Tourmaline Crystals Research Articles

Metasomatic tourmaline in hybrid contact-bands between gneiss and peridotite in the Ulten zone of the Eastern Italian Alps: chemistry and boron isotopic composition

Ultramafic rocks in the Ulten Zone (UZ) high-pressure melange, in the Eastern Italian Alps, embedded in crustal gneisses, preserve a series of metasomatic hybrid bands generated by infiltration of aqueous fluids. These hybrid bands at the peridotite–gneiss contact comprise phlogopite and anthophyllite with accessory zircon, apatite and allanite. Here, we report the rare occurrence of centimeter-sized crystals of metasomatic tourmaline in two different UZ outcrops, at Mt. Hochwart and Malga Preghena. Tourmaline has a dravite–uvite-rich composition and shows optical and chemical zoning, with slightly variable Ti, Ca and Fe contents. Secondary-ion mass spectrometry (SIMS) analyses provide information on the intragrain B-isotopic zonation of the grains, which is correlated with optical and chemical zoning. The range in δ 11 B is from −7.4 to +0.3‰ for the Mt. Hochwart tourmaline, generally with an isotopically lighter core and heavier rim. The δ 11 B range for Malga Preghena tourmaline is from +1.2‰ in the core to −3.9‰ in the rim. Variable proportions of fluids derived from both the local crustal gneisses and a subducting slab control the B-isotope composition in the metasomatic tourmaline from the UZ. Fluid mixing in hybrid crustal–ultramafic bands can be monitored by metasomatic tourmaline, which may also have a significant impact on the B isotope budget of the subducted slab and the slab–mantle interface.

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.

Synkinematic growth of tourmaline on brittle-ductile normal faults, Despotiko Island, Aegean Sea, Greece

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 μm in length and 50 μm in diameter, form slickenside fibers on these fault planes and therefore have grown during fault slip. They record a chemical variation from X (Na 0.66 Ca 0.20 □ 0.14 ) Y (Fe 2+ 1.17 Mg 0.92 Al 0.78 Ti 4+ 0.12 Mn 2+ 0.01 ) Z (Al 5.00 Mg 1.00 ) (BO 3 ) 3 T (Si 5.95 Al 0.05 )O 18 V (OH) 3 W [(OH) 0.88 F 0.12 ] to X (□ 0.50 Na 0.48 Ca 0.02 ) Y (Fe 2+ 1.75 Al 1.31 Mn 2+ 0.02 Ti 4+ 0.01 ) Z (Al 5.49 Mg 0.51 ) (BO 3 ) 3 T (Si 5.98 Al 0.02 )O 18 V (OH) 3 W (OH). The tourmaline compositions belong to the foitite – schorl – dravite series. On the basis of X □− X Mg , X □−Ca, Ca– X Mg 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°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.

Chemical and growth zoning in trapiche tourmaline from Zambia — a re-evaluation

Trapiche tourmalines from Zambia show the hexagonal prism a {112 - 0}, the positive pyramids r {101 - 1} and o {022 - 1} and the negative pyramid -r {011 - 1 } as dominant external crystal forms. The trapiche pattern is formed by the positioning of liquid and solid inclusions which are trapped at growth boundaries between pyramidal and prismatic growth sectors. Growth-induced elongate voids are also formed perpendicular to the dominant growth faces, which are inclined at 27°, at 46° or at 90° to the pedion {0001} of the tourmaline hosts. All the trapiche tourmalines are chemically zoned with isomorphic substitutions of Ca by Na and Mg by Al, which is characteristic for tourmalines of the uvite-dravite solid solution series. Content of Na increases and of Ca decreases from core to rim of all tourmalines. However, at the boundaries between different growth sectors (forming the geometric trapiche pattern of the samples), tourmaline compositions with higher Na and lower Ca contents than measured in adjacent pyramidal and prismatic growth sectors were found. The formation of this complex structural and chemical pattern is consistent with the formation of the tourmaline crystals in a two step process, with the skeletal growth of a sodium-rich dravite in a fi rst step and the subsequent, layer-by-layer growth of a second tourmaline generation. The growth of the second generation started with calcium-rich fl uor-uvite and was followed step-by-step by a more sodium-rich dravite. The green coloration of the samples is caused by minor contents of vanadium and chromium.

