Tourmaline Crystals Research Articles

Experimental investigation of reactions between two-mica granite and boron-rich fluids: Implications for the formation of tourmaline granite

The genetic relationship between different types of granite is critical for understanding the formation and evolution of granitic magma. Fluid-rock interaction experiments between two-mica leucogranite and boron-rich fluids were carried out at 600–700°C and 200 MPa to investigate the effects of boron content in fluid and temperature on the reaction products. Our experimental results show that tourmaline granite can be produced by reactions between boron-rich fluid and two-mica granite. At 700°C, the addition of boron-rich fluid resulted in partial melting of two-mica granite and crystallization of tourmaline from the boron-rich partial melt. Increasing boron concentration in fluid promotes the melting of two-mica granite and the growth of tourmaline. No melt was produced in experiments at 600°C, in which Fe, Mg and Al released from biotite decomposition combined with boron from the fluid to form tourmaline under subsolidus conditions. The Na required for tourmaline crystallization derived from Na/K exchange between feldspar and the K released by biotite decomposition. The produced tourmaline generally has core-rim structures, indicating that the composition of melt or fluid evolved during tourmaline crystallization. Based on the experimental results, we propose that tourmaline granite veins or dikes can be formed by the reactions between boron-rich fluids, presumably produced by devolatilization of boron-bearing granitic magma, and incompletely crystallized granite at the top of the magma chamber. This “self-metasomatism” involving boron-rich fluid in the late stage of magma crystallization could be an important mechanism for the formation of tourmaline granite.

New insights on Raman spectrum of K‐bearing tourmaline

AbstractRaman spectroscopy allows for semiquantitative analyses of major elements of tourmaline supergroup minerals. However, the new K‐bearing tourmaline (maruyamaite) species has not been calibrated yet and the effects of K in the X site of tourmaline on the Raman spectrum are still unclear. We investigated maruyamaite and K‐bearing dravite to test their Raman spectrum parameters, as a proxy for chemical composition of the X site. This study uses Raman imaging to obtain spectral data from the studied area of the zoned natural tourmaline crystals from Kumdy‐Kol (Kokchetav massif, Northern Kazakhstan). These data are correlated with chemical composition determined by electron probe microanalysis. Raman peak intensity best correlates with the K apfu at the X site. Peaks in the interval 100–250 cm−1 are aligned to the X‐site cation vibrations, and thus, their intensity positively correlates with K content. However, also, the more intensive peaks in the 650 to 850 cm−1 interval, which are controlled by the Si6O18 ring and AlO6 vibrations, show a positive correlation with the amount of K. The latter peak region is thus chosen here for the calibration due to its much higher intensity respect to the 100 to 250 cm−1 region. Furthermore, we discuss the influence of large K cations on the structure of Si6O18 rings in tourmaline. Our study demonstrates that Raman spectroscopy can be used as a reconnaissance tool for a semiquantitative discrimination of dravitic tourmalines with different X‐site occupation.

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.

Petrogenetic controls on the origin of tourmalinite veins from Mandrolisai igneous massif (central Sardinia, Italy): Insights from tourmaline crystal chemistry

An inclusive study of tourmaline, a well-known petrogenetic indicator, allowed the reconstruction of late-stage evolution of B-bearing Variscan granodioritic magmas in Sardinia batholith (Italy). Tourmaline samples from Mandrolisai igneous massif were chemically and structurally investigated by electron microprobe analysis, single-crystal X-ray diffraction, Mössbauer, infrared and optical absorption spectroscopy.Tourmaline aggregates occur both as large crystals displaying graphic textures with quartz, in aplite layers and pegmatite dykes within tonalitic granodiorite, and as fine-grained assemblages, in tourmalinite veins crystallized along fractures within metamorphic country rocks. Tourmaline was identified as schorl in pegmatites and as dravite in veins, both with relevant foitite and magnesio-foitite components. Petrological and mineralogical constraints based on mineral oxythermobarometry and tourmaline crystal chemistry converged towards crystallization temperatures in the range 650–400 °C at about 2.2 kbar, under NNO conditions that remained almost unvaried during the whole magma crystallization path. In the reconstructed scenario, Mandrolisai tourmaline recorded the late stages of consolidation of a single granodioritic magma batch, whose crystallization path locally led to residual concentration of B in the melt. Due to melt/hydrous fluid immiscibility processes, B enrichment promoted the crystallization of the large tourmaline + quartz assemblages in pegmatites. Late B-bearing fluids triggered metasomatic reactions and favored the precipitation of fine-grained tourmaline in tourmalinite veins under a brittle solid-state regime, which overprinted a fracture network of the country rocks previously formed under magmatic flow conditions.Mandrolisai granodiorites are metaluminous (ASI = 0.93–0.95), that is far from the compositional characters of typical B-bearing magmas, mostly felsic and markedly peraluminous (ASI > 1.2). The uncommon occurrence of tourmaline-bearing rocks in Mandrolisai may be an evidence of the limited control exerted by Al2O3 saturation on the origin of tourmaline. Conversely, a more important role of B contents, likely coming from crustal sources, may be invoked for tourmaline saturation in the magma.

