Tourmaline-supergroup Minerals Research Articles

Crystal chemistry of povondraite by single-crystal XRD, EMPA, Mössbauer spectroscopy and FTIR

AbstractFive povondraite crystals from San Francisco Mine, Villa Tunari, Bolivia, have been structurally and chemically characterised by single-crystal X-ray diffraction and electron microprobe analysis. For the first time, this characterisation is accompanied by Mössbauer spectroscopic and single-crystal infrared spectroscopic data, which show the exclusive presence of Fe3+ at both the octahedrally-coordinated Y and Z sites as well as slight disorder of (OH) and O over the O(1) and O(3) sites.The data obtained along with those for earlier-studied bosiite and oxy-dravite oxy-tourmalines show a complete substitution series described by the reaction YFe3+3 + ZMg + ZFe3+4 ↔ YAl2 + YMg + ZAl5 (i.e. Fe3+Al–1) with variation of the structural parameters dominated by Fe3+ (or Al). Povondraite is the tourmaline member having the largest unit-cell parameters due to the larger size of Fe3+ relative to other trivalent cations (V > Cr > Al). In the tourmaline-supergroup minerals, the a and c unit-cell parameters vary from ~15.60 Å to ~16.25 Å and ~7.00 Å to ~7.50 Å, respectively. Their values increase with increasing Fe3+ or decreasing Al. End-member compositions related to the smallest and largest a and c parameters are, respectively, NaAl3Al6(Si3B3O18)(BO3)3(OH)3(OH) (synthetic tourmaline) and NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O (povondraite).

Thermal expansion of minerals in the tourmaline supergroup

Abstract The thermal behavior of 15 natural tourmaline samples has been measured by X-ray powder diffraction from room temperature to ~930 °C. Axial thermal expansion is generally greater along the c crystallographic axis (αc 0.90–1.05 × 10–5/K) than along the a crystallographic axis and the symmetrically equivalent b axis (αa 0.47–0.60 × 10–5/K). Ferro-bearing samples show lower expansion along a than in other tourmalines. In povondraite the thermal expansion along the c axis is higher than in other tourmalines, whereas along a it is lower [αa = 0.31(2) and αc = 1.49(3) × 10–5/K]. Volume expansion in the tourmaline-supergroup minerals is relatively low compared with other silicates such as pyroxenes and amphiboles. Volume also exhibits a relatively narrow range of thermal expansion coefficients (1.90–2.05 × 10–5/K) among the supergroup members. An interpretation for the small changes in thermal expansion in a compositionally heterogeneous group like tourmaline is that all members, except povondraite, share a framework of dominantly ZAlO6 polyhedra that limit thermal expansion. Povondraite, with a framework dominated by ZFe3+O6 polyhedra, displays thermal expansion that is different from other members of the group. Unit-cell dimensions of tourmalines having significant Fe2+ deviate from linearity above 400 °C on plots against temperature (T); along with the resulting substantial reduction in unit-cell volume, these effects are likely the result of deprotonation/oxidation processes. Lithium-rich and Fe2+-free tourmalines deviate similarly at T > 600 °C. In Li- and Fe2+-free tourmalines, no such deviation is observed up to the highest temperatures of our experiments. It is not clear whether this is due to cation order-disorder over Y and Z sites that occurs during the highest temperature measurements, a phenomenon that is apparently inhibited (at least in the short term) in Li-free/Mg-rich samples. If so, this must occur at a relatively rapid rate, as no difference in unit-cell values was detected at 800 °C after heating in both one- and 12-h experiments on Na-rich rossmanite.

Mass Bias Corrections for Hydrogen and Oxygen Isotope Analysis of Tourmaline by Secondary Ion Mass Spectrometry