Optical absorption spectra of tourmaline crystals from Altay, China

Tourmaline is an important functional and gem material. The current study examines pink, green, and brownish-green tourmalines from Altay deposit. Based on X-ray fluorescence (XRF) quantitative analyses and ultrariolet-visible-near-infrared (UV-VIS-NIR) spectral analyses in combination with annealing experiments, the color center of tourmaline is found to be related to the d-d transitions of ions or the d-d transitions of exchange coupled ions. Annealing treatment affects the color improvement of tourmaline crystals.

Growth of tourmaline single crystals containing transition metal elements in hydrothermal solutions

Interest in the growth of tourmaline single crystals is based on the promising piezoelectric and pyroelectric properties of this material compared to quartz crystals currently in use. Moreover, synthetic tourmaline can be used as a substitute for the natural stone in the jewelry industry similar to other synthetic analogues of gemstones. Single crystals of colored Co-, Ni-, Fe-, (Ni,Cr)-, (Ni,Fe)-, and (Co,Ni,Cr)-containing tourmalines with concentration of transition metal elements up to 16wt% on a seed have been grown from complex boron-containing hydrothermal solutions at a range of temperatures 400–750°C and pressures 100MPa. Experiments were conducted under conditions of a thermal gradient in titanium and chromium–nickel autoclaves. Tourmaline growth on a seed crystal occurs only if separate tourmaline-forming components (monocrystalline corundum and quartz bars) are used as charge. All tourmalines specified above grow in analogous (+) direction of the optical axis with a speed of 0.05mm/day by faces of the trigonal pyramid, except tourmalines containing chromium. They grow in analogous (+0001) direction with a speed 0.05mm/day, and in antilogous (−0001) direction with a speed of 0.01mm/day by faces of the trigonal pyramid and in prism direction with a speed of 0.001mm/day. Along with the large single crystals, a great amount of finest (30–150μm in size) tourmaline crystals was formed during the runs by spontaneous nucleation both on the surface of the seed crystals and in the charge.

Influence of Polar Tourmaline Substrates on the Growth of ZnO Nanoplates

Nanoplate-shaped ZnO thin film was achieved by the ultrasonic spray pyrolysis technique,using polar(0001) tourmaline wafers as the growing substrates.The as-prepared nanoplates were cross-linked and upright-standing.XRD and Raman tests revealed the hexagonal structure of wurtzite.The electron probe microanalysis and Mssbauer spectroscope were utilized to explore the chemical composition and the fine structures of Fe ions in tourmaline substrates.The plate-like growth of ZnO nanocrystals are related to the surface electric field effect of tourmaline crystals.With the variation of Fe ions contents,valence states and occupation characters,the ratio of thickness to diameter of the ZnO nanoplates decreases with the increase of the intrinsic dipole moments along the c axis and the strengthening of surface electric field effect in tourmaline crystals.

Cr-bearing tourmaline associated with emerald deposits from Swat, NW Pakistan: Genesis and its exploration significance

Carbonate-talc altered ultramafic (and mafic) rocks of the obducted oceanic lithosphere along the Main Mantle Thrust in the Swat Valley host emerald deposits and are invaded by quartz veins and stockworks. Both the quartz veins and carbonate-talc altered rocks contain discrete grains and clusters of tourmaline crystals, which may be associated with emerald and/or chromian muscovite. Tourmalines from the Mingora and Gujar Kili emerald mines and the Spin Obo-Kuh area are Nibearing, Cr-rich dravites and “oxy-dravites” that exhibit substantial chemical variability with 2.0-2.5 atoms per formula unit (apfu) Mg, relatively low Al (4.8-5.8 apfu), high Cr (0.5-1.8 apfu), high Cr/(Cr+Al) ratios of 0.08-0.28, significant amounts of Ni (up to 0.11 apfu), and variable calculated W site O2- (0.2-0.8 apfu). The mechanisms that appear to control the tourmaline chemistry at these localites are combinations of the dominant substitution Al = Cr with lesser amounts of the Mg+OH1- = (Al,Cr)+O2- and Al3+ = Fe3+ substitutions. Relative to the Mingora, Gujar Kili, and Spin Obo-Kuh areas, the Charbagh tourmalines are “oxy-dravites” that have comparable Mg contents (2.2-2.3 apfu) with variable calculated W site O2- (0.6-0.9 apfu), but are more Fe-rich (0.5 apfu) and Al-rich (6.0-6.3 apfu) with low Cr (0.00-0.01 apfu) and Ni contents (0.01-0.02 apfu). The mechanism that appears to control the variable chemistry of the Charbagh tourmalines is primarily Mg+OH1- = Al+O2-. The chemical characteristics of tourmalines from the various localities relate most clearly to differences in host rock chemistry.

