bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diffusion in the titanomagnetite solid solution series
Authors: Price G. D. 
Published: December 2006

AbstractIn order to be able to use the nature and scale of the exsolution microstructures developed in titanomagnetites as quantitative indicators of thermal history, it is necessary to have available accurate diffusion data for the system. Diffusion data for pure magnetite and Ti-poor titanomagnetites are available, but no data for diffusion in the centre of the solid-solution series exist. In order to obtain values for the activation energy (ΔE) and the pre-exponential factor (D0) for the interdiffusion of Fe and Ti in ulvöspinel-rich titanomagnetites, the natural microstructures developed in titanomagnetites from the Taberg intrusion, Sweden, were homogenized over a range of temperatures from 490 to 730 °C. From the model describing homogenization, values of 49.8 kcal mole−1and 2.38 × 10−3cm2s−1were calculated for ΔEandD0respectively. Although the results obtained from these homogenization experiments are slightly less accurate than those which could be obtained by more conventional methods, the homogenization technique has several advantages which outweigh this drawback, namely the ease with which the experiment can be performed and the fact that the diffusion data can be obtained at significantly lower temperatures than is usually possible with more conventional methods.
Awaruite, iridian awaruite, and a new Ru-Os-Ir-Ni-Fe alloy from the Sakhakot-Qila complex, Malakand Agency, Pakistan
Authors: Ahmed Zulfiqar , Bevan J. C. 
Published: December 2006

AbstractThe authors report the first occurrence of awaruite in Pakistan, in the Sakhakot-Qila ultramafites. Electron-microprobe analysis reveals a considerable variation in the compositions of awaruites from different parts of the complex. The presence of copper in awaruite, in amounts up to 5 atomic percent, is also reported for the first time. In one chromitite body, an iridian awaruite with up to 27 wt% of iridium occurs in association with ‘normal’ awaruite and a new Ru-Os-Ir-Ni-Fe alloy. The awaruites are discussed in relation to those from other localities.
The textures and genesis of metamorphic pyroxene in the Freetown Intrusion
Authors: Wells M. K. , Bowles J. F. W. 
Published: December 2006

AbstractThe Freetown Intrusion, Sierra Leone, evolved by recurrent injection of olivine-bearing magma producing an aggregate thickness greater than 6000 m of rhythmically layered rocks. Sedimentation and constrained viscous flow of crystallizing magma concentrated olivine crystals during initial formation whilst concentration of volatiles locally lowered residual melt temperatures. This affected the crystallization of pyroxenes especially in pyroxene-troctolites where abundance of olivine caused deferred pyroxene growth. Three examples are described: (1) Flattened and elongated schlieren of augite enclosing plagioclase in ophitic intergrowths show preferred orientation cutting across primary layering defined by olivine, proving that pyroxene growth was secondary and influenced by stress directions in a relatively rigid rock. (2) In some pyroxene-troctolites, augite is segregated into coarse ophitic nodules which are widely and evenly spaced in a troctolitic host rock. Successful pyroxene nucleation and extensive diffusion caused development of domains of equilibration within a ‘cotectic’ pyroxene-plagioclase-volatile system. Crystallization within this was deferred compared with the surrounding olivine-plagioclase assemblage in which effects of granulitization are conspicuous. (3) Occasionally the pyroxenes are segregated in dispersed lenticular bodies of noritic pegmatite in troctolite. As in the previous cases, growth of pyroxene involved resorption and regrowth of olivine. This reflects the highest temperatures of crystallization in equilibrium with volatile-enriched pegmatite fluids. Temperatures decreased inwards giving a zonal structure and late growth of biotite, orthoclase, and apatite.The formation of late crystalline pyroxenes and pegmatites thus involved extensive diffusion of volatile and other components accompanying the establishment of domains of reduced pressure in hot and slowly cooled rocks subjected to gradual deformation due to progressive subsidence of the central parts of the intrusion. The final fabric is therefore metamorphic resulting from annealing rather than crystal accumulation.
Cesanite, Ca2Na3[(OH)(SO4)3] , a sulphate isotypic to apatite, from the Cesano geothermal field (Latium, Italy)
Authors: Cavarretta G. , Mottana A. , Tecce F. 
Published: December 2006