ABSTRACT Hydrogen (δ2H) and oxygen (δ18O) stable isotopes are used to trace fluid sources, metals, and contaminants in the environment and Earth's subsurface. Tourmaline-supergroup minerals provide an opportunity to quantify both δ2H and δ18O from the same grain using in situ analytical techniques (e.g., Secondary Ion Mass Spectrometry – SIMS). These minerals occur in a wide variety of geological environments and have a wide range of chemical compositions. However, large differences in chemical composition are problematic during SIMS analysis, as instrumental mass fractionation (IMF) often varies with the chemical composition of the mineral. Therefore, calibration models derived by analyzing tourmalines of different chemical composition must be developed for accurate analysis by SIMS. Hydrogen and oxygen isotope analysis was done on six reference tourmaline samples using a CAMECA 7f SIMS instrument operating at extreme energy filtering. Spot-to-spot repeatability for tourmalines was in the range 4–5‰ and 0.6–1.0‰ for δ2H and δ18O, respectively. There is a strong correlation between IMF and several elements (B, Si, Ca, Fe, and Fe#). Iron content is the most robust predictor of IMF, and we report two calibration curves for the correction of δ2H and δ18O measured by SIMS using reference tourmaline crystals with different Fe contents, ranging from 0.00 to 14.00 wt.% Fe. This is the first calibration curve used to correct for the fractionation of hydrogen isotope ratios in tourmaline as measured by SIMS. Tourmaline-supergroup minerals require a suite of at least three, with a range of Fe content, to ensure accurate and precise H and O analysis by SIMS. Crystallographic orientation effects were not observed for these tourmalines.

Cation partitioning among crystallographic sites based on bond-length constraints in tourmaline-supergroup minerals

Abstract Theoretical bond-length calculations from ideal bond valences for each ion and coordination allow for the prediction of ion site preference and partitioning in tourmaline structures at low-pressure conditions. A comparison of calculated data with published bond-length values enables the determination of the range of structurally stable bond lengths with a minimal induced distortion—the “Goldilocks zone.” The calculations provided the following conclusions: the B-site occupancy is strictly limited to B3+; the T site can freely accommodate not only Si4+ but also B3+ and Al3+, although these substituents require shrinkage and expansion of the TO4 tetrahedron, respectively; and the Be2+ substitution results in a significant difference in charge. Satisfactory bond lengths for octahedral sites were calculated for Al3+ (Z-site preference), Ti4+, Mn3+, Ga3+, V3+, Fe3+ (mixed preference), Mg2+, Fe2+, Li+, Mn2+, Ni2+, Zn2+, Cu2+, Sc3+, and Zr4+ (Y-site preference). Another group of cations, which includes U4+, Th4+, Y3+, and lanthanoids from Tb to Lu and Ce4+, have significantly longer bonds than typical Y-O and very short bonds for the X site; therefore, it is likely they would prefer an octahedron. The empirical bond length for the X site is met with Na+, Ca2+, Sr2+, Pb2+, and lanthanoids from La to Gd, while K, Rb, and Cs are too large in the low-pressure conditions. However, the final tourmaline composition results from the interaction of the structure with the genetic environment in terms of P-T-X and geochemical conditions. This results in structural and environmental constraints that limit the incorporation of elements into the structure. Consequently, major elements, such as Si, Al, B, Mg, Fe, Na, and Ca usually occur in abundance, whereas other elements (V, Cr, Mn, Ti, Pb) could form end-member compositions, but rarely do because of their low abundance in the environment. The elements with contents limited to trace amounts have either structural (Be, C, REE, Rb, Cs, U, Th) or geochemical (Zr4+, Sc3+, and Sr2+) limits. However, environmental properties, such as high pressure or specific local structural arrangements, can overcome structural constraints and enable the incorporation of elements (K).

Experimental Study of the Stability and Synthesis of the Tourmaline Supergroup Minerals

Abstract—Tourmaline is one of the widest spread minerals in nature and one of the most popular gems and a promising piezoelectric material. The growth of large crystals is today a topical task. Tourmaline monocrystals are difficult to synthesize owing to its complex chemical composition, the high chemical stability in hydrothermal solutions, and the low growth rate. The paper reviews recent data on the tourmaline synthesis and the results obtained at the Institute of Experimental Mineralogy, Russian Academy of Sciences.

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.