Tourmaline geochemistry and boron isotopic variations as a guide to fluid evolution in the Qiman Tagh W–Sn belt, East Kunlun, China

The Qiman Tagh W–Sn belt lies in the westernmost section of the East Kunlun Orogen, NW China, and is associated with early Paleozoic monzogranites, tourmaline is present throughout this belt. In this paper we report chemical and boron isotopic compositions of tourmaline from wall rocks, monzogranites, and quartz veins within the belt, for studying the evolution of ore-forming fluids. Tourmaline crystals hosted in the monzogranite and wall rocks belong to the alkali group, while those hosted in quartz veins belong to both the alkali and X-site vacancy groups. Tourmaline in the walk rocks lies within the schorl–dravite series and becomes increasingly schorlitic in the monzogranite and quartz veins. Detrital tourmaline in the wall rocks is commonly both optically and chemically zoned, with cores being enriched in Mg compared with the rims. In the Al–Fe–Mg and Ca–Fe–Mg diagrams, tourmaline from the wall rocks plots in the fields of Al-saturated and Ca-poor metapelite, and extends into the field of Li-poor granites, while those from the monzogranite and quartz veins lie within the field of Li-poor granites. Compositional substitution is best represented by the MgFe−1, Al(NaR)−1, and AlO(Fe(OH))−1 exchange vectors. A wider range of δ11B values from −11.1‰ to −7.1‰ is observed in the wall-rock tourmaline crystals, the B isotopic values combining with elemental diagrams indicate a source of metasediments without marine evaporates for the wall rocks in the Qiman Tagh belt. The δ11B values of monzogranite-hosted tourmaline range from −10.7‰ and −9.2‰, corresponding to the continental crust sediments, and indicate a possible connection between the wall rocks and the monzogranite. The overlap in δ11B values between wall rocks and monzogranite implies that a transfer of δ11B values by anataxis with little isotopic fractionation between tourmaline and melts. Tourmaline crystals from quartz veins have δ11B values between −11.0‰ and −9.6‰, combining with the elemental diagrams and geological features, thus indicating a common granite-derived source for the quartz veins and little B isotopic fractionation occurred. Tourmalinite in the wall rocks was formed by metasomatism by a granite-derived hydrothermal fluid, as confirmed by the compositional and geological features. Therefore, we propose a single B-rich sedimentary source in the Qiman Tagh belt, and little boron isotopic fractionation occurred during systematic fluid evolution from the wall rocks, through monzogranite, to quartz veins and tourmalinite.

Towards Zn-Dominant Tourmaline: A Case of Zn-Rich Fluor-Elbaite and Elbaite from the Julianna System at Piława Górna, Lower Silesia, SW Poland

