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71.
An end member of the tourmaline series with a structural formula □(Mg2Al)Al6(BO3)3[Si6O18](OH)4 has been synthesized in the system MgO-Al2O3-B2O3-SiO2-H2O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B2O3, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B2O3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al2O3-B2O3-SiO2-H2O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions.The (3a)-vacant tourmaline has the space group R3m with , , and . However, these values vary at room temperature with the pressure-temperature conditions of synthesis by , , and , probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are no = 1.631(2), nE = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca. 相似文献
72.
Günter Krumme 《GeoJournal》1984,9(2):111-119
The paper attempts to bridge some of the gaps between (macro) regional-economic and (micro) corporate-geographic perspectives and research approaches. Local stakeholders of large manufacturing activities are tied to the corporate environment through complex linkages. Large corporations tends to perform various gate-keeping roles for the local community. External turbulence (e.g. in export markets) may be mitigated or accentuated by the decisions, strategies and spatial structures of the gate-keeping corporation. Examples are drawn from the aircraft industry and its particularly turbulent environments and significant local employment impacts. 相似文献
73.
Peter Hoppe Doug MacDougall Günter W. Lugmair 《Meteoritics & planetary science》2007,42(7-8):1309-1320
Abstract— Aqueous activity on meteorite parent bodies is indicated by the presence of carbonates. High spatial resolution ion microprobe analyses of nine individual carbonate grains (four dolomites, five breunnerites) from the Orgueil meteorite reveal linear correlations between 53Cr excesses and Mn/Cr ratios in all grains, indicative of in situ decay of radioactive 53Mn (half‐life 3.7 million years). The well‐defined isochrons appear to have chronological significance. If this is the case, then 53Mn/55Mn ratios between 2.1 and 4.7 × 10?6 are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 Ma are calculated (systematic uncertainty of ±2 Ma). Dolomites tend to have formed slightly earlier than the breunnerites. Our data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ~3–4 Myr after formation of the solar system, that most likely was the result of impact heating and latent heat from the decay of radioactive 26Al and 60Fe. 相似文献
74.
Birgit Scheibner Gerhard Wörner Lucia Civetta Heinz-Günter Stosch Klaus Simon Andreas Kronz 《Contributions to Mineralogy and Petrology》2007,154(1):55-74
Igneous garnets have the potential to strongly fractionate rare earth elements (REE). Yet informations on partition coefficients
are very scant, and criteria for distinguishing between hydrothermal and magmatic garnets are ambiguous. To fill this gap,
we present trace element and isotopic data for two types of Ca-rich garnets from phonolites (Mt. Somma-Vesuvius). Both Ca-garnet
populations are different in their style and dynamics of fractionation: one population is progressively strongly depleted
in HREE from core to rim, reflecting REE fractionation in the host phonolite via earlier-crystallized garnets. Such examples
for extreme changes in HREE in garnets are only known for hydrothermal grandites by REE-bearing fluids. The second garnet
population is homogeneous and formed in a closed system. Near-flat patterns between Sm and Lu confirm experimental data indicating
lower D(Sm)/D(Lu) for Ca-rich garnets than for e.g. pyrope-rich garnets. It follows: D
Grt/PhMelt for La = 0.5, Sm = 48 and Yb = 110. 相似文献
75.
76.
77.
Werner Skrotzki Roland Tamm Carl-Georg Oertel Jens Rseberg Heinz-Günter Brokmeier 《Journal of Structural Geology》2000,22(11-12)
Lead sulfide (galena) of different purity and grain size was extruded through a round and rectangular die at temperatures between 773 and 923 K. Global and local lattice preferred orientations (here referred to as textures) were measured by neutron and electron back-scattering diffraction. Tension leads to a <100> <111> double fibre texture. Pure shear deformation yields texture components near the ideal face-centered cubic metal brass, copper, Goss and cube positions. The intensity of the components depends on the purity and/or grain size. The microstructure is partially recrystallized. Electron back-scattering diffraction indicates that in tension the <100> and in pure shear the Goss and cube components are associated with dynamic recrystallization. The deformation texture can be qualitatively explained by the full and relaxed constraints Taylor model using slip on {100}<110>, {110}<110> and {111}<110> systems. The texture formation in lead sulfide compares well with that observed for other ionic crystals with the NaCl-structure as well as for face-centered cubic metals with a high stacking fault energy. 相似文献
78.