Oxy-foitite, [] (Fe2+ Al2)Al6 (Si6 O18 )(BO3)3 (OH)3 O, a new mineral species of the tourmaline supergroup

Oxy-foitite, □(Fe 2+ Al 2 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral of the tourmaline supergroup. It occurs in high-grade migmatitic gneisses of pelitic composition at the Cooma metamorphic Complex (New South Wales, Australia), in association with muscovite, K-feldspar and quartz. Crystals are black with a vitreous luster, sub-conchoidal fracture and gray streak. Oxy-foitite has a Mohs hardness of ∼7, and has a calculated density of 3.143 g/cm 3 . In plane-polarized light, oxy-foitite is pleochroic ( O = dark brown and E = pale brown), uniaxial negative. Oxy-foitite belongs to the trigonal crystal system, space group R 3 m , a = 15.9387(3) A, c = 7.1507(1) A and V = 1573.20(6) A 3 , Z = 3. The crystal structure of oxy-foitite was refined to R 1 = 1.48% using 3247 unique reflections from single-crystal X-ray diffraction using Mo K α radiation. Crystal-chemical analysis resulted in the empirical structural formula: X ( □ 0.53 Na 0.45 Ca 0.01 K 0.01 ) ∑ 1.00 Y ( Al 1.53 Fe 2 + 1.16 Mg 0.22 Mn 2 + 0.05 Zn 0.01 Ti 4 + 0.03 ) ∑ 3.00 Z ( Al 5.47 Fe 3 + 0.14 Mg 0.39 ) ∑ 6.00 [ ( Si 5.89 Al 0.11 ) ∑ 6.00 O 18 ] ( BO 3 ) 3 V ( OH ) 3 W [ O 0.57 F 0.04 ( OH ) 0.39 ] ∑ 1.00 . Oxy-foitite belongs to the X -site vacant group of the tourmaline-supergroup minerals, and shows chemical relationships with foitite through the substitution Y Al 3+ + W O 2− → Y Fe 2+ + W (OH) 1− .

A multiple regression method for estimating Li in tourmaline from electron microprobe analyses

AbstractLithium cannot be determined by electron microprobe, but it may be an essential component in tourmalinesupergroup minerals. Therefore, its estimation is important for structural formula calculation and nomenclature. In this paper, we present a method to estimate Li content in tourmaline from microprobe data based on a multiple linear-regression model, which is not reliant on a particular normalization scheme. The results derived from this model are reasonably accurate, particularly for low-Mg tourmalines (<2 wt.% MgO) with Li2O contents higher than ∼0.3 wt.%. Furthermore, it provides a better fitness compared with estimations of Li assuming that Li fills any cation deficiency at the Y site.

Bosiite, NaFe3+ 3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new ferric member of the tourmaline supergroup from the Darasun gold deposit, Transbaikalia, Russia

Bosiite, NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral species of the tourmaline supergroup from the Darasun gold deposit (Darasun mine), Vershino-Darasunskiy, Transbaikal Krai, Eastern-Siberian Region, Russia (52°20′24″N, 115°29′23″E). Bosiite formed as a hydrothermal phase in a gold-bearing quartz-vein spatially related to the Amudzhikan–Sretensky subvolcanic K-rich granodiorite-porphyry intrusion. Ores of this deposit are enriched in sulfides (up to 60%). Bosiite is intimately associated with other tourmalines. The first tourmaline generation is bosiite, which is followed by a second generation of oxy-dravite and a third generation of dravite. Bosiite also coexists with quartz and pyrite; further associated minerals in the vein are gangue minerals (quartz, calcite, and dolomite), sulfides (pyrite, arsenopyrite, chalcopyrite, pyrrhotite, tetrahedrite, sphalerite, and galena) and native gold. Crystals of bosiite are dark brown to black with a pale-brown streak. Bosiite is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and cleavage. It has a measured density of 3.23(3) g/cm3 (by pycnometry) and a calculated density of 3.26(1) g/cm3. In plane-polarized light, it is pleochroic, O = yellow-brown, E = red-brown. Bosiite is uniaxial negative, ω = 1.760(5), ϵ = 1.687(5). The mineral is trigonal, space group R 3 m , a = 16.101(3), c = 7.327(2) A, V = 1645.0(6) A3. The eight strongest X-ray diffraction lines in the (calculated) powder pattern [ d in A( I ) hkl ] are: 2.606(100)(50-1), 8.051(58)(100), 3.008(58)(3-1-2), 4.025(57)(4-20), 3.543(50)(10-2), 4.279(46) (3-11), 2.068(45)(6-1-2), 4.648(28)(300). Analysis by a combination of electron microprobe (EMPA), inductively coupled plasma mass spectrometry (ICP-MS), Mossbauer spectroscopic data and crystal-structure refinement results in the empirical structural formula: ![Formula][1] According to the IMA-CNMNC guidelines, the dominant valence at the Y site is 3+ and the dominant cation is Fe3+. To accommodate the disorder and allocating cations to the Z and Y sites, the recommended procedure leads to the optimized empirical formula (based on 31 O): X (Na0.73Ca0.23□0.04) Y (Fe3+2.40Fe2+0.59Ti4+0.01) Z (Al3.36Mg1.69Fe3+0.95) T (Si5.92Al0.08O18) (BO3)3 V (OH)3 W [O0.85(OH)0.15]. Bosiite, ideally NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is related to end-member povondraite, ideally NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O, by the substitution Z Al4 → Z Fe3+4. Further, bosiite is related to oxy-dravite, ideally Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3O, by the substitutions [6]Fe3+3 → [6]Al3. Bosiite is named after Dr. Ferdinando Bosi, researcher at the University of Rome La Sapienza, Italy, and an expert on the crystallography and mineralogy of the tourmaline-supergroup minerals and the spinels. [1]: /embed/mml-math-1.gif