Tourmalines are a group of minerals which may concentrate various accessory components, e.g., Cu, Ni, Zn, Bi, Ti, and Sn. The paper presents fluor-elbaite and elbaite from a dyke of the Julianna pegmatitic system at Piława Górna, at the NE margin of the Bohemian Massif, SW Poland, containing up to 6.32 and 7.37 wt % ZnO, respectively. Such high amounts of ZnO are almost two times higher than in the second most Zn-enriched tourmaline known to date. The compositions of the Zn-rich tourmalines from Piława Górna, studied by electron micropropy and Raman spectroscopy, correspond to the formulae: X(Na0.733Ca0.013☐0.254)Σ1Y(Al1.033Li0.792Zn0.755Fe2+0.326Mn0.094)Σ3ZAl6(TSi6O18)(BO3)3V(OH)3W(F0.654OH0.344), and X(Na0.779Ca0.015☐0.206)Σ1Y(Al1.061Li0.869Zn0.880Fe2+0.098Mn0.094)Σ3ZAl6(TSi6O18)(BO3)3V(OH)3W(OH0.837F0.163), respectively, with Zn as one of the main octahedral occupants. A comparison with other tourmalines and associated Zn-rich fluor-elbaite and elbaite from the pegmatite indicates that atypically high Zn-enrichment is not a result of Zn-Fe fractionation, but dissolution and reprecipitation induced by a late (Na,Li,B,F)-bearing fluid within the assemblage of gahnite spinel and primary schorl-type tourmaline. This strongly suggests Na-Li-B-F metasomatism of gahnite-bearing mineral assemblages as that is the only environment that can promote crystallization of a hypothetical Zn-dominant tourmaline. The compositions of the Zn-rich fluor-elbaite and elbaite suggest three possible end-members for such a hypothetical tourmaline species: NaZn3Al6(Si6O18)(BO3)3(OH)3(OH), ☐(Zn2Al)Al6(Si6O18)(BO3)3(OH)3(OH) and Na(Zn2Al)Al6(Si6O18)(BO3)3(OH)3O by analogy with other tourmalines with divalent Y occupants, such as schorl/foitite/oxy-schorl and dravite/magnesio-foitite/oxy-dravite.

Late magmatic controls on the origin of schorlitic and foititic tourmalines from late-Variscan peraluminous granites of the Arbus pluton (SW Sardinia, Italy): Crystal-chemical study and petrological constraints

Tourmalines from the late-Variscan Arbus pluton (SW Sardinia) 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 from schorl 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 volume decrease. 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 significance of contribution from a metapelitic crustal component during the evolution of magmas in the middle-upper crust.

Accuracy of the hypothetical sky-polarimetric Viking navigation versus sky conditions: revealing solar elevations and cloudinesses favourable for this navigationmethod.

According to Thorkild Ramskou's theory proposed in 1967, under overcast and foggy skies, Viking seafarers might have used skylight polarization analysed with special crystals called sunstones to determine the position of the invisible Sun. After finding the occluded Sun with sunstones, its elevation angle had to be measured and its shadow had to be projected onto the horizontal surface of a sun compass. According to Ramskou's theory, these sunstones might have been birefringent calcite or dichroic cordierite or tourmaline crystals working as polarizers. It has frequently been claimed that this method might have been suitable for navigation even in cloudy weather. This hypothesis has been accepted and frequently cited for decades without any experimental support. In this work, we determined the accuracy of this hypothetical sky-polarimetric Viking navigation for 1080 different sky situations characterized by solar elevation θ and cloudiness ρ, the sky polarization patterns of which were measured by full-sky imaging polarimetry. We used the earlier measured uncertainty functions of the navigation steps 1, 2 and 3 for calcite, cordierite and tourmaline sunstone crystals, respectively, and the newly measured uncertainty function of step 4 presented here. As a result, we revealed the meteorological conditions under which Vikings could have used this hypothetical navigation method. We determined the solar elevations at which the navigation uncertainties are minimal at summer solstice and spring equinox for all three sunstone types. On average, calcite sunstone ensures a more accurate sky-polarimetric navigation than tourmaline and cordierite. However, in some special cases (generally at 35° ≤ θ ≤ 40°, 1 okta ≤ ρ ≤ 6 oktas for summer solstice, and at 20° ≤ θ ≤ 25°, 0 okta ≤ ρ ≤ 4 oktas for spring equinox), the use of tourmaline and cordierite results in smaller navigation uncertainties than that of calcite. Generally, under clear or less cloudy skies, the sky-polarimetric navigation is more accurate, but at low solar elevations its accuracy remains relatively large even at high cloudiness. For a given ρ, the absolute value of averaged peak North uncertainties dramatically decreases with increasing θ until the sign (±) change of these uncertainties. For a given θ, this absolute value can either decrease or increase with increasing ρ. The most advantageous sky situations for this navigation method are at summer solstice when the solar elevation and cloudiness are 35° ≤ θ ≤ 40° and 2 oktas ≤ ρ ≤ 3 oktas.

