Iron titanium oxides in equilibrium with metallic iron

The iron titanium oxide phases ulvite, ilmenite and ferropseudobrookite were synthesized in equilibrium with metallic iron at 1000 ° C, 1130 ° C and 1300 ° C in CO₂/H₂ gas mixtures.The composition of the phases were determined by wet chemical and electron microprobe analyses and by direct oxygen determination.The chemical composition of the Fe-Ti oxide phases in equilibrium with metallic iron is sensitively influenced by temperature and by bulk composition.Ulvite in equilibrium with wüstite does not contrivalent titanium in the whole temperature range up to the eutectic temperature at 1312 °C.Ulvite+ilmenite phase assemblages contain trivalent titanium only at temperatures above 1200 ° C.A ferropseudobrookite phase is stable under the given conditions at temperatures above 1068 °C (Ender and Woermann, 1977).Ferropseudobrookite in equilibrium with metallic iron always contains trivalent titanium.Deviations from stoichiometric compositions of the solid solution phases are generally small. Thus recalculation of microprobe data to stoichiometric solid solutions does not involve a major error.From: Bruno Simons, Diplomarbeit Aachen, October 1974     

A new phase transition in MnTiO3: LiNbO3-perovskite structure

The stable polymorph of MnTiO₃ at room temperature and pressure has the ilmenite structure. At high temperatures and pressures, MnTiO₃ ilmenite transforms to a LiNbO₃ structure through a cation reordering process (Ko and Prewitt 1988). Single crystals of both phases have been studied with X-ray diffraction to 5.0 GPa. We have obtained the first experimental verification of the close relationship between the LiNbO₃ and perovskite structures, first postulated by Megaw (1968). MnTiO₃ with the LiNbO₃ structure (MnTiO₃ II) transforms directly to an orthorhombic perovskite structure (MnTiO₃ III) between 2.0 and 3.0 GPa. The transition involves a change of volume of -5%, is reversible and has pronounced hysteresis. Only pressure is required to drive the transition because it involves no breaking of bonds; it simply involves rotation of the [TiO₆] octahedra about their triad axes accompanied by displacement of the Mn cations to the distorted twelve-coordinated sites formed by the rotations. An unusual aspect of this transition is that twinned MnTiO₃ II crystals transform to untwinned MnTiO₃ III crystals with increasing pressure. The twin plane of MnTiO₃ II, , corresponds to the (001) mirror plane of the orthorhombic perovskite structure. MnTiO₃ III examined at 4.5 GPa is very distorted from the ideal cubic perovskite structure. The O(2)-O(2)-O(2) angle describing the tilting in the ab plane is 133.3(7)°, in contrast to 180° for a cubic perovskite and the O(2)-O(2)-O(2) angle describing the tilting in the ac plane is 109.3(4)°, as opposed to 90° in a cubic perovskite.     

High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure

Crystals of a high-pressure phase of MnTiO₃ have been synthesized at pressures of 60 kbar using the SAM-85 cubic-anvil high pressure apparatus. Although all crystals examined were twinned on (10 \(\bar 1\) \(\bar 2\) ), a set of diffraction intensities that are essentially unaffected by the twinning were obtained. Three possible structure models were considered: (1) the corundum (completely disordered Mn and Ti), (2) the partially-disordered ilmenite, and (3) the LiNbO₃ structures. The R factors of the corundum and the disordered ilmenite models were much larger than that of LiNbO₃. Using structure factors unaffected by twinning, the final LiNbO₃-type refinement gave R _w=0.037 and R=0.034. The averaged bond lengths for Mn-O and Ti-O were consistent with ones calculated using Shannon and Prewitt's (1969) radii. The study concludes that MnTiO₃ II actually has an ordered LiNbO₃-type structure rather than the disordered one as reported previously. From the analysis of the two MnTiO₃ structures, the transition can be related to a cation reordering process, in which half of the cations participate, accompanied by the rotation of oxygens to accommodate the cations.     

Geochemical characteristics of Kırka (Sarıkaya) borate deposit,northwestern Anatolia,Turkey

The K?rka borate deposit was deposited in a Miocene lacustrine basin which is closely associated with volcanic activity which lasted from Paleogene to the beginning of Quaternary. Borate mineralization alternates with claystone, mudstone, tuff and fine-layered limestone and mostly shows a lenticular structure. The mineral paragenesis is composed of borax, tincalconite, ulexite, kurnakovite, probertite, tunellite, colemanite, dolomite, smectite group minerals, illite and some firstly reported minerals for the K?rka deposit including hydrochloroborite, brianroulstonite, hilgardite-4M and searlesite minerals. In comparison to average values of earth crust, concentrations of Cs, Sr, Li, As and Se were significantly enriched with respective rates of 21, 15, 14, 3 and 188 folds. Regarding KY, KS1 and KS2 locations, there are differences in both element abundances and their geochemical tendencies which are attributed to variations in discharge regime and physico-chemical conditions of the depositional environment. Independent behaviour of B₂O₃ might indicate that boron is not associated with clays and carbonates and, therefore, most part of boron must be derived from volcanic activity (hydrothermal solutions, gases). REE data indicate that the K?rka borate deposit was formed in a sedimentary environment where highly alkaline (high pH) hydrothermal solutions also took part in borate precipitation process.     

