Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US http://www.jgeosci.org/img-system/jgeosci_cover.jpghttp://www.jgeosci.org Ryznar J, Prsek J, Włodek A, Heredia B, Thomsen TB, Uher P; Vol. 69, issue 3, pages 129 - 150
This study provides detailed mineralogical, textural and compositional features of cassiterite from the Bugarura-Kuluti Nb-Ta-Sn deposit located in the Mezoproterozoic Karagwe-Ankole Belt of eastern Rwanda. The cassiterite occurs within two-mica, peraluminous “tin” granites, lithium-caesium-tantalum (LCT) pegmatites, greisens, and hydrothermal quartz-muscovite veins. The cassiterite from quartz-muscovite veins exhibit progressive oscillatory zonation and contains rutile and ilmenite inclusions. The chemical signature of the vein cassiterite is marked by elevated Ti and low Ta, Nb and Fe concentrations. The cassiterite from the pegmatites and associated greisens contains numerous Ta-Nb oxide inclusions. Both types expose complex and patchy textures, often associated with younger generations of cassiterite. Contrary to cassiterite from quartz-muscovite veins, the pegmatitic/greisen cassiterite has elevated Ta, Nb and Fe and low Ti contents. The cassiterite from the granite is anhedral, exhibits irregular patchy textures, and lacks zonation. Mineral inclusions in the two-mica granite cassiterite were not observed. In comparison to cassiterite from veins, greisens and pegmatites, granitic cassiterite is characterised by chemical compositions showing intermediate concentrations of minor and trace elements. The origin of Sn mineralisation involves segregation of rare lithophile elements into residual pegmatite melts and hydrothermal fluids and their crystallization in Sn and Nb-Ta minerals in LCT pegmatites and quartz veins. The majority of the cassiterite in pegmatites precipitated during the process of greisenisation by circulating aqueous solutions, probably exsolved from pegmatite melts. Most of hydrothermal fluids, enriched with volatiles and Sn, used tectonic fractures to escape farther to adjacent metasedimentary rocks allowing for the generation of quartz-muscovite veins by interaction with country rocks and/or mixing with meteoric waters. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.389 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.389 Janoušek V; Vol. 69, issue 3, pages 151 - 160
HafAn is a new R-language script for recalculation, statistical treatment and graphical presentation of the Hf isotopic data from whole-rock samples or, more commonly, obtained in situ (e.g. by LA MC ICP-MS) from igneous, metamorphic and detrital zircon. Besides the recalculation of the present-day Hf isotopic ratios to initial ones, epsilon values and various variants of model ages, it allows presentation of such univariate data in the form of histograms, boxplots, stripplots and violin plots. Arguably the most telling and sophisticated way of displaying the Hf isotopic data are the Hf isotopic growth (age-¹⁷⁶Hf/¹⁷⁷Hf or epsilon Hf) diagrams. The HafAn has been designed as a plugin module for the Geochemical Data Toolkit (aka GCDkit), a well-established system for interpretation of the whole-rock geochemical data. This approach potentially allows interpretation (and plotting) of the Hf isotopic data jointly with the rest of the chemical and/or isotopic signature, as well as merging or overplotting several datasets on a single diagram. Moreover, R/GCDkit contains a plethora of additional statistical tools that can be newly applied to the Hf isotopic data as well. Our HafAn also profits from a wide palette of the file formats available for reading data input, as well as exporting results and graphical output. The HafAn plugin, together with all the GCDkit-family tools, can be downloaded from https://gcdkit.org ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.391 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.391 Juroszek R, Prusik K, Schäfer C; Vol. 69, issue 3, pages 161 - 172
The new mineral alfredcasparite, ideally Sr₂TiO(Si₂O₇), is a Sr-analogue of fresnoite Ba₂TiO(Si₂O₇). It is an accessory phase, which was found in a silicate xenolith within the tephritic lava from the Caspar quarry, Bellerberg volcano, Germany. Usually, alfredcasparite occurs together with potentially new mineral (Sr₂Na)Ti(Si₂Fe)Si₂O₁₄, in small cracks in the K-feldspar (sanidine)-quartz-pyroxene (aegirine-diopside)-matrix, and wollastonite. In the type material, alfredcasparite rarely forms flattened crystals up to 30 μm in diameter; more common are irregular grains which do not exceed 15 μm in size. Alfredcasparite is colourless, transparent, and has a vitreous lustre and white streak. It is brittle with an irregular fracture and exhibits good cleavage on (001). The calculated density equals 3.950 g‧cm^-3 and the Mohs hardness is ˜3-4. Optically, alfredcasparite is uniaxial and non-pleochroic under transmitted light (n_mean = 1.823). The empirical formula calculated based on 8 O atom per formula unit (pfu) is (Sr_1.54Ba_0.29Ca_0.10Na_0.03K_0.03)_∑1.99(Ti_0.94Fe_0.04Mg_0.03)_∑1.01(Si_2.04Al_0.01)_∑2.05O₈, which