Journal of GEOsciences Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US Journal of GEOscienceshttp://www.jgeosci.org/img-system/jgeosci_cover.jpghttp://www.jgeosci.org <![CDATA[ Foreword to the special issue arising from the international conference TUR2021 ]]> Cempírek J, Bosi F, Marschall H; Vol. 67, issue 2, pages 71 - 72
The contributions to this special volume of “Journal of Geosciences” are essentially based on talks and posters presented at the 3rd International Conference on Tourmaline (TUR2021), which was held from September 9 to 11, 2021 in Portoferraio, Elba Island, Italy (Henry et al. 2021).
Elba Island plays a particularly important role for tourmaline not only scientifically, but also as the cradle of the tourmaline scientific community. The idea to organize the 1st International Conference on Tourmaline in Nové Mĕsto na Moravĕ, Brno, Czech Republic (1997) was born during the field trip on the Elba Island after the 1994 IMA meeting in Pisa from a group of mineralogists from Italy, Canada, USA and Czech Republic (including Federico Pezzotta, Frank Hawthorne, Matt Taylor, Milan Novák). Milan Novák recalls that he and Frank were sitting on seats close to a gelateria in Sant’Ilario, and maybe stimulated by their surrounding of fantastic tourmaline localities, the idea of a conference dedicated to tourmaline was born.
The 2021 edition of the Tourmaline conference was moderately affected by the subsiding COVID-19 pandemic and resulting vaccination restrictions that made it very difficult for participants from outside the EU to travel to Italy. Conference organizers successfully overcame travel difficulties by a hybrid (in-person + online) regime of the talks; this made them globally accessible, even though some presenters had to wake up very early in the morning or be online late at night. Despite the difficulties, the conference brought together 78 attendants from 12 countries, including 27 online participants, and resulted in fruitful discussions on diverse topics in boron mineralizations.
Post-conference field trips both to classical and new Elba tourmaline localities showed the large diversity of tourmaline-bearing mineral assemblages and tourmaline compositions on Elba. The field-trip guidebook was published as a special issue of Rivista Mineralogica Italiana (no. 3-2021) focused on Elba Island.
The special issue in your hands contains a diverse spectrum of tourmaline-related topics and delivers a range of remarkable papers pushing the boundaries of tourmaline-related research.
The paper “Perspectives on premetamorphic stratabound tourmalinites” by John Slack provides a comprehensive overview on tourmalinites, their diverse origins and geochemical characteristics. In addition, a unique review paper is presented by Paul Rustemeyer, who summarized his research on the inner architecture of tourmaline crystals in thin slices. The paper provides fundamental insight into a range of growth zoning features of tourmaline, which is the prerequisite for the interpretations of chemical and isotopic variations in mineralogical and geochemical patterns observed in zoned tourmaline.
An optical and structural study of a triclinic dimorph of schorl from Langesundsfjord (Norway) is presented by Fernando Cámara and co-authors; the paper shows a single crystal with trigonal-uniaxial core and triclinic-biaxial rim. Tourmaline will not stop surprising us by the wide range of elements it can accommodate in its structure. The study on Ni- and Fe3+-rich oxy-dravite by Daniela Mauro and co-authors enlarged the compositional space we normally regard in tourmaline-supergroup minerals and discusses the structure and properties of this unique tourmaline.
Paolo Ballirano and co-authors studied the high-temperature stability of Mn-bearing elbaite; their results show that its breakdown at 825 °C is preceded by structural adjustments causing Y,ZLi disorder. For the first time, the γ-LiAlSi2O6 polymorph of spodumene was noted among tourmaline-breakdown products.
Sometimes laboratory studies of exotic compositions are not far from discoveries in nature. Two papers in this issue are perfect examples of such a case. First, the experimental study of Oleg Vereschagin and co-authors presents the first results on the stability of Ti4+- and Sn4+-bearing tourmalines, showing that both elements enter the tourmaline structure favorably at low-pressure conditions. In the second paper, low PT-conditions were also derived by Kristian Drivenes on the first occurrence of Sn-rich tourmalines from Land’s End granite in SW England; multiple tourmaline generations were deciphered based on an outstanding amount of data and high-resolution elemental mapping.