Watermelon tourmaline from the Paprok mine (Nuristan, Afghanistan)

The “watermelon” tourmaline from the Paprok mine (Nuristan, Afghanistan) shows different tourmaline components. Four zones differ widely what is refl ected by different chemistry and various colours (pink core and green rim). All zones show a relatively low Mn content (~0.05 apfu) but different Fe contents. While Fe is in the (pink) core at the detection limit, it increases signifi cantly in the outer (green coloured) zones. In the core (zone I) “fl uor-elbaite” could be identifi ed for the fi rst time for the Nuristan region. Tourmaline compositions of this zone show an elbaitic component of ~55 mol%, ~30 mol% rossmaniteand ~15 mol% liddicoatite-component. Zone II shows an increasing schorland rossmanite-component (up to 40 mol%), while the liddicoatiteand elbaite-components are decreasing. The intermediate zone III shows the highest schorl-component of all zones (up to ~11 mol%), while the elbaite-component is decreasing. In the rim (zone IV) the schorl-component decreases to ~6 mol%, while simultaneously the elbaite-component increases. Our investigation shows that during tourmaline crystallization only the amounts of Fe and Li, which were available for the tourmaline crystallisation, have changed to a signifi cant degree. We believe that at beginning of the crystallisation of the Fe-bearing zones the formation of Fe-rich tourmalines (schorl, foitite) in this pegmatite was in a fi nal stage and therefore in this pegmatitic system Fe increasingly was available. The Mn/(Mn+Fe) ration ranges from ~0.10 to ~1.00. The presence of different mineral inclusions such as stannite and Ca carbonates point to a hydrothermal origin of this tourmaline.

Experimental study of stability and crystallization peculiarities of tourmaline in hydrothermal conditions

Tourmaline is a polygenetic mineral and often occurs in pegmatite and pneumatolyte‐hydrothermal (metasomatic) and metamorphic rocks as a rock-forming mineral [1]. Wide varieties of the tourmaline chemical composition, often within the limits of even one deposit, make it a good indication of the changing physicochemical conditions of mineral formation. Crystals of tourmaline formed in rare metal pegmatites, greisens, and hydrothermal veins are characterized by the extremely great varieties of composition. More than 14 mineral varieties of tourmaline have been identified [2], which is connected with the peculiarities of its structure. Its generalized crystal chemistry formula is the following: XY 3 Z 6 Si 6 O 18 (BO 3 ) 3 V 3 W , where X— Na + ,

EMERALD MINERALIZATION ASSOCIATED WITH THE MAVIS LAKE PEGMATITE GROUP, NEAR DRYDEN, ONTARIO

The Taylor 2 (also known as Ghost Lake) emerald showing near Dryden, in northwestern Ontario, is associated with a granitic pegmatite of the Mavis Lake Pegmatite Group proximal to the 2685 Ma Ghost Lake Batholith. The Taylor 2 pegmatite consists of three separate limbs that intruded a wide zone of chlorite schist near the eastern end of an altered ultramafic sill. Most of the beryl and emerald occurs in a “zone of mixing” between the southern and central limbs of the pegmatite. The rock in the “zone of mixing” consists of relict orange K-feldspar crystals (to 30 cm) in a matrix of anhedral bluish plagioclase, quartz, fine-grained black phlogopite, blue apatite crystals (to 1 cm), and black tourmaline crystals (to 2 cm). The beryl occurs as euhedral crystals up to 2.3 × 1.8 cm; most are opaque to translucent and white to pale green in color, but about 10% are emerald. The emerald compositions show average Cr concentrations of 0.25 wt.% Cr2O3 (maximum 0.42 wt.% Cr2O3 or 0.03 Cr apfu ) and maximum V concentrations of 0.05 wt.% V2O3. Primary fluid inclusions in the emerald crystals show variable compositions and high temperatures of homogenization (to 490°C). Oxygen isotope compositions of the emerald crystals, which average 9.6(7)‰, suggest a high degree of homogenization and fluid buffering during fluid–rock interaction. Whole-rock compositions show high concentrations of Be (89 ppm) in the the Taylor 2 pegmatite and elevated Cr in the chlorite schist (2610 ppm) and the altered ultramafic sill (3050 ppm). Geochemical similarities support the hypothesis that the chlorite schist is the faulted analogue of the altered ultramafic sill. The absence of beryl in the latter unit may be due to lower amounts of fluid or F concentrations (~150 ppm versus ~1300 ppm for the chlorite schist). The emerald in the Taylor 2 showing and the associated phlogopite most likely formed through metasomatism of the chlorite schist, which was driven by granitic magmatism.