leads to the ideal end-member formula Sr₂TiO(Si₂O₇). According to the electron back-scattered diffraction (EBSD) pattern, tetragonal alfredcasparite fit to the using structural model of synthetic Sr₂TiO(Si₂O₇) with the following parameters: space group P4bm, a = 8.32 Å, c = 5.02 Å, V = 347.77 ų, Z = 2. The crystal structure consists of layers composed of corner-sharing TiO₅ square-base pyramids and Si₂O₇ disilicate units, with interstices occupied by Sr²⁺ cations. The Raman spectrum of alfredcasparite is characterised by a sharp and intense Raman band at 863 cm^-1 with lower-intensity shoulder bands at ∼850 and 874 cm^-1, which have a complex nature and are assigned to both symmetric stretching SiO₃ vibrations of the disilicate group (Si₂O₇)^6- and Ti-O vibrations in the square pyramid (TiO₅)^6-. The authors assume that alfredcasparite forms as an effect of residual melt crystallisation enriched in incompatible elements like Ba, Sr, Ti, or P at a temperature around 1000 ˚C. In addition, based on chemical EPMA and Raman spectroscopy investigations, the second world occurrence of wesselsite, SrCuSi₄O₁₀, was confirmed in the same xenolith. Chemical analyses resulted in the empirical formula (based on 10 O pfu) (Sr_0.67Ba_0.28Ca_0.04Na_0.01K_0.01)_∑1.01 (Cu_0.93Mg_0.04Fe_0.02)_∑0.99Si_4.01O₁₀. Furthermore, the Raman spectrum of natural wesselsite was obtained and described in detail for the first time, in relation to its crystal structure. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.394 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.394 Sejkora J, Kristek J, Škácha P, Dolníček Z; Vol. 69, issue 3, pages 173 - 182
We have studied the rare ammonium uranyl phosphate mineral, uramphite, from the small uranium occurrence Nová Ves pod Pleší, central Bohemia (Czech Republic). It has been found on a few specimens and forms rare groups up to 1 mm in size in small vugs of limonite veins in altered rocks in association with meta-autunite, metatorbenite and churchite-(Y). Uramphite is pale yellow to light greenish yellow with a pale yellow streak and shows weak fluorescence in a pale yellow hue under 254 nm and 366 nm UV-radiation, respectively. Uramphite crystals are transparent to translucent and have an intensive vitreous luster. The mineral is very brittle and at least one system of perfect cleavage along {001} was observed. The quantitative electron-microprobe chemical analyses of uramphite agree well with the proposed ideal composition and correspond to the following empirical formula [(NH₄)_0.79Na_0.12K_0.07Ca_0.03]_Σ1.01(UO₂)_1.00(PO₄)_1.00·3H₂O (on the basis of 1 P atom pfu). Uramphite is tetragonal, space group P4/ncc, with the unit-cell parameters refined from X-ray powder diffraction data: a = 7.0292(11), c = 18.092(2) Å, V = 893.9(2) ų. Vibrational (Raman and infrared) spectroscopy documented the presence of molecular water, ammonium, uranyl, and phosphate groups in the crystal structure of uramphite. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.395 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.395 Alexa M, Mysliveček J, Franěk J, Valenta J, Pécskay Z; Vol. 69, issue 3, pages 183 - 197
The Bídnice hill near Litoměřice, Czech Republic, has been recognized as a volcanic structure during a survey for a new railway connection between Prague and Dresden, where after several campaigns of combined geophysical survey, it has been concluded, that Bídnice is actually a deeply eroded peanut-shaped maar-diatreme volcano forming a positive morphology. The reason for the higher resistance of the disintegrated rock in the diatreme fill against the erosion when compared to surrounding country rocks was the main question of our presented research. While other positive-morphology diatremes in the region are mostly associated with high-viscosity differentiated alkaline rocks (namely phonolites), the Bídnice diatreme comprises intrusions of basic rock (SiO₂ ca 38 wt. %), classified as olivine nephelinite. From the geophysical survey of the internal structure, comprising magnetometry and electrical resistivity tomography, a spoon-like subsurface intrusions of the olivine nephelinite were found. A smaller outcrop of the intrusive system penetrating the diatreme provided a K-Ar age of 27.06 ± 0.57 Ma (Oligocene). The results of the geophysical survey were then used to create a 3D geological model to better understand and interpret the geological setting. A system of coherent sills, even with a rather low thickness in the case of low-viscosity (ultra)mafic rocks, may be responsible for the reinforcement of the diatreme leading in slower erosion, and therefore resulting in the significant positive topography of eroded maar-diatreme volcano. In addition, sub-horizontal intrusions provide more homogeneous reinforcement of the diatreme against erosion compared to subvertical dykes, those would be exposed by the selective erosion in the form of small ridges. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.396 Original paper http://www.jgeosci.org/rss.php?ID=jgeosci.396