The petrology of tourmaline in meta-evaporite rocks was studied by Barbara Dutrow and Darrell Henry; their study shows that in the sulfate-rich meta-evaporite of the Arignac, tourmaline preserves a record of HT-LP conditions, whereas the surrounding mineral assemblage was subsequently overprinted by retrograde reactions. Tourmaline parageneses in polymetamorphic rocks are rarely simple; this is shown by Peter Bačík et al., who documented a very large textural and compositional variability of tourmaline minerals in tourmalinites from the Gemeric Unit of western Slovakia.
We especially thank Lenka Skřápková, who took care of all essential formatting checks and adjustments of all manuscripts in this issue. We thank the Editorial Board of the Journal of Geosciences and Chief Editor Jakub K. Plášil, who made this issue possible. Finally, we thank all referees for their careful work and devotion in helping papers published in this special issue to achieve the highest quality. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.360 Editorial http://www.jgeosci.org/rss.php?ID=jgeosci.360
<![CDATA[ Perspectives on premetamorphic stratabound tourmalinites ]]> Slack JF; Vol. 67, issue 2, pages 73 - 102
Stratabound tourmalinites are metallogenically important rocks that locally show a close spatial association with diverse types of mineralization, especially volcanogenic massive sulfides (VMS) and clastic-dominated (CD) Zn-Pb deposits. These tourmalinite occurrences pan the geologic record from Eoarchean to Jurassic. Host lithologies are dominated by clastic metasedimentary rocks but in some areas include metavolcanic rocks, marble, or metaevaporites. Stratabound and stratiform (conformable) tourmalinites commonly display sedimentary structures such as graded beds, cross-beds, and rip-up clasts. In most cases, field and microtextural relationships are consistent with a synsedimentary to the early diagenetic introduction of boron as a precursor to tourmaline formation.
Whole-rock geochemical data for major, trace, and rare earth elements (REE) provide valuable insights into tourmalinite origins. Al-normalized values relative to those for least-altered host metasedimentary rocks suggest that tourmalinites in proximal settings at or near hydrothermal vent sites characterized by high fluid/rock regimes (e.g., Sullivan Pb-Zn-Ag deposit, Canada) have very different signatures than those in low fluid/rock, distal settings (e.g., Broken Hill Pb-Zn-Ag deposit, Australia). The high fluid/rock regimes at Sullivan show large mass changes of +60 % for Mg and +180 % for Mn, as well as large variations in abundances of light and middle REE. In contrast, tourmalinite formation in low fluid/rock regimes yields minimal Al-normalized changes in major elements, trace elements, and REE. Boron isotope values of tourmalinite-hosted tourmaline vary widely from -26.1 to +27.5 ‰, and are attributed mainly to boron sources (e.g., sediments, evaporites) with generally minor influence from processes such as formational temperature, fluid/rock ratio, and secular variation in seawater δ11B values.
Laterally extensive stratiform tourmalinites formed mainly by syngenetic or early diagenetic processes on or beneath the seafloor. The syngenetic process is attributed to the interaction of vented B-rich brines with aluminous minerals in sediments, whereas the diagenetic process involves the selective replacement of aluminous sediments by B-rich fluids. Modern examples of tourmalinites, as yet undiscovered, may exist in metalliferous sediments of the Red Sea and the eastern Pacific Ocean, in altered volcaniclastic sediments within active seafloor-hydrothermal systems of the South Pacific, and in hydrothermal mounds and vents associated with mafic sill complexes in extensional basins as in the North Sea and South China Sea. Stratabound tourmalinites that contain base-metal sulfides, high Mn concentrations (>1 wt. % MnO), or positive Eu anomalies can be valuable exploration guides for base-metal sulfide deposits in sedimentary and volcanic terranes. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.349 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.349
<![CDATA[ The inner architecture of tourmaline crystals, as inferred from the morphology of color zones in thin slices ]]> Rustemeyer P; Vol. 67, issue 2, pages 103 - 128
Tourmaline crystals are known for their variety of morphologies, intricate and complex growth features, and wide spectrum of colors. If dark tourmalines crystals are sliced and ground to an optimum thickness, intensely colored intrasectoral zones and other fine features become visible. Evaluating a series of slices lead to an understanding of the three-dimensional inner morphology of the crystal.
A screening of thousands of dark tourmaline crystals led to a collection of more than 30 000 slices, from which the most important morphological features are revealed. These features include sector zones, intrasectoral color zones, concentric growth-zones, diverse types of delta-shaped features as second intrasectoral triangular color domains, various mechanisms of repair of fracture surfaces (healing) and of corroded crystals. Six mechanisms are documented in subdivision phenomena, in which a single crystal is subdivided into a bundle of parallel needle-shaped crystals.
