Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.https://www.jgeosci.orgen-US https://www.jgeosci.org/img-system/jgeosci_cover.jpghttps://www.jgeosci.org Cempírek J, Bosi F, Bačík P; Vol. 71, issue 1, pages 1 - 2
]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0071.26 Editorial https://www.jgeosci.org/rss.php?ID=jgeosci.0071.26 Bačík P, Skřápková L; Vol. 71, issue 1, pages 3 - 19
Tourmalines commonly contain minor amounts of Ti. However, Ti-rich compositions are relatively rare, with only two approved tourmaline species - dutrowite and magnesio-dutrowite, showing significant Ti⁴⁺ occupancy at the Y site. According to the currently valid tourmaline nomenclature, Ti⁴⁺ is assigned exclusively to the Y site. To evaluate the structural mechanisms capable of introducing Ti⁴⁺ into the tourmaline structure, a series of modelled mechanisms was examined to identify the most plausible and locally stable arrangements involving Ti. For this purpose, short-range bond-valence requirements, distortion indices characterizing local polyhedral geometry, and long-range constraints expressed through the <Y-O> and <Z-O> ratios were applied to assess the likelihood of individual components containing Ti⁴⁺ at various crystallographic sites and within different local environments. Most tested structural mechanisms exhibited severe anion underbonding, overbonding, or produced unrealistic distortion indices. Nevertheless, the calculations indicate that Ti⁴⁺ can, in principle, be accommodated at the Y, Z, and even T sites, and prove that its higher valence and larger cation size are efficiently redistributed within the structure. The total amount of Ti⁴⁺ appears to be structurally limited to approximately one apfu, as higher Ti⁴⁺ contents (>1 apfu) lead to substantial structural deformation resulting in an unstable arrangement. ]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0052.25 Original paper https://www.jgeosci.org/rss.php?ID=jgeosci.0052.25 Luppi R, Skogby H, Pezzotta F, Hålenius U, Di Benedetto F, Zoleo A, Bosi F; Vol. 71, issue 1, pages 21 - 37
Tourmalines, with their wide range of colors, are among the most sought-after and highly valued minerals in the gemstone market. For this reason, they are often subjected to laboratory treatments to enhance their color. Among the rarest and most expensive tourmaline varieties, the Cu-bearing Paraíba tourmaline stands out for its distinctive and unique neon blue color. In this study, we investigate a multicolored Cu-bearing tourmaline from the Mavuco deposit in Mozambique. Unlike most Paraíba tourmalines, which are typically classified as fluor-elbaite, this sample is compositionally identified as elbaite. Electron microprobe analysis, optical absorption, Fourier-transform infrared, and electron paramagnetic resonance spectroscopy are used to clarify the causes of its natural color zoning and the mechanisms underlying color changes induced by heat treatment. The untreated crystal exhibits distinct color zones: a dark green root (R1), a yellowish light green band (R2), intermediate light blue (I1) and blue violetish (I2) zones, and a greyish violet terminal portion (T). Chemical analyses show that Fe, Mn, Cu, and Ti concentrations change systematically along the crystal, with Fe²⁺/ Fe³⁺ exchange-coupled pairs interactions and Fe²⁺- Ti⁴⁺ intervalence charge transfer (IVCT) controlling the dark green color of the crystal root (R1), while Mn²⁺- Ti⁴⁺ IVCT and Fe²⁺- Ti⁴⁺ IVCT producing the yellowish light green color (R2). Cu²⁺ is the only chromophore in the light blue (I1) zone, classifying it as a natural Paraíba-type tourmaline. In the blue violetish (I2) and greyish violet (T) zones, the presence of Mn³⁺, is likely the result of an interaction with localized natural radioactive fluids, and in a combination with Cu²⁺ absorptions produces violetish color. The heat treatment under reducing conditions (˜500 °C, 18 h) selectively reduces Mn³⁺ to Mn²⁺, homogenizing the color of the intermediate (I2) and terminal (T) zones to light blue. The resulting Paraíba-like color is therefore attributed solely to Cu²⁺.