Experimental growth and structural-morphological characteristics of co-tourmaline

Experiments aimed at investigating opportunities of Co-tourmaline growth onto a seed crystal in aqueous boron-chloride cobalt-containing solutions under temperatures of 400‐500 ° C and pressure up to 1000 bar were performed. Experiments were conducted under thermogradient conditions in titanium autoclaves, and in several cases with the help of a dividing diaphragm. Tourmaline growth onto a seed crystal occurs only with the use of separate tourmaline-forming components (monocrystal corundum and quartz rods) as the melting stock. The layer (up to 600 micron) of Co-tourmaline grown is intensively crimson-colored and occurs only at the face of positive monohedron {+0001}. Along with the monocrystal outgrowth, a great amount of very fine (30‐150 micron in size) tourmaline crystals of selfgeneration nucleation occurred on the seed crystals and in the melting stock. Their composition does not differ from the grown layer composition, and their unit cell parameters ( a = 15.82638 ± 0.00215 A, c = 7.09434 ± 0.00186 A) correspond to intermediate values of the tourmaline group minerals olenite and elbaite. Tourmaline is a complex borosilicate of the general crystal chemical formula XYZ , where X corresponds to large voids filled with Na, Ca, K, or a vacancy; Y corresponds to closely packed octahedrons of Mg, Fe 2+ , Li, Al, Mn, and Cr; and Z corresponds to deformed octahedrons of Al and Fe 3+ . In nature tourmaline rather often occurs in granite pegmatites and pneumatolyte‐hydrothermal and hydrothermal formations. Its presence is not unusual in such granites as well as in metasomatites and metamorphic rocks. This indicates the diversity of the tourmaline formation conditions. Interest in growing tourmaline monocrystals is connected with the task of obtaining new promising piezoelectric and pyroelectric materials possessing enhanced characteristics in comparison with those already known. Moreover, synthetic tourmaline can be used as a substitute for the natural form in the jewelry industry similar to other synthetic analogues of precious stones. There are many questions concerning the tourmaline formation conditions in nature that have remained unclear up until the present despite numerous geological, mineralogical, geochemical, and crystal chemical investigations. Safe and reproducible methods of growing its monocrystals have also not been developed. This situation is connected in general with the tourmaline complex chemical composition resulting in the existence of numerous varieties according to shades of color, which often change even within the single crystal. Obviously, this is a consequence of complex and changing physicochemical crystallization conditions of this mineral during its growth as well as special predisposition of its structure to a wide exhibition of iso- and heterovalent isomorphism. Investigations in the sphere of synthesis and growth

La nature des fluides hydrothermaux des tourmalinites du massif granitique permien du Ment (Maroc central). Couplage de la minéralogie des tourmalines et de l'étude des inclusions fluides

The Permian Ment massif is famous for its hydrothermal alterations. The massif comprises a main intrusion of biotite granodiorite and several minor peraluminous granite plutons, both affected by the whole spectrum of alterations. In addition, the surrounding Namurian-Westphalian flyschs display huge bodies of massive tourmalinites. Two alteration styles are recognized : (i) early quartz-stable associations (greisens, quartz-tourmalinites) developed at ca. 500 °C and 50 MPa from C02-rich C-O-H-N fluids, and (ii) later quartz-unstable assemblages (muscovitites, tourmalinites). Late stage hydrothermalism was studied combining a classical microthermometric fluid inclusion study and a detailed mineralogi-cal study of zoned tourmaline crystals (SEM, electron microprobe). Late tourmalinisation occurred in a high enthalpy geothermal system at low pressure and high temperature (ca. 400 ° to 200 °C, 30 to 10 MPa), and was driven by mixing processes between threee hydrothermal end-members : a magmatic fluid, a fluid derived from the metamorphic surrounding rocks and a surficial fluid infiltered at various depths in the granite masses and experiencing re-equilibration with the granite rocks. Mixing proportions were extremely variable in time and space, being controlled by episodic tectonic activity.