Characterizing Tourmaline in Metapelitic Schists from The Gasht Area, North Iran and Boron Source in Metamorphic Fluids

Metapelitic rocks in the Gasht area include micaschist, kyanite schist, andalusite schist, garnet schist, staurolite schist, cordierite schist and sillimanite schist. Tourmaline occurs as accessory mineral in all of these rock types. These schists are metamorphosed regionally and are affected by contact metamorpism subsequently. Based on the textural relations and the fact that CaO and TiO2 contents in the studied tourmalines are low, they are formed during regional metamorphism. They appear in the rock matrix and as inclusions in other minerals, especially biotite and albite. The studied tourmalines are of alkali type and are rich in dravite end-member. Cl and F contents are below the detection limit testifying for hydro-tourmaline nature of the studied minerals. Na and K are higher in the X-site in comparison with Ca. Low Ca shows neglegible amounts of Ca end-member or uvite. Mg content is much higher than Fe in the structure of the studied tourmalines, which are not zoned or show weak zoning and are grown at nearly constant P-T conditions. These tourmalines are crystallized from pelitic to psammitic protoliths in equlibrium with a fluid phase, rich in Al. Boron in fluid more likely was from the boron adsorbed on clay minerals surface in the protolith, released during metamorphism and boron from B-bearing mica breakdown during high tempertaure metamorphism, while boron released from the subducting oceanic crust is not a likely source for tourmaline crystallization in the studied rocks since there is no evidence for tourmaline in the associated oceanic crust mafic rocks.

Foitit-magneziofoitit a rózsa-hegyi ércesedésből, avagy a nagybörzsönyi turmalin újravizsgálata

Turmalin jelenléte a nagybörzsönyi ércesedésben már 60 éve ismert. A Rózsa-hegy É-i felén, az ún. rózsabányai ércesedés területén erősen kovásodott kőzet üregeiben, illetve később képződött ásványok (kvarc, kalcit, sziderit, pirrhotin, szfalerit) zárványaiként általában 1 mm-nél rövidebb, színtelen, tűs kristályokat, tűs-sugaras, vagy nemezszerű halmazokat alkot. A korábban készült nedveskémiai elemzés alapján a szakirodalomba drávitként bekerült turmalin az újabb elektron-mikroszondás vizsgálatok alapján zónás, és a foitit-magneziofoitit ásványfajokat képviseli. A vasban és magnéziumban legdúsabb elemzési pontokon mért összetétel (31 anionra számolva): ([]0,63Na0,29Ca0,08)Σ=1,00 (Fe2+1,26Mg0,89Al0,79)Σ=2,94 Al6,00 (Si5,88Al0,12)Σ=6,00 O18 (BO3)3 (OH)4,00, illetve ([]0,45Na0,37Ca0,18)Σ=1,00 (Mg1,55Fe2+0,48Al0,83Ti0,01)Σ=2,87 Al6,00 (Si5,67Al0,33)Σ=6,00 O18 (BO3)3 (OH)4,00. A röntgen-pordiffrakciós felvételből számított rácsállandók: a = 15,901(6) Å; c = 7,190(4) Å, V = 1574(1) Å3. A turmalin hidrotermás oldatból vált ki, melynek hőmérséklete 260°C fölött lehetett. A képződő turmalin kémiáját elsősorban a hidrotermás oldat összetétele határozta meg, ebben a tekintetben a mellékkőzetek összetétele, illetve az uralkodó hőmérséklet- és nyomásviszonyok csak másodrangúak voltak. A kristályokon megfigyelhető kémiai zónásság elsősorban a Fe ↔ Mg helyettesítésből ered. A zónásság oka az lehet, hogy a Fe-gazdag hidrotermás oldatok összetételét a Mg-ban és Ca-ban gazdagabb leszivárgó vizek megváltoztatták. A turmalin tűs habitusa kristálykémiai okokra vezethető vissza, ugyanis a magas Al-tartalom és a viszonylag alacsony Fe+Mg-tartalom a c-tengely irányában való gyors növekedését segítette elő.

