bluuebunny/crossref_metadata_2025_split huggingface dataset:
The descloizite-mottramite series of vanadates from the Otavi Mountain Land, South West Africa: an X-ray study
Authors: van der Westhuizen W. A. , de Bruiyn H. , Tordiffe E. A. W. , Botha B. J. V. 
Published: December 2006

AbstractDescloizite, cuprian descloizite, and mottramite from most of the known prospects and mines in the Otavi Mountain Land were sampled and subsequently analysed chemically and by X-ray diffraction. Unit cell parameters were refined with the aid of a computer program. It was found that, contrary to previous opinion, chemical parameters of these vanadates can be deduced from their X-ray powder data according to the equations (correlation coefficients in parenthesis):ZnO = 75.369c−0.119b−31.177a−456.512(0.9634)Zn2+ = 369.568c + 236.583b − 146.42a − 3726.344 (0.9642)CuO = − 74.991c + 0.086b + 29.419a + 486.204 (0.9649)Cu2+ = − 369.568c − 236.583b + 146.420c + 3826.344 (0.9642).
The crystal structure and chemical composition of cumengéite
Authors: Hawthorne F. C. , Groat L. A. 
Published: December 2006

AbstractThe crystal structure of cumengéite, Pb21Cu20Cl42(OH)40, tetragonal, a= 15.065(2), c= 24.436(5) Å, V = 5546(2) Å3, space group I4/mmm, Z = 2, has been solved by direct methods and refined by least-squares to an R index of 7.1 % for 1158 observed (I > 2.5σ I) reflections. The cumengéite structure shows a very diverse range of cation coordinations. There are five distinct Pb sites with the following coordination polyhedra: octahedron, square antiprism, augmented trigonal prism, a very distorted biaugmented trigonal prism and a fairly regular biaugmented trigonal prism, the latter being only half-occupied; there are two distinct Cu sites with octahedral and square pyramidal coordination respectively. The coordination polyhedra share elements to form prominent columns or rods of polyhedra parallel to the c-axis and centred on the 4-fold axes of the unit cell. These rods form a body-centred array, and link by sharing elements of their coordination polyhedra.
Reconsideration of ‘soda-minettes’
Authors: Bachinski Sharon W. 
Published: December 2006

AbstractIt is shown here that dyke rocks of type localities for ‘soda-minettes’ are not minettes (= lamprophyres dominated by phlogopite and K-rich feldspar). It has long been suggested that ‘soda-minette’ also be applied to otherwise normal minettes from elsewhere that carry modally variable amounts of groundmass aegirine and/or, more commonly, arfvedsonitic to riebeckitic amphibole. Despite the presence of sodic pyriboles, these rocks are poorer in Na2O then minettes lacking these phases. ‘Soda-minette’ is thus misleading and self-contradictory to the many petrologists to whom ‘soda-’ legitimately connotes that the rock in question either has K/Na < 1 and/or is rich in albitic feldspar. To eliminate this ambiguity it is recommended, with the approval of twenty-six other petrologists, that ‘soda-minette’ no longer be used as a rock name. As minettes with Na-pyribole(s) are markedly K-rich and most are both ultrapotassic and peralkaline, independent of the modal abundance of the Na-pyrobile, it is suggested that one of these three chemical characteristics be utilized adjectivally in naming these minettes.
Pyrite-pyrrhotine redox reactions in nature
Authors: Hall A. J. 
Published: December 2006