Hollow skeletal crystals and monocrystalline dendritic growth were found in the core of freely crystallized parallel aggregates. Such parallel aggregates with normal compact crystals in the same pocket could support the hypothesis that crystallization occurred from coexisting media, a melt and an aqueous phase that had undergone phase separation. In addition, structures similar to the parallel aggregates were found in graphic intergrowths of tourmaline with quartz, feldspar or mica. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.348 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.348
<![CDATA[ Schorl-1A from Langesundsfjord (Norway) ]]> Cámara F, Bosi F, Skogby H, Hålenius U, Celata B, Ciriotti ME; Vol. 67, issue 2, pages 129 - 139
A crystal fragment of schorl from Langesundsfjord (Norway), showing a zonation with a biaxial optic behavior in the rim, was studied by electron microprobe analysis, single-crystal X-ray diffraction, Mössbauer, infrared and optical absorption spectroscopy and optical measurements. Measured 2Vx is 15.6°. We concluded that biaxial character of the sample is not due to internal stress because it cannot be removed by heating and cooling. Diffraction data were refined with a standard R3m space group model, with a = 16.0013(2) Å, c = 7.2263(1) Å, and with a non-conventional triclinic R1 space-group model keeping the same hexagonal triple cell (a = 16.0093(5) Å, b = 16.0042(5) Å, c = 7.2328(2) Å, α = 90.008(3)°, β = 89.856(3)°, γ = 119.90(9)°), yielded Rall = 1.75% (3136 unique reflections) vs. Rall = 2.53% (17342 unique reflections), respectively. The crystal-chemical analysis resulted in the chemical formula X(Na0.98K0.01 0.01)Σ1.00 Y(Fe2+1.53Al0.68Mg0.35Ti0.20Fe3+0.20Mn0.02V0.01Zn0.01)Σ3.00 Z(Al5.10Fe2+0.50 Mg0.40)Σ6.00 (Si6O18)(BO3)3(OH)3 [(OH)0.39F0.22O0.39]Σ1.00, which agrees well in terms of calculated site-scattering (X 10.9 epfu, Y 63.7 epfu, Z 83.7 epfu) and refined site-scattering (X 11.4 epfu, Y 63.4 epfu, Z 83.6 epfu). About 0.19 apfu Fe2+ is at the Z sites in the R1 model that showed that one out of six independent Z sites (Zd) has higher refined site scattering [15.5 eps vs. mean 13.7(2) eps for the other five sites] and larger mean bond length [1.969 Å vs. 1.927(6) Å for the other five sites] and larger octahedral angle variance [53° vs. 42(3)°]. All these features support local order of Fe2+ at the Zd site. Optical absorption spectra also show evidence of Fe2+ at the Z sites. The elongation of the Zd-octahedron is along a direction that forms an angle of ca. 73° with a unit-cell edge and is coincident with the direction of the γ-refraction index. All these data support the triclinic character of the structure of the optically biaxial part of the tourmaline sample from Langesundsfjord and provide evidence that even in the presence of excellent statistical agreement factors from excellent X-ray diffraction data, the lowering of symmetry due to cation ordering may have been overlooked in many other tourmaline samples in the absence of a check of the optical behaviour. According to the nomenclature rules, the studied triclinic schorl, should be named schorl-1A. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.344 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.344
<![CDATA[ Nickel- and Fe3+-rich oxy-dravite from the Artana Mn prospect, Apuan Alps (Tuscany, Italy) ]]> Mauro D, Biagioni C, Hålenius U, Skogby H, Dottorini V, Bosi F; Vol. 67, issue 2, pages 141 - 150