This study demonstrates that the color of Cu-bearing tourmalines is governed by the combination, concentration and oxidation state of Cu, Mn, Fe and Ti, as well as by post-crystallization redox processes. It also highlights the importance of tourmaline color as a petrogenetic recorder of subtle physicochemical changes in its crystallization environment. These findings provide interesting insights into pegmatitic evolution, the role of natural irradiation in gemstone coloration, and the gemological distinction between natural and treated Paraíba-type tourmalines. ]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0058.25 Original paper https://www.jgeosci.org/rss.php?ID=jgeosci.0058.25 Macek I, Novák M, Gadas P, Bačík P, Vašinová-Galiová M; Vol. 71, issue 1, pages 39 - 56
Late fibrous tourmalines from three granitic pegmatites with similar size, internal structure and compositionally contrasting Mg- and Mg,Ca-rich host rocks (serpentinite - barren pegmatite Dolní Bory North, dolomite-calcite marble + calc-silicate rock - elbaite-subtype pegmatite Bližná I, both localities in Czech Republic, and spodumene-subtype pegmatite Tamponilapa Ambany, Sahatany Valley, Madagascar) were examined. We focused on their morphology, zonal patterns, chemical composition (EMPA, LA-ICP-MS), powder XRD and RAMAN spectroscopy as well as on the chemical composition of the associated early tourmalines (schorl - Dolní Bory, Mg-rich liddicoatite-elbaite - Bližná, dravite-schorl - Tamponilapa) to reveal textural and compositional relations. Fibrous tourmalines are Mg-rich and F- and Ca-poor: Dolní Bory - greyish Fe-rich dravite with Al_tot 6.07-6.33 apfu, Mg 1.45-1.62 apfu, Fe_tot 0.97-1.26 apfu; Bližná - blue to colorless dravite with Al_tot 6.15-6.36 apfu, Mg 1.97-2.22 apfu, Fe_tot 0.20-0.34 apfu; Tamponilapa - colorless to brownish dravite-oxy-dravite and rare magnesio-foitite with Al_tot 6.42-6.77 apfu, Mg 1.78-2.23 apfu, Fe_tot 0.16-0.24 apfu; low Ca ≥ 0.13 apfu, moderate X-site vacancy 0.31-0.51 pfu, very low F ≥ 0.01 apfu (Dolní Bory, Tamponilapa) are typical. The contents of major, minor and trace elements manifest that the following factors play a substantial role for composition of fibrous tourmalines: external fluids from host rocks with contrasting chemistry (e.g., Mg, Cr, Ni, V), internal residual fluids from pegmatite (H₂O, B, Li, Si, Al), and the chemical compositions of early tourmalines (B, Si, Al, Fe). Fibrous habitus of tourmalines was facilitated by crystallization from low-T aqueous fluids and controlled chiefly by crystal-structural constraints and rate of crystallization. The complex textural relations do not allow to reliably define their origin - direct replacement of early tourmaline, its overgrowing after its partial dissolution or combination of both processes. ]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0062.26 Original paper https://www.jgeosci.org/rss.php?ID=jgeosci.0062.26 Rustemeyer P; Vol. 71, issue 1, pages 57 - 77
Innumerable dark tourmaline crystals from pockets in pegmatites around Skardu in Northern Pakistan have entered the mineral market as by-products of the trade of rare gem minerals. The author has searched this material for 40 years and thus found a great diversity of external and internal corrosion phenomena. Numerous crystals exhibit etch pits, hillocks, typical examples of which are presented from different types of crystal faces. Very special are regular linear etch grooves forming a trigonal pattern on the Pedion face and a checkerboard pattern on the adjacent trigonal pyramid face. A second group of tourmaline crystals exhibit more extreme dissolution phenomena, from crystals with a selectively dissolved pyramidal core zone, deeply etched faces with small remnants of the original external faces to the extreme of dissolving bodies. The observations of strongly corroded crystals support the model of an initially tourmaline-undersaturated hydrous fluid that separated from the pegmatite melt. The mechanisms of the formation of various etched morphologies on the tourmaline crystals as well as form-specific corrosion rates are discussed.
This part of the morphological study is the basis for the second part, in which similar corroded crystals are found as fossil complex color zones within outwardly perfect tourmaline crystals. ]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0056.25 Original paper https://www.jgeosci.org/rss.php?ID=jgeosci.0056.25 Rustemeyer P; Vol. 71, issue 1, pages 79 - 99
From several hundred dark tourmaline crystals from Skardu, Pakistan, stacks of tourmaline thin sections perpendicular to the c-axis (“cross-cuts”) were prepared. In contrast to standard methods, the thickness of the slices was adjusted to their optical density, as they have been ground thin until the colour zoning became visible and colour depth reached its optimum. Some of these outwardly inconspicuous tourmaline crystals show quite complex corrosion patterns in their cross-cuts. The images of the stacks of thin section deliver for each position in the crystal information about the morphology of the original crystals, the usually complex contours of the corroded crystal and the stages of growth on the way to the healed crystal. With this morphological information from the stacks of cross-cuts profiles of the crystals parallel to the c-axis were derived, which then allowed to reconstruct simplified growth histories of the tourmaline crystals.
The examples show that corrosion may take place during early, intermediate, and late stages of tourmaline growth. Some crystals exhibit even two corrosion and subsequent healing episodes. Twelve examples of such crystals are presented, illustrating the diversity and different intensity of corroded and healed tourmaline. This inner architecture of crystals is a good recorder of the discontinuous growth and dissolution history of tourmaline, giving valuable information on the sequence and intensity of supersaturated and undersaturated, corrosive fluids in the crystallizing pegmatite pocket. ]]> https://www.jgeosci.org/rss.php?ID=jgeosci.0055.25 Original paper https://www.jgeosci.org/rss.php?ID=jgeosci.0055.25