AbstractThe origin in rocks of the common iron sulphides, pyrrhotine, Fe1-xS and pyrite, FeS2 and their behaviour during geochemical processes is best considered using the simplified redox reaction: 2FeS ⇌ FeS2 + Fe2+ + 2e−.Thus pyrrhotine is more reduced than pyrite and is the stable iron sulphide formed from magmas except where relatively high oxygen fugacities result from falling pressure or hydrothermal alteration. Pyrite, on the other hand, is the stable iron sulphide in even the most reduced sedimentary rocks where it usually forms during diagenesis through bacteriogenic reduction of sulphate; it is stable throughout the pressure/temperature range endured by normal sedimentary rocks. Pyrrhotine after pyrite or sulphate in metasediments of regional metamorphic origin results mainly from progressive reduction on metamorphism due to the presence of graphite-buffered fluids. Pyrrhotine and/or pyrite may be precipitated from hydrothermal solutions on epigenetic or syngenetic mineralization but pyrrhotine will only be preserved if protected from oxidation to pyrite or to more oxidized species. Exhalative pyrrhotine appears to have been more common in Precambrian times and/or in depositional environments destined to become regionally metamorphosed. FeS can be considered to be the soluble iron sulphide, rather than FeS2, in reduced aqueous systems although pyrite may precipitate from solution as a result of redox reactions. The relatively soluble nature of FeS explains the observed mobility of iron sulphides in all rock types.
Spheniscidite, a new phosphate mineral from Elephant Island, British Antarctic Territory
Authors: Wilson M. J. , Bain D. C. 
Published: December 2006

AbstractSpheniscidite is a new phosphate mineral occurring in a soil profile in an area of nesting penguins on Elephant Island. It is the NH4-dominant analogue of leucophosphite and has the formula (NH4,K)(Fe3+,Al)2 (PO4)2(OH) · 2H2O. It is monoclinic, P21/n with unit cell parameters a = 9.75, b = 9.63, c = 9.70 Å, β = 102° 34′, and Z = 4. The strongest X-ray lines are 6.79(100), 5.99(90), 3.05(45), 7.62(40) Å. Spheniscidite is thought to have formed by the interaction of ammonium phosphate solutions from penguin guano with micaceous and chloritic minerals in the soil. The name is for Sphenisciformes, the order name for penguins, and has been approved by the IMA Commission on New Minerals and Mineral Names.
Two types of metal particle in the Bachmut (L6) chondritic meteorite
Authors: Semenenko V. P. , Samoilovich L. G. , Tertichnaya B. V. 
Published: December 2006

AbstractMetal particles in the Bachmut chondrite may be subdivided into large (hundreds of μm in size) and small (1–10 µm in size) varieties. They differ in the ratio of their constituent phases, in structure, and in chemical composition. Large particles are richer in Ni than small ones, the latter being characterized by an abnormally high Cu content. It is suggested that different pre-accretion histories were responsible for producing the various types of metal grains, and that, during post-accretion thermal history, temperatures were too low to cause elemental redistribution among the grains.
Metamorphic reaction rate laws and development of isograds
Authors: Lasaga Antonio C. 
Published: December 2006

AbstractNew data on the kinetics of dehydration of muscovite + quartz suggest the necessity for a careful treatment ofbothsurface kinetics and diffusion processes in metamorphic reactions. A new model is proposed that illustrates the relative role of diffusion and surface reactions in the overall metamorphic process. The rate law for the reaction at mineral surfaces derived from the experimental data is shown to be probably non-linear and similar to rate laws derived from Monte Carlo calculations. The experimental rate data is then used in a heat flow calculation to model the evolution of the muscovite isograd in the field. The position of the isograd, the temperature oversteps above equilibrium, and the width of ‘reaction zones’ are then analysed as a function of intrusion size and kinetic parameters.
An electron optical study of muscovite breakdown in pelitic xenoliths during pyrometamorphism
Authors: Brearley A. J. 
Published: December 2006

AbstractTransmission electron microscopy and analytical electron microscopy have been used to study the breakdown reaction of muscovite during pyrometa-morphism. The transformation has been shown to be topotactic in nature, with the orientations of the product biotite and alkali feldspar being strongly controlled by the precursor phase. Microprobe investigations indicate that the reaction is probably isochemical with no detectable interaction with adjacent phases. Chemical analyses of the products carried out by AEM have enabled a possible balanced reaction to be calculated. It is proposed that the spatial distribution of the product minerals can be interpreted in terms of diffusion of species from one domain in the crystal to another over distances of 5 to 10 µm as a result of chemical potential gradients which develop during the reaction. The reaction probably occurred under conditions of declining temperature between 900 and 750 °C over a period of 4–5 days. This time period was insufficient to enable the reaction to reach completion as indicated by the presence of relics of muscovite coexisting with K-feldspar, corundum, biotite, hercynite, and mullite.
Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning
Authors: Mørk Mai Britt E. 
Published: December 2006