Use of tourmaline as a potential petrogenetic indicator in the determination of host magma: CRS, XRD and PED-XRF methods

Tourmaline defines a group of complex borosilicate forms as accessory mineral in igneous and metamorphic rocks and they act an important role in the interpretation of the chemical composition changes of the composition of the host fluid of the magma.The variety of tourmaline can be identified by using optical microscopy, X-Ray Diffraction (XRD) and by determining its chemical composition through Polarized Energy Dispersive X-Ray Fluorescence (PED-XRF) methods. However, microscopic investigations and XRD analyses are not quite adequate for detailed determination of tourmaline sub-groups. In addition, the use of chemical composition of tourmaline as a strict indicator of geochemical processes might be a misleading method. In this study, variable tourmaline crystals were collected from three different pegmatitic occurrences in Behrekdag, Yozgat and Karakaya granitic bodies of Central Anatolia to identify their chemical properties through Confocal Raman Spectroscopy (CRS), PED-XRF and XRD analyses.The confocal Raman spectrometry of collected tourmalines from the Behrekdag, Yozgat and Karakaya granites are in the compositions of schorl, schorl and elbaite respectively. The dominant compositional groups of these tourmalines are in the form of schorl. Raman shift values of tourmalines revealed four bands centered at almost 1050, 750, 400 and 300cm−1. The first group of the band arises from SiO stretching, the second from BO stretching and the other two belong to bending modes of OBO and BOAl with symmetrical deformation of SiOSi. The strongest spectra near 360cm−1 should belong to the bonding of AlO. As a result, the confocal Raman studies are more sensitive for identification of tourmaline subgroup compositions and have a quite important in the explaining source of the magma.

Tourmaline as a petrogenetic indicator in the Pfitsch Formation, Western Tauern Window, Eastern Alps

Tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch (Passo de Vizze) area in the Tauern Window of the Eastern Alps. These post-Variscan metasedimentary units experienced peak metamorphic conditions of ~550°C, 1.0GPa during the Alpine orogeny. Tourmaline is most abundant in a ~25m thick unit of feldspathic gneiss (~20–200μg/g B), where it occurs as idiomorphic crystals typically 10mm in length. The abundance and size of the tourmaline crystals increase near coarse-grained quartzofeldspathic segregations (~1200μg/g B), reflecting the mobilization and concentration of B by metamorphic fluids. Near segregations, individual tourmaline crystals have up to three growth zones, recording pro- (~350–500°C, 0.7–1.0GPa) and retrograde (~400°C, 0.2GPa) growth as determined by combining textural information and sector-zoning thermometry. Retrograde tourmaline occurs as individual crystals as well as overgrowths on prograde tourmaline crystals, especially on the surface of extensional fractures formed by E–W extension during regional decompression.Tourmaline in the Pfitsch Formation is dravitic with a variable Fe content that correlates with the Fe content of its respective host unit. Charge balance calculations suggest that a significant proportion of Fe in tourmaline is ferric, supporting the interpretation of a subaerial continental sedimentary protolith. Tourmaline near segregations has the highest inferred ferric iron content, which decreases across growth zones, potentially reflecting a reduction of the fluid during metamorphism. The Mg/(Mg+Fe) ratio increases with prograde tourmaline growth and decreases in retrograde overgrowths. In contrast, the Ca/(Ca+Na) ratio increases gradually from 0.05 to 0.20 with prograde growth and continues to increase up to 0.25 in the retrograde overgrowths, recording the maximum Ca/(Ca+Na) ratio of the fluid during Alpine metamorphism.The metasediments of the Pfitsch Formation have very low and variable whole-rock δ11B values (−14.1 to −33.6‰), with the highest values (−17.7 to −14.1‰) found in B-rich samples (165–1200μg/g B) containing abundant tourmaline, and the lowest values (−24.2 to −33.6‰) in B-depleted samples (21–40μg/g), which lack tourmaline. This observation supports preferential loss of 11B from the rocks during prograde metamorphism. Zoned tourmaline crystals in the Pfitsch formation show successively decreasing B isotope ratios from −7.8 to −11.2‰ in their cores and −17.3 to −20.3‰ in their rims. As supported by a Rayleigh fractionation model, the B-isotope values of the host rocks and the tourmaline crystals are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline during Alpine metamorphism.

Chemical composition and evolution of tourmaline-supergroup minerals from the Sb hydrothermal veins in Rožňava area, Western Carpathians, Slovakia