Nickel- and Fe3+-rich oxy-dravite was identified on a specimen collected in the Artana Mn prospect, Carrara, Apuan Alps, Tuscany, Italy. Oxy-dravite occurs as brownish-orange prismatic crystals, up to 0.3 mm in length, associated with quartz, carbonates, and hematite. Electron microprobe analysis gave (in wt. % - average of 7 spot analyses): SiO2 35.81, TiO2 0.41, B2O3(calc) 10.38, Al2O3 29.36, V2O3 0.78, Cr2O3 0.09, Fe2O3 3.32, FeO 0.33, MgO 8.04, CaO 0.39, MnO 0.34, NiO 3.46, ZnO 0.40, Na2O 2.84, F 0.29, H2O(calc) 3.00, O = F -0.12, total 99.12. The Fe3+/Fetot ratio was calculated based on optical absorption spectroscopy. The empirical ordered formula of the studied sample is (with rounding errors) X(Na0.92Ca0.070.01)Σ1.00 Y(Mg2.01Ni2+0.47Fe3+0.33Ti0.05 Mn2+0.05Fe2+0.05Zn0.05)Σ3.00 Z(Al5.80V0.10Cr0.01Fe3+0.09)Σ6.00 Si6O18(BO3)3 V(OH)3W[O0.50(OH)0.35F0.15]Σ1.00. This is an intermediate member of the dravite-oxy-dravite series. In naming it, the prefix oxy- was preferred since WO is very close to being larger than 0.5 atoms per formula unit. Infrared spectroscopy revealed the occurrence of significant amounts of W(OH), and allowed to propose of specific short-range arrangements around the O(1) and O(3) sites. Unit-cell parameters are a = 15.9349(11), c = 7.2038(5) Å, V = 1584.1(2) Å3, space group R3m. The crystal structure was refined by single-crystal X-ray diffraction data to R1 = 0.0146 on the basis of 1138 unique reflections with Fo > 4σ(Fo) and 94 refined parameters. The optimized crystal-chemical formula is X(Na0.92Ca0.070.01)Σ1.00 Y(Mg1.21Al0.80Ni2+0.47Fe3+0.26 Ti0.05Mn2+0.05Zn0.05V0.10Cr0.01)Σ3.00 Z(Al5.00Mg0.80Fe3+0.16Fe2+0.05)Σ6.00 Si6O18(BO3)3O(3)(OH)3O(1) [O0.50(OH)0.35F0.15]Σ1.00. Nickel is ordered at the Y site, in agreement with results obtained on synthetic tourmalines. Oxy-dravite is likely the result of the metamorphic recrystallization of Mn-rich layers at the top of the Liassic carbonates belonging to the Marble Formation of the Apuan Alps Metamorphic Complex. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.346 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.346
<![CDATA[ HT breakdown of Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar ]]> Ballirano P, Celata B, Skogby HK, Andreozzi GB, Bosi F; Vol. 67, issue 2, pages 151 - 161
The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition X(Na0.410.35Ca0.24)Σ1.00 Y(Al1.81Li1.00Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00ZAl6[T(Si5.60B0.40)Σ6.00O18](BO3)3(OH)3 W[(OH)0.50F0.13O0.37]Σ1.00 was investigated using both in situ High-Temperature X-Ray powder diffraction (HT-pXRD) and ex situ X-Ray single-crystal diffraction (SC-XRD) on two single crystals previously heated in the air up to 750 and 850 °C. The first occurrence of mullite diffraction peaks allowed us to constrain the breakdown temperature of Mn-bearing elbaite at ambient pressure, at 825 °C. The breakdown products from the HT-pXRD experiments were cooled down to ambient temperature and identified via pXRD, represented by B-mullite and &gama;-LiAlSi2O6. A thermally induced oxidation of Mn2+ to Mn3+ was observed with both in-situ and ex-situ techniques; it started at 470 °C and is assumed to be counterbalanced by deprotonation, according to the equation: Mn2+ + (OH)- → Mn3+ + O2- + 1/2H2. At temperatures higher than 752 °C, a partial disorder between the Y and Z sites is observed from unit-cell parameters and mean bond distances, possibly caused by the inter-site exchange mechanism YLi + ZAl → ZLi + YAl.  ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.347 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.347
<![CDATA[ Ti4+ and Sn4+-bearing tourmalines - pressure control and comparison of synthetic and natural counterparts ]]> Vereshchagin OS, Wunder B, Baksheev IA, Wilke FDH, Vlasenko NS, Frank-Kamenetskaya OV; Vol. 67, issue 2, pages 163 - 171
Me4+-bearing (Me4+ = Sn, Ti) dravite analogs were synthesized in the system MeO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O at 700 °C and 4 / 0.2 GPa in four hydrothermal experiments. Tourmalines form rosette-like aggregates and needle-like crystals that are chemically homogeneous. Tourmaline crystals obtained in high-pressure runs (4 GPa) are much smaller (up to 0.1 × 2 μm) and have lower Me4+ (0.27 wt. % SnO2, 0.57 wt. % TiO2) than those from the low-pressure (0.2 GPa) runs (up to 1 × 5 μm; 1.77 wt. % SnO2, 2.25 wt. % TiO2). Synthetic analogs of rutile, quartz and coesite were obtained in the system TiO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O, whereas synthetic analogs of cassiterite, tin-rich (up