Chemical composition of tourmaline from the Asarcık Pb–Zn–Cu ± U deposit, Şebinkarahisar, Turkey

Abundant tourmaline, commonly as rosettes up to 1.5 cm in diameter, is present in the Upper Cretaceous Asarcik granitoid (Şebinkarahisar), Northeastern Turkey. The tourmalines also occur in quartz–tourmaline veins up to 4 m thick that cut the Asarcik granitoid. This granitoid was emplaced in the eastern Pontide Metallogenic Belt, where accompanying tourmalines are found in close association with a vein-type, uranium-bearing Pb–Zn–Cu deposit. Tourmaline crystals are generally optically zoned from core to rim with a bluish green core. Electron-microprobe studies show that tourmalines from the Asarcik granitoid and quartz–tourmaline veins display mainly dravite–schorl solid solutions with a tendency to schorl compositions. The tourmaline from the Asarcik granitoid has Fe/(Fe + Mg) ratios from 0.28 to 0.69 (mean = 0.52) and Na/(Na + Ca) ratios from 0.57 to 0.93 (mean = 0.76). In contrast, tourmaline in the quartz–tourmaline veins has Fe/(Fe + Mg) ratio from 0.14 to 0.92 (mean = 0.59) and Na/(Na + Ca) ratio from 0.78 to 1.00 (mean = 0.96). Tourmaline in the veins has a more alkali-deficient trend than that in the granitoid. Substitution mechanisms for the chemical evolution of tourmalines at Asarcik are the MgFe−1, (Fe2+Fe3+)(MgAl)−1, □Al(NaR)−1, AlOR2+−1(OH)−1, □Al2ONa−1R2+−2(OH)−1, Ca0.5□0.5Na−1, CaMg2□−1Al−2, CaMgO□−1Al−1(OH)−1, CaMg3OH□−1Al−3O−1, CaMgNa−1Al−1, CaONa(OH)−1, CaMg2OHNa−1Al−2O−1, CaMg2□−1Al−2, and CaMg3OH□−1Al−3O−1 exchange vectors. Chemical zoning in the tourmalines is consistent with these substitutions. Stoichiometric calculations of electron-microprobe data in the Asarcik granitoid suggest a relatively oxidizing environment during tourmaline crystallization.

VIGEZZITE AND ASSOCIATED OXIDES OF Nb-Ta FROM EMERALD-BEARING PEGMATITES OF THE VIGEZZO VALLEY, WESTERN ALPS, ITALY

Two sequences of crystallization involving oxides of Nb–Ta occurred during the albitization process of three emerald-bearing pegmatites that cut serpentinites at Mount Pizzo Marcio and Alpe Rosso, in Vigezzo Valley, Western Alps, Italy: 1) a higher-temperature sequence, characterized by an alkaline micro-environment [high a (Na + )], allowing the crystallization of Mg-rich columbite, vigezzite, thorian vigezzite and U-rich microlite, and 2) a lower-temperature sequence, characterized by high a (Ca 2+ ), producing replacements of tapiolite-(Fe) and ferrowodginite by microlite, of tantalite-(Mn) by microlite and (later) fersmite, and of columbite-(Mn) by fersmite. Chromium-bearing beryl crystals (emerald) occur along the exocontacts characterized by a green to grey, sheared micaceous unit composed of phlogopite + talc + tremolite. Beryl crystals commonly show a green Cr-rich vitreous rim and a whitish milky core, partially replaced by beryllium silicates such as bavenite, bertrandite and bityite. Tourmaline crystals vary from schorl–dravite to Fe-rich dravite. In the model of evolution envisaged, we propose an anatectic origin of the pegmatite-forming melts during the latest stages of the Alpine tectonometamorphic event. The emplacement of the pegmatitic dikes in the serpentinites was followed by intense circulation of supercritical to hydrothermal fluids enriched in Mg, B and Be, and depleted in F, producing albitization of pegmatites and generating the aforementioned mineral reactions and replacements.

