a moderate salinity (∼20–25 wt.% NaCl eq.) brine which periodically boiled following pressure drops within the vein system. The QCT show rheomorphic and lobate textures on their inner margins indicating selective dissolution of their brown, relative Mg-, Ti- and Ca-rich tourmaline cores and replacement with quartz. This presents a problem in terms of the nature of the fluid responsible for such selective dissolution because tourmaline is generally highly resistant under the normal range of hydrothermal fluid conditions. It is proposed that the relatively high concentrations of Ti, Mg and Ca in the brown tourmaline caused significant lattice strain, which together with an increase in pH, and probably Al, in the boiling hydrothermal fluid caused the brown cores to become unstable compared with the Fe-rich blue tourmaline rims.

FENÔMENOS DE ORDEM-DESORDEM LOCAL EM TURMALINAS NATURAIS E TRATADAS DA SÉRIE SCHORLITA-ELBAÍTA

We report a comparative study between natural and treated green, blue and black color tourmalines from granitic pegmatites of the Minas Gerais state, Brazil, using Electron Microprobe, Electron Paramagnetic Resonance (EPR) and Mössbauer spectroscopy. Electron Microprobe analysis confirms that all studied samples are schorl species. The Electron Paramagnetic Resonance spectra of green and blue tourmaline are characterized by line widths of about 1000 Gauss. These unusual broad lines are probably due to spin-spin interactions caused by the high Fe concentrations and structural disorder. Mössbauer spectra collected at room temperature confirm that iron is dominantly present as Fe 2+ . It is incorporated at two or three different Y sites for the green and blue tourmalines, respectively. From those features, we conclude that blue tourmalines are characterized by a high degree of disorder, whereas a smaller degree of disorder is observed in green tourmalines. In addition, heat treatments in oxidizing and reducing atmospheres yielded structural readjustments at the Fe sites, de-hydroxylation, oxidation and generation of a second, different phase.

FENÔMENOS DE ORDEM-DESORDEM LOCAL EM TURMALINAS NATURAIS E TRATADAS DA SÉRIE SCHORLITA-ELBAÍTA

We report a comparative study between natural and treated green, blue and black color tourmalines from granitic pegmatites of the Minas Gerais state, Brazil, using Electron Microprobe, Electron Paramagnetic Resonance (EPR) and Mossbauer spectroscopy. Electron Microprobe analysis confirms that all studied samples are schorl species. The Electron Paramagnetic Resonance spectra of green and blue tourmaline are characterized by line widths of about 1000 Gauss. These unusual broad lines are probably due to spin-spin interactions caused by the high Fe concentrations and structural disorder. Mossbauer spectra collected at room temperature confirm that iron is dominantly present as Fe 2+ . It is incorporated at two or three different Y sites for the green and blue tourmalines, respectively. From those features, we conclude that blue tourmalines are characterized by a high degree of disorder, whereas a smaller degree of disorder is observed in green tourmalines. In addition, heat treatments in oxidizing and reducing atmospheres yielded structural readjustments at the Fe sites, de-hydroxylation, oxidation and generation of a second, different phase.

Fluid inclusion microthermometry in Be–Ta–(Li–Sn)-bearing pegmatites from the Borborema Province, Northeast Brazil

Fluid inclusions in Be–Ta–Li–Sn-bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low-salinity aqueous or aqueous–carbonic fluids. The evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare-element pegmatites do not seem to be unique. The aim of this work is to contribute to this discussion, on the basis of fluid inclusion information from the Borborema Pegmatite Province (BPP) in Northeast Brazil. Some well-known heterogeneous Be–Ta–(Li–Sn) mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones (I, muscovite-rich contact zone; II, homogeneous or graphic pegmatite; III, blocky feldspar zone; and IV, quartz core). The observed fluid inclusions (mainly in quartz) were almost always secondary or pseudosecondary, grouped along healed fractures with random orientation. Even some trails paralleling quartz grain limits were not undoubtedly primary. Almost all these fluid inclusions were filled with low-salinity aqueous or aqueous–carbonic fluids. Primary inclusions were recognized only in some well-formed crystals from druses with erratic distribution in the pegmatites. Microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low-salinity aqueous or aqueous–carbonic fluids (with insignificant N 2 or CH 4 contents) with frequent accidentally trapped solids. The various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections. The relative age of the crystals hosting primary fluid inclusions, the relative age of primary inclusion types in successive crystal-growth zones, the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization. An early low-salinity aqueous–carbonic fluid with CO 2 contents of 20 mol% was present during the crystallization of early minerals, such as garnet and tourmaline of the border zone and graphic quartz, beryl, manganotantalite, and blue tourmaline of zones II and III. The end of zone III crystallization occurred in the presence of a low-salinity, aqueous fluid with very low CO 2 contents, as indicated by inclusions in the smoky core of quartz crystals at the limit between zones III and IV. Later aqueous fluids barren in CO 2 and with salinities increasing from 5% to 20% coexisted with the formation of the pegmatite nucleus and replacement bodies, as indicated by inclusions in the border of these crystals and in euclase. The estimated trapping conditions of 3.5 kbar and 580°C for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare-element-bearing pegmatite crystallization. The trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core, at 3.5 kbar and 400°C are slightly below the conditions admitted for the solidus of rare-element pegmatites and indicate an isobaric cooling during pegmatite crystallization.