AbstractGabbros and dolerites in the Western Gneiss Region of Norway have been subjected to a high P-T regime and preserve evidence of the existence of several stages in the consequent reaction sequence. Incomplete reaction stages are characterized by corona structures between relict igneous phases and by frequent pseudo-morphs after igneous olivine, plagioclase and augite. The increasing degree of reaction is recorded by successive increases of modal garnet, omphacite and phlogopite (±orthopyroxene) to produce eclogite, often with excellent pseudomorphic preservation of the igneous fabric.Pseudomorphic replacement of olivine by orthopyroxene (in coronas and aggregates) and of plagioclase by local assemblages of garnet, spinel, and sodic plagioclase, is interpreted as a transient reaction stage with restricted and selective diffusion between the original mafic and felsic domains. Complete eclogitization is compatible with more extensive diffusion, especially of Na and Al (on a mm scale) leading to omphacite production by replacement of olivine, augite, and orthopyroxene. Concomitant reactions in the felsic domains lead to total replacement of the transient phases by garnet, with or without inclusions of minute omphacite grains.Strong Ca zoning of garnet in the coronites is inter-preted as a relict growth zoning, attributed to local controls by diffusion and subreactions in the plagioclase host and to local garnet + plagioclase + spinel equilibria. In contrast, complete eclogitization is associated with diffusional homogenization of garnet by (Mg, Fe)Ca−1 and MgFe−1 exchange with omphacite ± phlogopite.
Compositional variation of some rare earth minerals from the Fen complex (Telemark, SE Norway): implications for the mobility of rare earths in a carbonatite system
Authors: Andersen Tom 
Published: December 2006

AbstractRare earth minerals in ankeritic ferrocar-bonatite and associated magnetite ore, as well as in hematite-calcite-(dolomite) carbonatite (rødberg) derived from the ferrocarbonatite by post-magmatic alteration, have been studied by electron microprobe. The minerals found are monazite, parisite, and bastnäsite (identifications confirmed by X-ray diffraction) as well as an aluminous silicate with allanite composition, and possibly, synchysite. The rare earth element (REE) distribution patterns of the minerals change from light rare earth (LREE)-enriched in magnetite ore and ferrocarbonatite to less LREE-enriched, or middle REE-enriched in the most strongly re-equilibrated carbonatites. Two distinct mineral varieties, monazite-(Ce,Nd) and a Nd-fluorocarbonate, have been found in rødberg. The LREE were leached from their host minerals during re-equilibration of the carbonatite, most likely because of the influence of fluoride-bearing hydrothermal solutions.
Cation exchange in ganophyllite
Authors: Guggenheim Stephen , Eggleton Richard A. 
Published: December 2006

AbstractGanophyllite has been shown to have alkali cation exchange capability. Partial cesium exchange for K + Na in large (> 0·3 mm) grains shows that the exchangeable cations migrate parallel to X, the crystallographic direction for the ‘interlayer’ tunnels. Such exchange capability supports the suggestion that the alkali elements are located in zeolite-like sites attached to the sides of the tunnels. Exchange experiments show that complete sodium substitution for potassium is possible also. Eggletonite, the Na analogue of ganophyllite, is shown to have an identical superlattice to ganophyllite, indicating that eggletonite differs from ganophyllite only by the exchangeable cation.
Thermal decomposition ractions of caledonite and their products
Authors: Morgan D. J. , St. J. Warne S. , Warrington S. B. , Nancarrow P. H. A. 
Published: December 2006

AbstractThe thermal decomposition of caledonite has been examined by simultaneous differential thermal analysis, thermogravimetry and mass spectrometry. Structural H2O and CO2 are liberated endothermically between 300 and 400°C leaving a residue of lead sulphate, oxysulphate, and Cu(I) and Cu(II) oxides. A series of sharp endothermic peaks between 850 and 950°C correspond to phase transition and melting reactions of the PbO-PbSO4 mixture. The sulphate anion breaks down above 880 °C. Mass spectra of the gaseous decomposition products show SO2, SO, and O2, although SO is an artefact arising from ion fragmentation of the SO2 within the mass spectrometer. The residue at 1060 °C is composed predominantly of 2PbO · PbSO4 and Cu(I) and Cu(II) oxides.
Magnetite exsolution in almandine garnet
Authors: Brearley A. J. , Champness P. E. 
Published: December 2006