AbstractCesanite occurs both as a solid vein (1 cm thick) and as a cavity-filling of an explosive breccia in core samples of the Cesano 1 geothermal well (Cesano area, Latium, Italy).Cesanite is coloudess, medium to coarse grained, soft (H = 2 to 3) and light (ρmeas 2.786±0.002 g cm−3). It is uniaxial negative, ε = 1.564, ω = 1.570, with space group P63/m and cell parameters a = 9.442 (4), c = 6.903 (3) Å, for c/a = 0.730. Identifying spacings are 8.161, 2.822, 2.727,1.844 Å, in X-ray powder patterns strikingly similar to those of apatite. The chemical formula (microprobe analyses on two grains) is Ca1.53Sr0.03Na3.42K0.02[(Cl0.06F0.06OH0.44)(S2.99O12)]·0.44H2O, while the theoretical formula, derived from considerations on structural identity with apatite, is Ca2Na3[(OH)(SO4)3]. Cesanite is the end member of the apatite-wilkeite ellestadite series where [PO4]3− is entirely substituted by [SO4]2−, the charge balance being made up by Na+ substituting for Ca2+.
Chemical effects of deuteric alteration in some Kenyan trachyte lavas
Authors: Jones W. B. 
Published: December 2006

AbstractThe Kenyan trachyte volcanoes Kilombe and Londiani show some unusual chemical features. Analyses of elements normally regarded as residual in salic peralkaline rocks do not show the linear covariance characteristic of these elements in other East African volcanoes. The scatter from expected trends is greatest for Y and the lanthanides, less for Nb and Rb, and not apparent for Zr. The lavas also show variable loss of Na, P, and Fe. It is suggested that this chemical deviation is the result of a deuteric event during the later stages of crystallization of the lavas. The flows showing the chemical deviations, in particular negative Ce anomalies, also tend to have their ferroaugite phenocrysts deuterically altered to a mineraloid closely resembling iddingsite.During the crystallization of these trachyte lavas an H2O-, Cl-, and F-rich vapour separates from the liquid. This absorbs some elements, particularly Na, P, Fe, Y, and the lanthanides from the silicate liquid as water-soluble or volatile halides. Some of the absorbed material is redeposited in other parts of the flow and the rest is lost altogether as the vapour escapes from the surface.
Zoned garnets in some Scottish Dalradian pelites
Authors: Sivaprakash C. 
Published: December 2006

AbstractZoning in garnets in Barrovian garnet zone rocks of central Perthshire is shown to be dependent on the continuous prograde reaction histories in which the garnet was involved. In highly aluminous rocks, garnet grows at the expense of chloritoid and staurolite whereas in less aluminous rocks garnet grows at the expense of chlorite. Minerals included in garnet such as ilmenite, plagioclase, chlorite, and chloritoid show compositional changes which with the garnet zoning elucidate the garnet-forming reactions. Where garnet is resorbed to produce staurolite the former loses Fe at its rim to staurolite so that ‘reverse’ zoning of decreasing Fe and increasing Mn and Ca towards garnet rim develops.
Partition geochemistry of sediments from DSDP 424 in the Galapagos Hydrothermal Mounds Field
Authors: Varnavas S. P. , Cronan D. S. 
Published: December 2006

AbstractSediments from DSDP 424, a core taken in a hydrothermal mound in the Galapagos Mounds Field, comprise biogenic oozes, siliceous ash-rich layers and two varieties of hydrothermal precipitates, an Fe-rich clay (nontronite), and Mn oxides. Compositionally, these sediments differ considerably from one another and vary in proportion down the core to basement. Geochemical partition studies on the sediments have indicated a variable fractionation of the elements that they contain between clays, carbonate, and Mn oxides with the hydrothermal phases being strongly depleted in trace elements. The hydrothermal sediments are thought to have been formed as a result of the fractional precipitation of Fe silicates and Mn oxides from ascending hydrothermal solutions which are depleted in trace elements due to subsurface precipitation of sulphides, and have had insufficient time to scavenge trace metals from sea water due to their rapid precipitation.
Hydrohonessite—a new hydrated Ni-Fe hydroxy-sulphate mineral; its relationship to honessite, carrboydite, and minerals of the pyroaurite group
Authors: Nickel Ernest H. , Wildman John E. 
Published: December 2006