Tourmaline-supergroup minerals are common gangue minerals in Sb-hydrothermal veins on Betliar – Strakova, Cucma – Gabriela and Rožňava – Peter-Pavol vein deposits in the Rožňava area, Slovakia. Tourmaline-supergroup minerals form relatively large prismatic to radial aggregates of parallel black to greyish-black crystals. Tourmaline-supergroup minerals from Betliar – Strakova and Rožňava – Peter-Pavol are almost homogeneous with intermediate schorl-dravite composition. Cucma – Gabriela tourmaline have distinct zoning with massive core of the schorlitic-to-feruvitic shifting to schorlitic-to-dravitic composition, and dravitic to magnesio-foititic rim. The tourmaline composition is influenced by two main substitutions, namely Ca(Mg,Fe)Na−1Al−1 and X □AlNa−1(Mg,Fe)−1. Betliar – Strakova and Rožňava – Peter-Pavol tourmaline-supergroup minerals exhibit only small extents of the X □AlNa−1(Mg,Fe)−1 substitution. This substitution shifts the composition to magnesio-foitite in Cucma – Gabriela tourmaline. The decrease of Al in the core of Cucma – Gabriela tourmaline crystals is caused by extensive Ca(Mg,Fe)Na−1Al−1 substitution. The unit-cell dimensions of all investigated tourmaline-supergroup minerals indicate an octahedral disorder with the Z (Fe3++Mg) proportion calculated from empirical equations varying between 0.85 and 0.87 apfu (atoms per formula unit). Based on Mossbauer spectra, the Z Fe3+ content varied between 0.25 apfu in Betliar – Strakova tourmaline and 0.45 apfu in Cucma – Gabriela sample. Based on Fe/(Fe + Mg) ratio, Betliar – Strakova tourmaline is slightly enriched in Fe compared to Rožňava – Peter-Pavol, suggesting the impact of the host-rock composition; first are grown in Fe-richer acidic metarhyolitic rocks, latter in metapelites. In Cucma – Gabriela, the variations in Fe/(Fe + Mg) are very likely reflecting the change in fluid composition. Magnesio-foitite is the product of second-stage crystallization forming rims and crack fills. The relatively low Fe3+/Fe2+ ratio suggests only minor proportion of meteoric fluids forming tourmaline.

Mineral chemistry of tourmaline from Mashak Pahar, South Purulia Shear Zone (SPSZ), eastern Indian Shield

The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebble conglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (∼Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F1, F2 and F3. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolution of tourmaline during prograde metamorphism (∼400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals.

Source of boron in the Palokas gold deposit, northern Finland: evidence from boron isotopes and major element composition of tourmaline

The recently discovered Palokas gold deposit is part of the larger Rompas-Rajapalot gold-mineralized system located in the Paleoproterozoic Perapohja Belt, northern Finland. Tourmaline is an important gangue mineral in the Palokas gold mineralization. It occurs as tourmalinite veins and as tourmaline crystals in sulfide-rich metasomatized gold-bearing rocks. In order to understand the origin of tourmaline in the gold-mineralized rocks, we have investigated the major element chemistry and boron isotope composition of tourmaline from three areas: (1) the Palokas gold mineralization, (2) a pegmatitic tourmaline granite, and (3) the evaporitic Petajaskoski Formation. Based on textural evidence, tourmaline in gold mineralization is divided into two different types. Type 1 is located within the host rock and is cut by rock-forming anthophyllite crystals. Type 2 occurs in late veins and/or breccia zones consisting of approximately 80% tourmaline and 20% sulfides, commonly adjacent to quartz veins. All the studied tourmaline samples belong to the alkali-group tourmaline and can be classified as dravite and schorl. The δ11B values of the three localities lie in the same range, from 0 to −4‰. Tourmaline from the Au mineralization and from the Petajaskoski Formation has similar compositional trends. Mg is the major substituent for Al; inferred low Fe3+/Fe2+ ratios and Na values (<0.8 atoms per formula unit (apfu)) of all tourmaline samples suggest that they precipitated from reduced, low-salinity fluids. Based on the similar chemical and boron isotope composition and the Re–Os age of molybdenite related to the tourmaline-sulfide-quartz veins, we propose that the tourmaline-forming process is a result of a single magmatic-hydrothermal event related to the extensive granite magmatism at around 1.79–1.77 Ga. Tourmaline was crystallized throughout the hydrothermal process, which resulted in the paragenetic variation between type 1 and type 2. The close association of tourmaline and gold suggests that the gold precipitated from the same boron-rich source as tourmaline.