AbstractThree almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe2+ show that there are no significant departures from stoichiometry due to the presence of Fe3+ in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400 Å in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 55°C. The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe3+ content of the host garnet.
An occurrence of ilvaite layers in the Cinco Villas metasomatic rocks, Western Pyrenees (Spain)
Authors: Pesquera A. , Velasco F. 
Published: December 2006

AbstractIn the Artikutza area, near the contact with the southern Aya granite, skarns containing hedenbergite, grandite, epidote, quartz, calcite, actinolite, idocrase and magnetite occur and have been found to contain ilvaite layers. This is the second reported occurrence of ilvaite in Spain; spectral reflectances, colour values. Vickers hardness numbers and chemical composition are given. The monomineralic ilvaite lens intercalation developed by hydrothermal alteration of hedenbergite in a second stage of skarn formation.
Metavivianite and kerchenite: a review
Authors: Rodgers K. A. 
Published: December 2006

AbstractMetavivianite has been shown to be not dimorphous with vivianite. Assuming the homogeneity and uniformity of the original type samples, it is a triclinic hydrated ferri-ferrous hydroxy phosphate whose formula may be given as Fe2+3-xFe3+x(po4)2(OH)x(8-x)H2O where x > 1.4. The precise oxidation limits between which the triclinic lattice is stable are not known but the structure persists close to total oxidation of all iron. The structure of metavivianite was established using a fragment of kerchenite whose formula as originally given is covered by the general metavivianite formula. Assuming the homogeneity of the original (1907) samples, kerchenite and metavivianite appear to be identical.
Quantitative X-ray microanalysis of thin specimens in the transmission electron microscope; a review
Authors: Lorimer G. W. 
Published: December 2006

AbstractIn a thin specimen X-ray absorption and fluorescence can, to a first approximation, be ignored and the observed X-ray intensity ratios, IA/IB, can be converted into weight fraction ratios, , can be converted into weight fraction ratios, CA/CB, by multiplying by a constant , by multiplying by a constant kAB;kAB values can be calculated or determined experimentally. The major correction which may have to be made to the calculated weight fraction ratio is for X-ray absorption within the specimen. The activated volume for analysis in a thin specimen is approximately 100 000 × less than in a bulk sample. Beam spreading within the specimen can be estimated using a simple formula based on a single elastic scattering event at the centre of the specimen. Examples are given of the application of the technique to obtain both qualitative and quantitative analyses from thin mineral specimens. The minimum detectable mass and the minimum mass fraction which can be measured using the technique are estimated.
The 1875 eruption of Askja volcano, Iceland: combined fractional crystallization and selective contamination in the generation of rhyolitic magma
Authors: Macdonald R. , Sparks R. S. J. , Sigurdsson H. , Mattey D. P. , McGarvie D. W. , Smith R. L. 
Published: December 2006

AbstractMajor and trace element and Sr, Nd and O isotopic data are presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja. Trace element modelling and Sr and Nd data largely confirm previous major element calculations that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb (and Th and U?), depletion in Cs, and low values of δ18O/16O, in the rhyolites are not explained by this mechanism alone. The silicic magmas were selectively contaminated by diffusion from partially molten granitic wall rocks, now found as xenoliths in the eruptive products, the process being particularly marked by lower δ18O and Cs/Rb ratios in the rhyolites than in the associated basalts. This is the first record of a combined fractional crystallization-selective contamination process in an Icelandic silicic complex.
The occurrence and significance of xenocrysts of apatite, ilmenite, and Na-Fe-Ti oxide in ultrapotassic lavas from the Leucite Hills, Wyoming
Authors: Barton Michael 
Published: December 2006