Chemical and boron isotopic variations of tourmaline in the Hnilec granite-related hydrothermal system, Slovakia: Constraints on magmatic and metamorphic fluid evolution

Multi-stage formation of tourmaline occurs in the Hnilec granite-related hydrothermal tin mineralisation system from the Western Carpathians, Slovakia. The tourmalines belong to the schorl–dravite series and have two major stages of formation: the majority crystallized during the first stage (defined as M-stage), forming zoned tourmaline crystals with the cores being generally more Fe, Al, and Mn rich than the rims. During the second stage (defined as L-stage), tourmaline formed as small veins or irregular patches along fractures and cracks in the M-stage tourmaline grains. In the contact metapelites near the granite body, the L-stage tourmalines are more Mg-rich and Fe, Al, Mn depleted than the M-stage tourmalines. In the granites, the L-stage tourmalines have generally similar compositions to those of the M-stage tourmaline rims. The boron isotopic compositions of the M-stage tourmalines vary from − 10.3‰ to − 15.4‰; with no clear variation between the cores and the rims, however, some of the tourmaline grains from the contact metapelites show a slightly higher δ 11B in the cores than in the rims. The L-stage tourmalines have lower δ 11B value of − 16.0‰ to − 17.1‰. We suggest that these trends reflect a changing fluid source from a dominant magmatic–hydrothermal fluid derived from the granites to a late-stage metamorphic fluid derived from the regional metamorphism (chlorite and biotite zone) in the metapelites. The significantly higher Fe 3+ in the L-stage than the M-stage tourmalines reflect changing redox conditions towards a more oxidising environment. This redox condition change may have important implications for the hydrothermal tin mineralisation in the area.

Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]fluid/[B]tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. 11B always preferentially fractionated into the fluid. For experiments where [B]fluid/[B]tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ11B(tour–fluid) = −4.20 · [1,000/T (K)] + 3.52; R2 = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]fluid/[B]tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.

CHEMICAL SUBSTITUTIONS IN OXIDIZED TOURMALINE IN GRANITE-RELATED MINERALIZED HYDROTHERMAL SYSTEMS, WESTERN TURKEY

Tourmaline is the most common borosilicate mineral in granite-related magmatic-hydrothermal ore deposits in the Aegean region of western Turkey. Hydrothermal deposits there include the mesothermal argentiferous Pb–Zn–Cu deposit at Kadikalesi (Bodrum–Mugla) and the porphyry Au deposit at Kisladag (Esme–Usak), with related potassic igneous rocks and accompanying Tertiary strata of western Anatolia. At Kadikalesi, mesothermal vein-type mineralization is linked to monzodioritic intrusions in a subvolcanic suite, which is overlain by volcanic rocks ranging in composition from calc-alkaline to potassic. In the non-brecciated hydrothermal systems, tourmaline occurs as subhedral to anhedral crystals in monzodiorite or as an essential mineral in veins and their reaction zones. At Kisladag, magmatic-hydrothermal brecciation associated with multiphase intrusions of latite porphyry is related to the Kisladag caldera. Tourmaline is an abundant and widespread alteration-induced mineral throughout the Kisladag porphyry system. Electron-microprobe analyses of the tourmaline crystals collected from both hydrothermal deposits show significant chemical variation within the range of buergerite compositions. The main substitution mechanism involves the CaFe 2+ [Na −1 (Al,Fe 3+ ) −1 ] exchange vector. Compositions deviate from ideal schorl–dravite along a trend that closely approximates the uvite exchange-vector. Mossbauer spectroscopy shows the presence of significant Fe 3+ in all the tourmaline crystals, suggesting a high oxidation state for the ore-forming fluids, and for related hydrothermal systems associated with emplacement of subvolcanic intrusions.

