On zeophyllite from Radejčín, České středohoří Mts.: X-ray and IR-investigations

Summary A redetermination of the crystal structure of zeophyllite, idealized Ca₁₃Si₁₀O₂₈F₁₀.6H₂O [a=9.377(2), c=36.57(2) Å; Z=3], space group R3 (No. 148) yielded R = 0.033 for 2238 observed single crystal X-ray data (Fo > 3_Fo). The atomic coordinates in combination with the anisotropic thermal displacement parameters as well as the internal R-value from merging equivalent intensities gave no evidence for a violation of trigonal symmetry. The structure is characterized by SiO₄ tetrahedra combined to open branched vierer single layers in an arrangement normal to the threefold axes. These layers are connected via the Ca(O,F)_x polyhedra and hydrogen bridges to a framework with a pronounced cleavage parallel to (00.1). The statistic . occurrence of Si-OH groups in parts of the structure was confirmed by IR-investigations.

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Über Zeophyllit aus Radzein, Böhmisches Mittelgebirge: Röntgen- und IR-Untersuchungen

Zusammenfassung Eine Neubestimmung der Kristallstruktur an Zeophyllit, idealisiert Ca₁₃Si{ion10}O₂₈F₁₀· .6H20 [a=9.377(2), c=36.57(2) Å; Z=3], Raumgruppe R3 (No. 148), ergab R = 0.033 für 2238 beobachtete Röntgeneinkristalldaten (Fo > 3_Fo). Die Koordinaten der Atome in Verbindung mit den Parametern der anisotropen thermischen Schwingung sowie der interne R-Wert nach Mittelung äquivalenter Intensitäten ergaben keinen Anhaltspunkt für eine Verletzung der trigonalen Symmetrie. Die Struktur ist charakterisiert durch SiO₄-Tetraeder, verknüpft zu open branched vierer single layers, die normal zur dreizähligen Achse angeordnet sind. Diese Schichten werden über die Ca(O,F)_x Polyeder und Wasserstoffbrücken zu einem Gerüst mit ausgeprägter Spaltbarkeit parallel (00.1) verbunden. In der Struktur statistisch auftretende Si-OH-Gruppen konnten durch IR-Untersuchungen belegt werden.

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With 4 Figures     

Earth's surface movements in the hazardous area of Jezeří Castle,Krušné hory Mountains

Abstact The results are summarized of the correlation between the detailed geomorphological analysis and the geodynamic interpretation of the twelve high-precision levelling measurements of the Earth's recent surface movements (1983–1989) in the hazardous area of Jezeí Castle in the Kruné hory Mts. Indicated are the types and régime of these movements (Tab 1, Fig 3) which are climatic, tectonic and anthropogenous in origin. Both slightly and strongly deformed zones in the near-surface part of crystalline rocks massif were formed (Fig 1). The landform patterns (Fig 2), as well as the present-day activity of geological processes are described.     

Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Ohře (Eger) Rift,Western Bohemia

High-Ti melanephelinite (3.8–5.9 wt% TiO₂), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO₂), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO₂) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7034–0.7038 and ε_Nd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7035–0.7036 and ε_Nd of 4.3–5.5. The large variation in initial ⁸⁷Sr/⁸⁶Sr ratios at similar ε_Nd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial ⁸⁷Sr/⁸⁶Sr ratios of ca. 0.7036 and ε_Nd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.     

Re–Os isotopic constraints on the source of platinum-group minerals (PGMs) from the Vestřev pyrope-rich garnet placer deposit,Bohemian Massif

Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt–Fe alloy grains are accompanied by Os–Ir–Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt–Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os–Ir–Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re–Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (¹⁸⁷Os/¹⁸⁸Os = 0.12082–0.12505 ± 0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (T_MA) and Re-depletion model age (T_RD) model ages range from ~ 0.4 to ~ 1.0 Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.     

Macrocrystic corundum and Fe–Ti oxide minerals entrained in alkali basalts from the Eger (Ohře) Rift: Mg−Fe3+-rich ilmenite as tracer of an oxidized upper mantle

Macrocrysts of corundum, ilmenite, and spinel-group minerals from alluvial deposits of the Eger Rift were studied for composition, texture, and mineral inclusions. All macrocrysts show usually magmatic corrosion textures indicating disequilibrium with the transporting alkali-basalt magma. Corundum grains, exclusively sapphires, were classified by trace-element signatures as magmatic and metamorphic types. Some sapphire grains show erratic compositions that may have resulted from a metasomatic overprint. The inclusion inventory of magmatic corundum suggests crystallization from a differentiated alkaline silicate melt. Corundum itself was never observed as an inclusion mineral. Magnesium- and Fe³⁺-rich ilmenite, described as typical mantle-derived species, is the dominant heavy mineral in almost all alluvial deposits of the Eger Rift. Most discrete macrocrysts are similar in appearance and composition to kimberlite- and basanite-related ilmenite. Ilmenite included in alluvial corundum and zircon grains differ from the bulk of discrete ilmenite grains by larger concentrations of Nb and Mn. The mantle origin of the Mg–Fe³⁺-rich ilmenite is confirmed by compositional and thermo-barometric comparison with ilmenite from clinoproxenitic and hornblenditic xenoliths, which probably originated in the Moho region. The Fe–Ti two-oxide geothermometry and oxygen-barometry of coexisting ilmenite–magnetite pairs yield equilibrium temperatures between 900 and 1,080 °C and oxygen fugacities log₁₀fO₂ between ?0.1 and 1.1 (relative to the NNO buffer), which indicate that the upper mantle as well as the mantle/crust transition zone below the rift is at least partially oxidized. The ilmenite macrocrysts were transported from the source region to the surface by explosive alkali-basalt magmas, as implied by the presence of basaltic-pipe breccias in close vicinity to some placer deposits.