to ˜19.55 wt. % SnO2) Na-pyroxene, MgSn(BO3)2 (Mg-analogue of tusionite), quartz and coesite were synthesized in the system SnO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O. We suggest that at a high temperature (≤ 700 °C), the pressure negatively affects the Ti incorporation into the tourmaline structure. In contrast, at relatively low pressures, the Ti incorporation in tourmaline structures is governed by the Ti content in the mineral-forming medium. Low-pressure conditions are feasible for Sn incorporation in the tourmaline structure. The presence of Ti4+ and Sn4+ cations in structures of the synthesized tourmalines (probably at octahedrally coordinated sites), is also indicated by changes in the unit-cell parameters. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.345 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.345
<![CDATA[ Sn-rich tourmaline from the Land’s End granite, SW England ]]> Drivenes K; Vol. 67, issue 2, pages 173 - 189
Multiple generations and growth stages of tourmaline from a hydrothermal quartz-tourmaline rock from the Land’s End granite, SW England, were investigated by Electron Probe MicroAnalyzer (EPMA) to reveal details of the variation in tourmaline composition with emphasis on the distribution of Sn. Tourmaline shows a large range in chemical composition, mostly on the dravite-schorl solid solution and towards more Fe-rich compositions. Several growth zones have very high Fe levels (> 3.5 apfu) with a significant amount of Fe3+ coupled with low Al. The main substitution vectors controlling the major element composition are Fe2+Mg-1 and Fe3+Al-1. The Fe-Mg exchange is the main substitution in the earlier growth stages, whereas the Fe-Al substitution becomes more important towards the end of the crystallization sequence. Tin is commonly associated with the high Fe-zones, but all Fe-rich zones do not necessarily have elevated Sn content. Octahedral sites in tourmaline, most likely the Y-site, host Sn through the proposed coupled substitution YZSn4+ + 2YZFe2+ + 5YZFe3+ + WO2- ↔ 2YZMg2+ + 6YZAl3+ + WOH-. The thin Sn-rich zones, hosting up to 2.53 wt. % SnO2, are interpreted to coincide with the onset of cassiterite crystallization, and the lower Sn content in subsequent growth zones reflects the fluid chemistry and Sn solubility in a cassiterite-buffered hydrothermal system. This study demonstrates the suitability of quantitative X-ray mapping in identifying and quantifying minor elements in finely-spaced growth zones. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.351 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.351
<![CDATA[ Calcium-rich dravite from the Arignac Gypsum Mine, France: Implications for tourmaline development in a sulfate-rich, highly magnesian meta-evaporite ]]> Dutrow B, Henry D; Vol. 67, issue 2, pages 191 - 207
Tourmaline occurs in a wide range of compositional environments, but its occurrence in meta-evaporites is less commonly investigated. Highly magnesian (XMg = 0.90-0.98), poikiloblastic tourmaline occurs in a sulfate-rich, anhydrite-gypsum-bearing meta-evaporite in the Arignac Gypsum Mine, France and preserves a petrologic record of this unusual geochemical environment. Originally a Triassic evaporite deposit, the sample is interpreted to have undergone high-temperature-low-pressure (HT-LP) metamorphism and subsequently experienced low-grade, highly deformed overprints. Poikiloblastic tourmaline preserves relicts of the HT-LP mineral assemblage, as inclusions of anhydrite, phlogopite, dolomite, tremolite, Cl-rich scapolite (71-85 % marialite component), rutile, zircon, and fluor-apatite. The low-grade deformational overprints are characterized by partial replacement of anhydrite by gypsum, phlogopite by clinochlore, dolomite by talc, and scapolite by mixtures of near end-member albite and K-feldspar with later crosscutting calcite and celestite. Tourmaline develops two textural zones - zone 1 with few mineral inclusions and zone 2 with abundant inclusions and/or complex chemical zoning.