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 (

Evolution of Structural Complexity In Boron Minerals

Abstract Boron minerals are among the most structurally and chemically complex naturally occurring inorganic compounds. Of the 291 B minerals recognized as valid or potentially valid, structures are known for 256 species (plus three polytypes); for 245 of these the age of the earliest reported occurrence in the geologic record has also been reported. The earliest B minerals are four tourmaline species formed during metamorphism at 3550 Ma in the Isua supracrustal belt (Greenland), but many ephemeral B minerals are restricted to Late Cenozoic and Holocene deposits. We analyzed structural complexity with the program package TOPOS, using complexity parameters that provide Shannon information content per atom and per unit cell (Krivovichev 2013). The average content per unit cell, 343 bits/unit cell (hereafter noted simply as “bits”), is significantly greater than the mode complexity ( ca . 175 bits) because distribution of complexity is strongly right-skewed. Qingsongite, cubic BN (2 bits), is the simplest boron mineral. Seventeen B minerals possess information content per unit cell > 1000 bits. Rogermitchellite, Na 6 (Sr,Na) 12 Ba 2 Zr 13 Si 39 (B,Si) 6 O 123 (OH) 12 ·9H 2 O, which is the most complex (2321 bits), owes its structural complexity in part to the incorporation of eight essential chemical elements. However, chemically simpler hydrated Ca borates (with four or five essential elements), e.g ., ginorite, Ca 2 B 14 O 20 (OH) 6 ·5H 2 O (1506 bits); ruitenbergite, Ca 9 B 26 O 34 (OH) 24 Cl 4 ·13H 2 O (1492 bits); and alfredstelznerite, Ca 4 B 16 O 16 (OH) 24 ·19H 2 O (1359 bits), are also structurally complex, which we attribute to the interplay of borate polyanions and hydrogen bonding networks. The earliest complex borates are takeuchiite (1179 bits), blatterite (1656 bits), and the Mg analogue of blatterite (1612 bits), which are reported uniquely from 1825 Ma deposits at Langban and Nordmark (Sweden). There is little evidence for a significant increase in maximum structural complexity since 1825 Ma. Localities with high boron mineral diversity (nine to 25 species) are more likely to contain minerals with complex structures, which could be simply a matter of there being more minerals present, which increases the chance that one of these minerals would be structurally complex. The variation of structural complexity with time shows features of a “passive” trend in mineral evolution – more minerals with complex structures arise with the passage of geologic time, yet the simpler structures are not supplanted; instead, new simpler structures also appear, e.g ., tourmaline-supergroup minerals (176 to 207 bits).