Alexander Langosch Eberhard Seidel Heinz-Günter Stosch Martin Okrusch 《Contributions to Mineralogy and Petrology》2000,139(3):339-355
The ophiolitic mélange in the uppermost tectonic unit of the Cretan nappe pile contains crystalline slices which consist
of a low-pressure/high-temperature metamorphic sequence and synmetamorphic intrusions, ranging in composition from diorite
to granite. The plutonic rocks conform to two different igneous suites, dominated by diorites in eastern, and granites in
central Crete, displaying I-type and A-type characters, respectively. Some of the granites from central Crete are classified
as transitional I/S-type. They are closely associated with migmatitic paragneisses. Based on major and trace element, REE,
Sr- and Nd-isotope geochemistry, the mafic members of both suites are derived from a depleted mantle source. The higher concentrations
of P and Ti in the mafic members of the igneous suite in central Crete and the deviant trend of the whole suite may be explained
by a different mantle source or a lower degree of partial melting. In both suites, magmatic evolution was governed by fractional
crystallization of amphibole/clinopyroxene, plagioclase and minor phases. In addition, mixing or mingling of compositionally
different magmas is indicated for the intrusive suite of eastern Crete whereas in central Crete the magma composition was
at least partially modified through assimilation of (meta)pelites. The geochemical results suggest that the plutonic rocks
formed in a supra-subduction zone setting. However, a formation during continental lithospheric extension cannot be ruled
out. Published and new Rb–Sr and K–Ar dates on amphiboles and biotites from intrusive rocks and their metamorphic country
rocks show that the peak of the low-P/high-T metamorphism and the intrusion of the two igneous suites testify to the same
thermal event of Late Cretaceous age. A similar Late Cretaceous association of metamorphic and plutonic rocks has been described
from the uppermost tectonic unit in the Attic–Cycladic Crystalline Complex. Together with the Cretan occurrences, they form
a small sector radiating SSW along a distance of 300 km, across the general trend of the tectonic zones in the Hellenic orogen.
This N–S alignment is regarded as a primary feature which may delineate the frontier zone between the Hellenides and the Taurides.
The real paleogeographic position and geodynamic significance of the Late Cretaceous low-pressure/high-temperature belt, however,
remains enigmatic.
Received: 1 June 1999 / Accepted: 2 February 2000 相似文献
79.
80.
Joachim T. Haug Carolin Haug Dieter Waloszek Günter Schweigert 《Swiss Journal of Geoscience》2011,104(1):85-98
Developmental biology has become a major issue for understanding the evolution of Arthropoda. While usually only the ontogenies of extant species are studied, developmental information of fossil arthropods may exhibit developmental patterns not present in living ones. Crustacea possess, basically, a more gradual development than, for example, pterygote insects and would, therefore, be appropriate candidates for the study of fossil ontogenies. Remarkably, famous fossil deposits like the Devonian Rhynie Chert or the Early Palaeozoic ‘Orsten’-type deposits do not comprise the generally macroscopic malacostracan Crustacea (although most probably adult malacostracan fossils have already been found in the Cambrian). By contrast, the Late Jurassic Solnhofen Lithographic Limestones of southern Germany provide thousands of specimens (although only few morphotypes) that can be identified as malacostracan larvae, together with juvenile specimens differing in certain morphological aspects from their conspecific adults. More recent investigations with up-to-date imaging methodology on additional malacostracan crustacean larvae yielded also reconstructible developmental sequences of species from the Solnhofen deposits. The very similar fossil deposits of the Cretaceous lithographic limestones of Lebanon have also yielded malacostracan larvae and juvenile specimens. We present a summary of the occurrences of crustacean fossils providing developmental information and a demonstration of the potential of the lithographic limestones in this context. The importance of developmental data for understanding crustacean evolution is also highlighted. 相似文献