BEUSITE AND AN UNUSUAL Mn-RICH APATITE FROM THE SZKLARY GRANITIC PEGMATITE, LOWER SILESIA, SOUTHWESTERN POLAND

An assemblage of phosphate minerals, including beusite, Mn-rich fluorapatite, chlorapatite, hydroxylapatite and traces of alluaudite and mitridatite, usually accompanied by Mn-rich oxides also enriched in Ba, Ca, Mg and Ni, and Bi and Pb, has been recognized in a granitic pegmatite occurring within serpentinites of the Szklary massif in Lower Silesia, Poland. This pegmatite is a representative of the MSREL–REE subclass of the muscovite – rare-element pegmatites. The beusite usually occurs there as a Mn-dominant, (Ca,Fe)-bearing, Mg-enriched phase devoid of the typical lamellar intergrowths with triphylite or sarcopside, evolving toward more and more Mn-enriched varieties, stabilized by the earlier crystallization of tourmaline and fluorapatite. In terms of enrichment in Mn, the beusite from Szklary is on a level with that from the Cross Lake pegmatite in Canada, considered as the example richest in Mn worldwide. Similarly, Mn-rich fluorapatite containing up to 19.3 wt.% MnO, and a Mn-dominant apatite-group mineral containing up to 31.5 wt.% MnO, are two apatite species richest in Mn worldwide, the second close to the ideal Mn 3 Ca 2 (PO 4 ) 3 (Cl,F,OH) composition. The progressive removal of Fe over Mn, a decrease in the activities of F and Fe due to crystallization of abundant Fe-bearing tourmaline and fluorapatite, extraction of Mn from pegmatite-forming, H 2 O-undersaturated, Cl-enriched melts by an exsolved H 2 O-, F- and Cl-bearing volatile phase, and metasomatic alteration induced by residual Mn- and Cl-bearing fluids additionally enriched in Mn released during the dissolution of metastable beusite are the main reasons of such a high degree of Mn enrichment in the phosphates from the Szklary pegmatite.

TETRAHEDRALLY COORDINATED BORON IN Li-BEARING OLENITE FROM "MUSHROOM" TOURMALINE FROM MOMEIK, MYANMAR

So-called “mushroom” tourmaline, a botryoidal aggregate of pale pink tourmaline crystals, occurs in the Palelni mine, Khetchel village, Molo quarter, Momeik Township, northeast of Mogok, Mandalay Division, Shan State, Myanmar. It has been characterized by crystal-structure determination, electron-microprobe analysis (EMPA), secondary-ion mass spectrometry (SIMS), uranium extraction, and inductively coupled plasma – mass spectrometry (ICP–MS). This sample consists of sprays of radially arranged crystals up to ~9 mm in length and ~50–150 μm in diameter. Chemical data show that Al increases, whereas the Mn content decreases, from the center of the aggregate to the pyramidal faces. The composition at the center can be described as a Mn-bearing elbaite with ~1.6 wt% MnO. Near the pyramidal faces, the composition of these tourmaline crystallites is relatively Al-rich (up to 43.6 wt% Al 2 O 3 ), and Mn-poor (0.1–0.3% MnO). A crystal-structure refinement of the Li-bearing olenite (near the pyramidal faces), in combination with analytical results, gives the optimized formula X (Na 0.55 Ca 0.08 Pb 0.01 □ 0.36 ) Y (Al 2.13 Li 0.75 Mn 2+ 0.01 □ 0.11 ) Z Al 6 T (Si 5.34 B 0.66 )O 18 (BO 3 ) 3 V (OH) 3 W [(OH) 0.50 O 0.26 F 0.24 ], with a 15.7561(6), c 7.0703(5) A. The structure refinement yields a relatively high [4] B content (Si 0.87(1) B 0.13(1) ), and a T –O> distance of 1.604 A, the shortest observed in natural tourmaline to date. Together with Al-rich tourmalines from other localities, a positive correlation between Al at Y site and [4] B with r 2 ≈ 0.998 was found. Furthermore, this tourmaline aggregate shows a relatively high Pb content (1640 ppm) and a significant Bi content (153 ppm). The REE pattern (∑REE: ~7.2 ppm) exhibits a weak negative Eu anomaly, a strong positive Tm anomaly, and a positive Tb anomaly. Such a pattern cannot be derived simply from a late-stage fractionated magma of a S-type granite or from differentiated crustal melts alone.