Discriminating fluid source regions in orogenic gold deposits using B-isotopes

The genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions. However, there is considerable debate as to the ultimate source of these fluids and the metals they transport. Tourmaline is a common gangue mineral in orogenic gold deposits. It is stable over a very wide P–T range, demonstrates limited volume diffusion of major and trace elements and is the main host of B in most rock types. We have used texturally resolved B-isotope analysis by secondary ion mass spectrometry (SIMS) to identify multiple fluid sources within a single orogenic gold ore district. The Loulo Mining District in Mali, West Africa hosts several large orogenic gold ore bodies with complex fluid chemistry, associated with widespread pre-ore Na- and multi-stage B-metasomatism. The Gara deposit, as well as several smaller satellites, formed through partial mixing between a dilute aqueous-carbonic fluid and a hypersaline brine. Hydrothermal tourmaline occurs as a pre-ore phase in the matrix of tourmalinite units, which host mineralization in several ore bodies. Clasts of these tourmalinites occur in mineralized breccias. Disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation. Tourmalines show a large range in δ11B values from −3.5 to 19.8‰, which record a change in fluid source between paragenetic stages of tourmaline growth. Pre-mineralization tourmaline crystals show heavy δ11B values (8–19.8‰) and high X-site occupancy (Na±Ca; 0.69–1apfu) suggesting a marine evaporite source for hydrothermal fluids. Syn-mineralization and replacement phases show lighter δ11B values (−3.5 to 15.1‰) and lower X-site occupancy (0.62–0.88apfu), suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments. The large, overlapping range in isotopic compositions and a skew toward the opposing population in the δ11B data for both tourmaline groups reflects continual tourmaline growth throughout mineralization, which records the process of fluid mixing. A peak in δ11B values at ∼8‰ largely controlled by tourmalines of syn- to post-ore timing represents a mixture of the two isotopically distinct fluids. This paper demonstrates that B-isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems. The importance of B as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes B-isotope analysis a powerful tool for testing the level of source region variability in these fluids, and by extension, that of metal sources.

Origin of tourmaline from the Kolah Ghazi granitoid body (SE Isfahan, Iran)

Abundant tourmaline crystals, commonly up to 0.5 cm in diameter, are present in the middle Jurassic Kolah Ghazi granitoid body. They occur in the granitoid, quartz veins, and hornfels of the area. Electron microprobe analyses indicate that the tourmalines from the granitoid and hornfels display ferrous (ferrischorl), and those from quartz veins show tendency to magnesium (dravite) compositions. On the Al-Fe(tot)-Mg and Ca-Fe(tot)-Mg diagrams, the tourmalines from the quartz veins are related to metasediments (i.e., metapelites and metapsammites), while those of the granitoid and the hornfels related to granitoid, pegmatite, and aplite field. According to the petrographic observations as well as geochemical signatures, the investigated tourmalines are classified as magmatic and hydrothermal origin. The granitoid and hornfels tourmalines are chemically homogeneous and reflect high Fe/Mg, high F, and high Al values in R2 site (ave. 0. 20 apfu) with schorl end member, and so these may be considered as magmatic in origin. In contrast, tourmalines from quartz veins display a weak zoning pattern, dravite in composition with little Al values in R2 site (ave. 0.17 apfu). Moreover, its Mg and Fe concentration varies from 1.72 to 1.80 apfu and from 0.60 to 0.70 apfu, respectively, suggesting hydrothermal conditions of formation. Substitution mechanisms for the chemical evolution of tourmalines from the granitoid and the hornfels are the Fe3+ Al_1, AlOR(OH)−1, □Al(Na R)_1, □Al(Na Mg)_1, and for the quartz–tourmaline vein substitutions display by XNa1ZR2+1X□_1ZAl_1, XCa1ZR2+1XNa_1ZAl_1, □Al(Na Mg)_1 that can be the traces of metasomatism evidences.

Seismic slip recorded in tourmaline fault mirrors from Elba Island (Italy)