The effect of syndepositional deformation within the Upper Permian Capitan Platform on the speleogenesis and geomorphology of the Guadalupe Mountains, New Mexico, USA

The Guadalupe Mountains in New Mexico and Texas are home to more than 300 caves. Caves have been formed within the Upper Permian Capitan carbonate platform and are oriented along two structural trends, one of which is parallel to the platform margin and the other of which is roughly perpendicular to it. Our recent studies of the Capitan Platform have identified syndepositional faults associated with growth monoclines and synclines in Slaughter Canyon, New Mexico, and these are also parallel to the platform margin. In this study, we demonstrate that syndepositional faults and folds are also present in Rattlesnake and Walnut Canyons, as much as 19 km along strike, and that they have exerted control on karstification of the Guadalupe Mountains from the Upper Permian until present.Three distinctive episodes of karst formation have been recognised in outcrops on the basis of karst-filling deposits and crosscutting relationships. The syndepositional “Phase 1 karst” was formed along syndepositional faults and fractures and is filled by platform-derived sediments. The burial “Phase 2 karst” is filled by post-Permian siliciclastics and is limited to the youngest syndepositional faults and fractures that penetrate the platform in the proximity of its terminal margin. Connectivity of these youngest faults and fractures to the platform top and the overlying stratigraphy is inferred to have controlled the distribution of the Phase 2 karst. The “Phase 3 karst” includes the present cave systems, which were mainly formed by sulphuric acid produced by mixing of fossil and fresh underground waters in conjunction with the uplift of the Guadalupe Mountains in the Late Tertiary, and have since been modified by vadose karst processes. The Phase 3 karst caves are not solely developed along syndepositional faults and fractures as the earlier karst palaeocaverns are, but also follow another, uplift-related, structural trend.Syndepositional folds, faults, and fractures in the Capitan Platform have influenced the shaping of the modern surface geomorphology of the Guadalupe Mountains by controlling drainage and, hence, erosion. Trellis drainage parallel to the platform margin is developed where syndepositional folds, faults, and fractures occur. The morphology of the trellis drainage varies systematically across the range in response to the character of the deformation structures and karst features along which the drainage channels have developed.     

Sloshing behaviours of rectangular and cylindrical liquid tanks subjected to harmonic and seismic excitations

A numerical and experimental study on the sloshing behaviours of cylindrical and rectangular liquid tanks is addressed. A three‐dimensional boundary element method for space with the second‐order Taylor series expansion in time is established to simulate the sloshing phenomenon and its related physical quantities inside a liquid tank subjected to horizontal harmonic oscillations or recorded earthquake excitations. The small‐scale model experiments are carried out to verify some results of numerical methods in this study. The comparisons between numerical and experimental results show that the numerical method is reliable for both kinds of ground excitations. Finally, the water wave and the base shear force of a rectangular tank due to harmonic excitation are also presented at different frequencies. A huge cylindrical water tank subjected to a recorded earthquake excitation is used for application and discussion. Copyright © 2007 John Wiley & Sons, Ltd.     

Distribution of Al between coexisting micas in metamorphic rocks from the Central Alps

In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al _Mu ^tot /Al _Bi ^tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al _Mu ^IV /Al _Bi ^IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, Al^VI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the K_D values locates a 100 km long and 23 km broad central zone with low K_D. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The K_D values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.     

Inversion of the geomagnetic field determined by palaeomagnetic investigations of quaternary sediments

Résumé Au cours des recherches paléomagnétiques en Tchécoslovaquie, on a constaté und double polarité de la magnétisation rémanente des sédiments quaternaires. Des recherches plus détaillées de la localité Červeny kopec près de Brno, qui représente une région de sédiments avec une suite initerrompue de couches déposées au cours d'un million d'années dernières, datées par la méthode de correlation astronomique, fournissent certains résultats importants sur l'époque au cours de laquelle s'est passé l'inversion du champ géomagnétique. Selon nos recherches de jusqu'à présent, on peut déterminer la fin de l'époque inverse de Matuyama par 0,930 million d'années environ. Après elle succède la pèriode de transition, dont la durée est de 60 000 ans environ, au cours de laquelle se sont passé des changements rapides dans le champ géomagnétique et au cours de laquelle existent des roches possédant une magnétisation avec inclinaison soit positive, soit négative. L'époque de polarité normale de Brunhes commence selon nos constatations autour de 0,870 million d'années.

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Address: Bočni II, Praha 4-Spořilov     

Microencruster-microbial-cement framework of the Upper Jurassic reef developed on the slope of the intra-Tethyan carbonate platform (Bulgaria)

The carbonate succession in the Lyalintsi section of the western Moesian Platform (western Bulgaria) displays a shallowing-upward trend. Growth of the Tithonian–Valanginian coral biostromes and low-relief bioherms was preceded by Oxfordian–Kimmeridgian sedimentation of fine-grained peloidal-bioclastic limestones and Saccocoma-bearing limestones on the homoclinal ramp and the carbonate platform slope. In the late Kimmeridgian, boundstones with very rare corals, but with easily recognisable biohermal morphology, were developed. The main components of this reef are encrusting microorganisms, microbial crusts and synsedimentary cements. Microencrusters Labes atramentosa, Crescentiella morronensis, Perturbatacrusta leini and Radiomura cautica, as well as thin crusts of calcified sponges (sclerosponges), are the main biotic components. Corals (almost exclusively microsolenids) are sparse, whereas photophilic microencrusters (e.g., “Lithocodium–Bacinella”), are absent, although they are common in the overlying shallow-water part of the Lyalintsi sequence. Microbialites and synsedimentary cements provided additional support for the reef framework. The framework, especially the biotic components, and the reefal facies position within the sedimentary succession, implies that the high-energy upper slope of the carbonate platform was the depositional setting of the microencruster-microbial-cement reef studied. Encrusting microorganisms, except for C. morronensis and sponges, are only known from the intra-Tethyan platforms. This study supports conclusion of studies of coeval Alpine reefs that the presence of the microencruster-microbial-cement framework provides insight into the palaeobathymetry, palaeogeography and tectonic configuration of the intra-Tethyan carbonate platforms.