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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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.