This paper reports the first example of fault mirrors developed in an unusual protolith, consisting of tourmaline crystals with interstitial goethite. The deformation mechanisms active in the fault zone have been investigated from the outcrop to the nanoscale, aiming to identify possible traces of frictional heating at seismic slip rate, as observed for other fault mirrors in different protoliths. The investigation revealed the superposition of two main deformational stages. The first was dominated by brittle processes and produced a cataclastic/ultracataclastic principal slip zone, a few mm thick; the second was associated with seismic slip and produced a sharp discontinuity (the principal slip surface) within the cataclastic/ultracataclastic zone. The mirror-like coating, a few microns thick, occurs on the principal slip surface, and is characterized by 1) absence of interstitial goethite; 2) occurrence of truncated tourmaline crystals; 3) highly variable grain size, from 200 μm to 200 nm; 4) tourmaline close packing with interlobate grain boundaries, and 5) tourmaline random crystallographic orientation.Micro and nanostructural investigations indicate the occurrence of thermally-activated processes, involving both interstitial goethite and tourmaline. In particular, close to the principal slip surface, goethite is completely decomposed, and produced an amorphous porous material, with local topotactic recrystallization of hematite. Tourmaline clasts are typically characterized by strongly lobate boundaries, indicative of reaction and partial decomposition at grain boundaries. TEM observations revealed the occurrence of tourmaline nanograins, a few tens of nm in size, characterized by rounded shape and fading amorphous boundaries, that cannot be obtained by brittle processes. Lastly, the peculiar interlobate microstructure of the mirror surface is interpreted as the result of grain boundary recrystallization processes taking place by deformation at high-T conditions. Our results show that tourmaline fault mirrors recorded localized high-T processes triggered by frictional heating and can be therefore considered as reliable traces of ancient earthquakes.

Episodic tourmaline growth and re-equilibration in mica pegmatite from the Bihar Mica Belt, India: major- and trace-element variations under pegmatitic and hydrothermal conditions

AbstractMica pegmatites from the Bihar Mica Belt contain three distinct generations of tourmaline. The major-element composition, substitution vectors and trajectories within each group are different, which indicates that the three types of tourmalines are not a part of one evolutionary series. Rather, the differences in their chemistries as well their mutual microtextural relations, can be best explained by growth of tourmaline from pegmatitic melts followed by episodic re-equilibration during discrete geological events. The euhedral, coarse-grained brown type I tourmaline cores have relatively high Ca, Mg (XMgc. 0.37) and Al with correlated variation in Sr, Sc, Ti, Zr, Y, Cr, Pb and Rare Earth elements (REEs). They are inferred to have crystallized from pegmatitic melts. Monazites included within these tourmalines give chemical ages of 1290−1242 Ma interpreted to date the crystallization of the pegmatitic tourmaline. The bluish type II and greyish type III tourmalines with low Ca and Mg contents (XMg = 0.16−0.27) and high Zn, Sn, Nb, Ta and Na, formed by pseudomorphic partial replacement of the pegmatitic tourmaline via fluid-mediated coupled dissolution–reprecipitation, are ascribed to a hydrothermal origin. The ages obtained from monazites included in these tourmalines indicate two alteration events at c. 1100 Ma and c. 950 Ma. The correlated variation of Ca, Mg and Fe and the trace elements Sr, Sn, Sc, Zn and REE within the tourmalines indicates that the trace-element concentrations of tourmaline are controlled not only by the fluid chemistry but also by coupled substitutions with major-element ions.

Fibrous Tourmaline: A Sensitive Probe of Fluid Compositions and Petrologic Environments

Abstract Tourmaline supergroup minerals with fibrous morphology record and respond to changing conditions in fluid-rich hydrothermal environments. Based on published data, hand-specimen and optical observations, and new chemical analyses of fibrous tourmalines from localities worldwide, several commonalities are apparent. Fibers typically nucleate on a preexisting substrate of tourmaline, but the fibers generally have a dramatically different composition than the substrate tourmaline. When fibers nucleate on a single tourmaline crystal, fibrous growth is restricted to the +c pole of the tourmaline. Tourmaline fibers also nucleate on or in other minerals without preexisting tourmaline. Most fibrous tourmalines form late in the paragenetic sequence of a geologic environment and generally where there is an open fluid-filled space, e.g ., at the end of the tourmaline crystallization sequence in a pegmatite pocket or in a hydrous fluid-rich fracture system. Fibrous tourmaline compositions can be foititic, schorlitic, dravitic, or elbaitic, reflecting the dissolved components in the fluxing aqueous fluids. While some fibers are homogeneous, many fibers are chemically zoned with irregular, patchy, or oscillatory zoning, and the zoning tracks the chemical evolution trends in the host environment. Using newly derived expressions relating X-site cationic occupancy to aqueous fluid compositions, Na and Ca contents in aqueous fluids in local equilibrium with fibrous tourmaline suggest that in all petrologic settings fibrous tourmalines equilibrated with aqueous fluids having variable Na concentrations (0.07–0.48 mol/l Na with the lower ranges associated with foititic fibrous tourmaline) and with generally low Ca concentrations (