A Windows program for calculation and classification of tourmaline-supergroup (IMA-2011)

A Microsoft Visual Basic program, WinTcac, has been developed to calculate structural formulae of tourmaline analyses based on the Subcommittee on Tourmaline Nomenclature (STN) of the International Mineralogical Association's Commission on New Minerals, Nomenclature and Classification (IMA-CNMCN) scheme. WinTcac calculates and classifies tourmaline-supergroup minerals based on 31 O atoms for complete tourmaline analyses. For electron-microprobe-derived tourmaline analyses site occupancy can be estimated by using the stoichiometric H2O (wt%) and B2O3 (wt%) contents. This program also allows the user to process tourmaline analyses using 15 cations and 6 silicons normalization schemes. WinTcac provides the user to display tourmaline analyses in a various classification, environmental, substitution, and miscellaneous plots by using the Golden Software's Grapher program. The program is developed to predict cation site-allocations at the different structural positions, including the T, Z, Y, and X sites, as well as to estimate the OH1−, F1−, Cl1−, and O2− contents. WinTcac provides editing and loading Microsoft Excel files to calculate multiple tourmaline analyses. This software generates and stores all the calculated results in the output of Microsoft Excel file, which can be displayed and processed by any other software for verification, general data manipulation, and graphing purposes. The compiled program code is distributed as a self-extracting setup file, including a help file, test data files and graphic files, which are designed to produce a high-quality printout of the related plotting software.

MINERALOGY, CLASSIFICATION, AND TECTONIC SETTING OF THE GRANITIC PEGMATITES OF NEW YORK STATE, USA

Pegmatites in New York State are largely restricted to areas of exposed Precambrian basement rocks, including the Adirondack Mountains and the Hudson Highlands. In the Adirondacks, they were emplaced into rocks that were metamorphosed during the Grenville orogeny to either the upper amphibolite facies (Adirondack Lowlands) or the granulite facies (Adirondack Highlands). Their mineral assemblages range from simple to complex, with localized occurrences of: (a) Be-, Al-, and B-rich species such as beryl, chrysoberyl, sillimanite, dumortierite, and tourmaline-supergroup minerals, (b) rare-element- and rare-earth-element-bearing minerals such as columbite-(Fe), uranopolycrase, fergusonite-(Y), uraninite, xenotime-(Y), and monazite-(Ce), (c) Ca-, F-, and K-dominant amphiboles, (d) phosphates (fluorapatite, isokite, wagnerite), (e) sulfides (bismuthinite, molybdenite), and (f) tungstates (scheelite). The pegmatite bodies from southern New York State and from some of the southern Adirondack locations display mineralogical zoning, typically with a quartz-rich core. Post-emplacement metamorphic features are observed in pegmatites in the northwestern Adirondack Lowlands, but are negligible or obscured in most of those from the southern and eastern Adirondacks. On the basis of the metamorphic grade of the host rocks, all New York pegmatites belong to the Abyssal class, whereas on the basis of mineralogy and inferred tectonic setting, they show affiliations to the NYF and mixed NYF–LCT pegmatite families. Pegmatites in the Adirondack Lowlands are related to the calc-alkaline arc magmatism of the Antwerp–Rossie granite suite and yield U–Pb zircon crystallization ages that indicate intrusion during the late Shawinigan orogeny (~1195 Ma); currently there are no known pegmatites related to the Ottawan and Rigolet orogenies in the Lowlands. Pegmatites in the Highlands yield U–Pb zircon crystallization ages that correspond to: (a) emplacement during the late Elzevirian (~1222 Ma), and metamorphism during the late Shawinigan (~1178 Ma) orogenies; (b) emplacement during the early (1098 Ma) and mid to late Ottawan orogeny (~1062–1025 Ma); (c) intrusion during the early Rigolet orogeny (1009–1003 Ma), and finally, (d) intrusion related to a thermal pulse at 949 Ma, possibly associated with the Cathead Mountain leucocratic dike swarm (935 ± 9.2 Ma). Few, if any, granitic pegmatites were intruded between 1178 and 1098 Ma in the Adirondack Highlands. The main pegmatite bodies in the Adirondack Highlands occur in association with extensional A-type granites such as the Hawkeye and Lyon Mountain Granite suites. These rocks were emplaced in response to orogenic collapse following the Ottawan orogeny. At present, relatively little can be concluded about the tectonic setting of pegmatites in southern New York.