The tourmaline is mostly dravite or Ca-rich (> 0.25 apfu Ca) dravite with a few analyses of oxy-dravite and one attaining a high-Ti composition consistent with the hypothetical species “magnesio-dutrowite”. The average zone-1 tourmaline has an ordered structural formula of (Na0.61 Ca0.320.07) (Mg2.82 Fe3+0.11 Ti0.06) (Al5.87 Fe3+0.13) (Si5.93 Al0.07) (BO3)3 (OH)3 (OH0.51 O0.41 F0.07) and the average zone-2 tourmaline has an ordered structural formula of (Na0.72 Ca0.210.07) (Mg2.80 Ti0.20) (Al5.76 Fe3+0.17 Mg0.07) (Si5.91 Al0.09) (BO3)3(OH)3 (OH0.57 O0.39 F0.04). Inter- and intragranular chemical variability observed in the tourmaline is primarily consistent with the operation of combinations of the (CaMg)(NaAl)-1, (R3+O)[R2+(OH)]-1 and (TiO2)[R2+(OH)2]-1 exchange vectors.
Assuming 600 °C for tourmaline formation, the Na-Ca contents of the X-site yield fluid composition estimates of Na1+fluid of 480 mM and Ca2+fluid of 160 mM for average zone-1 and Na1+fluid of 460 mM and Ca2+fluid of 110 mM for average zone-2. Na is comparable to modern seawater, with Ca increased at least ten times. Partition coefficients involving aqueous fluids in equilibrium with tourmaline suggest that the dissolved constituents of the coexisting aqueous fluid were 144.7 ppm B, 3.0 ppm Cr, 8.6 ppm Mn and 53.6 ppm Ti for zone 1 compared to 145.4 ppm B, 0.7 ppm Cr, 13.6 ppm Mn and 16.6 ppm Ti for zone 2. Comparison of the Arignac tourmaline data with other meta-evaporites and sulfate- or scapolite-bearing lithologies demonstrate the remarkable commonalities and a few differences exhibited by tourmalines of these lithologies. These data contribute to our understanding of Mg-rich tourmaline formation and coexisting fluid compositions, require a modification of the AFM host rock compositional fields, and underscore the breadth of environments in which tourmaline forms. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.352 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.352
<![CDATA[ Crystal chemistry and evolution of tourmaline in tourmalinites from Zlatá Idka, Slovakia ]]> Bačík P, Odzín D, Uher P, Chovan M; Vol. 67, issue 2, pages 209 - 222
Tourmalinites occur in early-Paleozoic metamorphic rocks of the Gemeric Unit near Zlatá Idka village, Western Carpathians, eastern Slovakia. Tourmaline compositions, analyzed with the electron microprobe, include a wide range of tourmaline species. Tourmaline in tourmalinites from Zlatá Idka is compositionally variable, with the dominant substitution Mg-Fe2+ consistent with prevalent schorl-dravite compositions and their fluor- and oxy-dominant counterparts - fluor-schorl, fluor-dravite, oxy-schorl and oxy-dravite. Portions of tourmaline are enriched in Ca in the form of the fluor-uvite and magnesio-lucchesiite components. A subset of the compositions has Ti > 0.25 atoms per formula unit (apfu) and corresponds to the hypothetical “magnesio-dutrowite”, Mg-dominant analogue of dutrowite. In addition, some of the tourmalines are X-site vacant and classified as foitite. The crystal chemistry of tourmaline is complex and influenced by several exchange mechanisms, including Mg(Fe)-1, Al□(Mg,Fe)-1Na-1, AlO(Mg,Fe)-1(OH)-1 (Mg,Fe)CaAl-1Na-1, MgCaOAl-1-1(OH)--1, Ti0.5O(Fe,Mg)-0.5(OH)-1 and TiMg(Al)-2 substitutions. In general, tourmalines in all samples usually have oscillatory-zoned dravitic cores and schorlitic rims (Tur I). However, in ZLT-4 and ZLT-6 samples, some crystals have secondary Mg-dominant and Ca-enriched overgrowths (Tur II), partially replacing Tur I. Tourmalinites were most likely produced by regional or contact metasomatic processes, likely due to the intrusion of the Permian Poproč granitic massif. Origin of tourmalinites likely results from the flow of late-magmatic to early post-magmatic B,F-rich fluids from the granite intrusion into adjacent metamorphic rocks. The tourmaline crystallization and its resulting chemical composition were controlled by both the metapelitic host rock and the granitic intrusion; the Mg-rich cores of the Tur I are most likely compositionally related to the metapelitic host rock, whereas later schorlitic to foititic compositions in rims suggest origin due to the intrusion-triggered fluid flow. The significant changes and oscillations of tourmaline zoning imply a dynamic, unstable fluid regime. The late Ca-rich Tur II could result from subsequent metasomatic processes associated with the alteration of host-rock minerals. ]]>
http://www.jgeosci.org/rss.php?ID=jgeosci.350 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.350