AbstractThe occurrence and mineral chemistry of apatite, magnesian ilmenite, and an Na-Fe-Ti oxide in lavas from the Leucite Hills are reported. Magnesian ilmenite and apatite occur as xenocrysts and as crystals in amphibole-mica-pyroxenite xenoliths. Na-Fe-Ti oxide and also rutile occur as inclusions in ilmenite. The latter mineral contains up to 7.2% MgO and shows evidence of oxidation by, and reaction with, the host magma. The apatite differs from that which occurs as phenocrysts and microphenocrysts inasmuch as REE were not detected. The occurrence of these minerals, which are important repositories for REE and High Field Strength elements, together with phlogopite in the upper mantle source regions of ultrapotassic lavas, is important and may explain some unusual aspects of the geochemistry of such lavas (low K/Rb, P2O5/Ce, Ti/Zr, high Ti/V, Zr/Nb). The source regions must be grossly heterogenous and a two-component model is suggested for the source. This model is similar to that suggested for the source regions of other alkaline magmas and is capable of explaining the unusual Nd-Sr isotopic characteristics of the Leucite Hills lavas.
Natural As-Sb alloys: texture types, thermal behaviour and mechanism of formation
Authors: Bernardini G. P. , Cipriani C. , Corsini F. , Guarini G. G. T. , Mazzetti G. , Poggi L. 
Published: December 2006

AbstractThe thermal behaviour and mechanism of formation of different texture types of intergrown As-Sb alloys have been studied by DTA and annealing experiments performed on natural samples. The constant composition of the As-rich component and of the stibarsen in the intergrowths, and the large compositional range of the homogeneous solid solution obtained after heating in the DTA cycle, have been established using the linear relationship between cell volume and composition. The high-temperature features detected in the DTA studies of the natural samples confirm previously published phase relations for the synthetic As-Sb system. The low-temperature features can be correlated with the homogenization reaction which leads to the formation of a complete solid solution. Study of TTT plots based on the annealing experiments clearly shows that a diffusion mechanism is involved in the homogenization reaction. This has been further substantiated by fitting the experimental data to kinetic equations for diffusion-controlled processes. The kinetic parameters evaluated from the ending time for the 520, 480, and 420 °C annealing experiments, using both the Arrhenius and the transition state theory formalisms, suggests a rather rigid activated complex for the rate-determining step of the process.
‘On the eclogites of Norway’—65 years later
Authors: Griffin W. L. 
Published: December 2006

AbstractThe Western Gneiss Region (WGR) of Norway consists largely of Proterozoic orthogneisses, but also contains paragneisses, peridotites, anorthosites, gabbros and coarse-grained intermediate-acid ‘rapakivi granites’. All of these lithologies enclose eclogites. Structural and isotopic data suggest that many of these rocks were juxtaposed by early Caledonian thrusting prior to eclogite formation at ca. 425 Ma.Low-P protoliths can be demonstrated for many eclogites. Prograde metamorphism to eclogite facies is demonstrated by inclusion suites within garnet grains and zoning of eclogite minerals. The regional distribution of KD (gnt/cpx) and Xcpxjd shows a decrease in Tmax, and in the corresponding P, away from the present coastline. The lowest values (500 °C, 10 kbar) are found in the Sunnfjord area and the highest (∼ 800 °C) along the coast of Sunnmøre and Nordmøre.Maximum pressures were reached at temperatures 100–200° < Tmax. This P-T-t path is consistent with the preservation of jadeite-rich cpx (and possible coesite) in the coastal regions. Earlier overestimates of Pmax, based on partitioning of Al between opx and gnt, resulted from combination of early low-T (low-Al) opx and T values derived from cpx/gnt equilibrated at Tmax. Despite pervasive later amphibolitization, high-P assemblages (phengite + kyanite, omphacite + quartz) are locally preserved within gneisses near the coast. The high-P metamorphism can be explained by westward subduction of the Baltic continental plate beneath the Greenland plate, during the Caledonian orogeny.At least some of the Mg-Cr garnet peridotites of the WGR were derived from low-P protoliths (spinel ± chlorite peridotites, enclosing high-Al pyroxenites). While Sm-Nd mineral ages of most eclogites cluster around 425 Ma, garnet peridotites and their enclosed garnet pyroxenites give Proterozoic Sm-Nd mineral ages (1700–1000 Ma). The tectonic position of the Mg-Cr garnet peridotites, relative to the Caledonian high-pressure metamorphism, remains to be resolved.