Tourmaline as a prospecting guide for the porphyry-style deposits

Tourmaline-supergroup minerals are proposed as efficient indicators of porphyry-style Cu, Au and Sn deposits and therefore are considered as useful for both the prospection and exploration of these important mineral deposits. The tourmaline-supergroup minerals from the porphyry deposits exhibit some general features: (1) sector and oscillatory chemical zoning of crystals; (2) presence in several generations; (3) Fe → Al coupled substitution; (4) evolution from Fe-rich to Mg-rich varieties resulting from sulphide deposition at the late stage; (5) Li concentration ranging from a few to ~30 ppm. On the other hand, there are distinguishing features for tourmalines from different deposits: (1) total Mg content is ~2 apfu for tourmaline in the Cu deposits, 1–2 apfu in the Au deposits, and 0–1 apfu in Sn deposits; (2) maximum Fe tot ~3 apfu (Cu, Sn) and ~5 apfu (Au), (3) maximum F content up to 0.1 apfu (Cu, Au) and up to 0.5 apfu (Sn); (4) Fe 3+ /Fe tot ~0.5–0.8 (Cu, Au) and ~0.2 (Sn).

Mineralogy and Origin of the Dumortierite-Bearing Pegmatites of Virorco, San Luis, Argentina

The Virorco dumortierite-bearing pegmatites, in Sierra de San Luis, Argentina, are a group of thin, steeply dipping dikes one to 10 cm thick. The pegmatits are symmetrically zoned, with quartz, albite, oligoclase, tourmaline- and dumortierite-group minerals, muscovite and kyanite as the major phases; the accessory and trace minerals include beryl, chrysoberyl, garnet, fluorapatite, columbite-(Mn) to tantalite-(Mn), pollucite, gahnite, zircon, uraninite and thorite. Holmquistite was found in the exocontact assemblage. Primary textures of magmatic origin were partially disrupted by partial replacements by later minerals and incipient to strong deformation. Five textural and compositional types of tourmaline-supergroup minerals were identified in the different pegmatitic zones, ranging from dravite-rich compositions to rossmanite, passing through schorl and Mn-rich elbaite. At least four generations of the dumortierite-holtite minerals are texturally and compositionally represented in these dikes: the earliest dumortierite replaces muscovite and tourmaline, locally together with a second generation that grades into As-poor holtite. The third generation is represented by overgrowths or individual crystals of As-poor and As-rich holtite; it is commonly overgrown by the last generation of dumortierite enriched in As. The chemical evolution of dumortierite-group minerals is characterized by an increase of Ta, Nb and minor As, followed by an extensive enrichment in As (+ Sb + Bi) along with gradual decrease in Ta + Nb. The initial stage comprises the magmatic crystallization of a highly evolved and boron-rich peraluminous melt. The second stage was a prograde medium-pressure metamorphism, with a fluid-phase-related episode of crystallization. The most likely source of the initial melt is an extraction of residual melt from an almost completely crystallized rare-element parental pegmatite.

Nomenclature of the tourmaline-supergroup minerals

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

Tourmaline: an ideal indicator of its host environment

Tourmaline-supergroup minerals are ubiquitous accessory minerals in rocks of the Earth’s crust. They can adjust their composition to suit a wide variety of environments, and therefore display a remarkable range in stability in terms of pressure, temperature, fluid composition, and host-rock composition. Because of this compositional sensitivity, tourmaline is an excellent indicator of the environmental conditions in its host. This is further enhanced by negligible diffusion up to high temperatures and a strongly refractory character during subsequent host-rock alteration and weathering, as well as mechanical transport of grains. Whereas most prior research on tourmaline has focused on chemical and crystallographic characterizations and systematics of the tourmaline-supergroup minerals, recent studies are shifting the focus to a quantitative reconstruction of environmental conditions in the host using a combination of structural, compositional and crystallographic characteristics of the tourmaline. This thematic issue, which follows a special session at the 2009 GAC–MAC–AGU meeting in Toronto, highlights these exciting advances; here we discuss some of the obstacles that will need to be overcome to insure the practical applicability of tourmaline. The papers presented in this thematic issue of The Canadian Mineralogist show that we are standing on the brink of a major breakthrough in the use of tourmaline as a quantitative indicator of the chemical and physical properties of its host environment these properties may well make tourmaline the prime mineral for this purpose.