Transparent bright blue Cu-bearing tourmalines from Paraiba, Brazil

AbstractPhysical, crystal-chemical and absorption spectroscopical properties of blue tourmalines from a new occurrence in the State of Paraiba, Brazil, are described. Refractive indices, birefringence and specific gravity have been determined as ne = 1.615–1.620, no = 1.632–1.640, Δn=−0.017 to −0.020 and 3.04–3.07g/cm3. Microprobe analyses proved the samples to be elbaites, which are relatively Mn-rich and contain 2.1 wt. % CuO and 0.5 wt. % Bi2O3. Lattice constants are ao = 15.854(4) and co = 7.102(1) Å. The blue colour is due to pleochroic absorption bands of Cu2+ and Mn3+ with maxima at 700 and 520 nm respectively.

Fluid inclusion study of gem tourmaline from Xinjiang, China

Reported in this paper are the results of geological and fluid inclusion studies of gem-grade tourmaline in pegmatite veins from Altay, Xinjiang. The homogenization temperatures of fluid inclusions in the tourmaline range from 230° to 350°C with a salinity of 0.2−4.5 NaCl% equiv. The composition of fluids corresponds to the NaCl-CaCl2-H2O system in terms of the measurements of first melting temperature of fluid inclusions. By comparing the homogenization temperatures of different color zones in the gem tourmaline, it is suggested that it has undergone an intermittent growth process. Careful observation of the section parallel to thec axis of greenish blue tourmaline provides evidence of the occurrence of cat-eye tourmaline similar to that found in Brazil and other parts of the world. Attention should be paid to such kind of tourmaline as viewed from its economic importance.

New investigations of the strewn field of Libyan desert glass and its petrography

Twenty-six specimens of Libyan Desert glass were collected from the westernmost area of Libyan Desert glass occurrence during a joint expedition, 30 May to 5 June, 1971, of The University of Libya and The University of Texas. The Oasis circular structure, a suspected astrobleme southwest of the glass site, was also visited. In the glass, included minerals are partially altered through shock and temperature effects; microprobe analysis confirms one of these minerals as zircon and indicates that another is the elbaite variety of tourmaline. Tourmaline of the elbaite variety and zircon were also identified in a thin section of an analyzed sample of high-silica (99.6%) Nubian Sandstone collected in the area of glass occurrence. This silica content is higher than that of any Libyan Desert glass yet examined. In a heavy mineral separation of this sandstone, the minerals cordierite; green, brown, and blue tourmaline; and minor amounts of staurolite, zircon, and kyanite were identified. It is concluded, from the mineral and chemical evidence, that Libyan Desert glass was derived from Nubian Sandstone. Whether there is any connection between the origin of Libyan Desert glass and the origin of the nearby impact structures is undetermined.

The crystal field spectra and dichroism of tourmaline

Optical absorption spectra of colorless, pink, green, blue, brown and black tourmaline have been taken to determine the origin of the colors and to explain the color changes which occur on heating. Absorption data were taken with polarized radiation parallel and perpendicular to the c axis at 77 and 300°K. Cell dimensions supported by partial chemical analyses were used to characterize the specimens. The data combined to show that generally speaking the color is due to electronic transitions in and the amount of Fe 2+, Fe 3+ and Mn 2+ in the crystal. Specimens on the dravite-sehorl join are characteristically colored shades of brown. With small to moderate concentrations of transition metal ions, colors on the elbaite-schorl join are clear pinks, greens and blues according to the Fe 2+ (Fe 2++ Fe 3+ + Mn 2+) ratio in the tourmaline. Intensity changes of absorption peaks and shifts in the absorption edges, correlated with removal of hydrogen and consequent oxidation of Fe 2+ to Fe 3+ in the structure, account for color changes observed when tourmaline is heated in air. Blue tourmaline becomes green on short-term heating and redish-brown on further heating. Assignments of absorption peaks from 0·3 to 2·0μ to specific transitions in the appropriate cations have been made and the origin of the characteristic dichroism is discussed.