AbstractHydrohonessite has a composition that can be expressed by the formula [Ni8-x2+Fex+(OH)16] [x/2SO2-4.yH2O.zNiSO4], where x is approximately 2.6, y is 7, and z is 1. The X-ray powder diffraction pattern has strongest lines at 11.0 (10), 5.56 (5), 3.68 (4), and 2.709 Å (3), and can be indexed on a hexagonal unit cell with a = 3.09 Å and c = 10.80 Å. It is optically uniaxial negative with ɛ = 1.59 and ω = 1.63; bright yellow in hand specimen and transmitted light. Hydrohonessite is the hydrated equivalent of honessite, and is related to carrboydite, motukoreaite, and mountkeithite. These minerals are related to brucite and pyroaurite-type minerals in that they have a layered structure consisting of brucite-like layers separated by about 7 Å of interlayer material consisting predominantly of water, but also containing anions that balance the charge of the brucite-like layer. Hydrohonessite is a secondary mineral resulting from the weathering of Ni-Fe sulphides, and appears to be stable between pH 6 and 7.
Uranium in the Niger-Nigeria Younger Granite Province
Authors: Bowden P. , Bennett J. N. , Kinnaird Judith A. , Whitley J. E. , Abaa S. I. , Hadzigeorgiou-Stavrakis Penelope K. 
Published: December 2006

AbstractIn Niger, uranium occurs in upper Palaeozoic and lower Mesozoic continental sedimentary basins west of the Aïr mountains, but the source of the uranium had not been identified. Geochemical studies and fissiontrack observations on alkaline ignimbrites preserved in two Palaeozoic anorogenic centres in Aïr show that uranium is concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it is concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile. Tectonic uplift, anorogenic magmatism, followed by weathering and erosion of the volcanic cover, with sedimentation in nearby continental basins, have all contributed to the development of uranium mineralization in Niger. The petrological and geochemical similarities between the Palaeozoic ring complexes in Niger and the Nigerian Mesozoic ring structures suggest that sedimentary uranium deposits may also be found in Nigeria if the tectonic and sedimentological controls were favourable.Enriched concentrations of uranium have been discovered in the exposed granitic roof zones of the Nigerian subvolcanic centres, with Th/U ratios approaching unity. Thus local vein deposits of uraninite, as well as dispersed uranium in recent sedimentary horizons, could be discovered particularly in the drainage systems entering the Chad Basin.
Some aspects of the genesis of heavy mineral assemblages in Lower Proterozoic uranium-gold conglomerates
Authors: Clemmey H. 
Published: December 2006

AbstractSome genetic models for Lower Proterozoic gold- and uranium-bearing pyritic conglomerates favour a modified placer origin in which low levels of atmospheric oxygen are used to account for the survival of uraninite and pyrite. There are many difficulties with such models—for example magnetite is absent in the ore-bearing horizons although it is stable in anoxic conditions, while it is abundant in over- and under-lying strata. Compact and porous pyrite grains are not in hydraulic equivalence and the deposits lack a normal detrital heavy-mineral assemblage. Moreover uranium and gold show evidence of diagenetic remobilization, the uranium becoming associated with secondary titaniferous phases and uranium and gold being enriched in reduzate facies sediments.New evidence concerning the genesis of the deposits is derived from a clast of ferric iron clay thought to represent a precursor sediment of the Witwatersrand Basin. Reworking of such clays and transport of a magnetite and ferric clay assemblage with subsequent sulphidation, could account for the porous pyrites, the absence of magnetite and the lack of hydraulic equivalence of the mineral grains in the conglomerates. The presence of oxygen, as indicated by the ferric iron clasts, would account for the evidence of mobility of uranium and of gold and would enhance their extraction from source rocks; particularly the release of gold from a precursor auriferous iron formation source. It is suggested that some aspects of the genesis of uranium deposits of the Witswatersrand and Elliot Lake may be similar to those of the Phanerozoic ‘Roll Front’ ores involving interaction between oxidizing uraniferous groundwaters and previously sulphidized and reduzate facies sediments.
The relationship between uranium distribution and some major crustal features in Canada
Authors: Darnley A. G. 
Published: December 2006

AbstractThe availability of reconnaissance scale geochemical maps for large areas of Canada enables spatial associations between major crustal structures and surface uranium content to be identified. Maps of the distribution of uranium for an area greater than 2 million km2 compiled from airborne gamma-ray spectrometry data are supplemented by maps for uranium, based on stream and lake sediment and some bore hole sampling. These are examined in relation to gravity, aeromagnetic and geological maps.The radioelement distribution can be related in detail to exposed bedrock and surface geology, but in addition there is evidence of the control of uranium distribution by major structural features which are marked by granitoids containing elevated levels of radioelements; several of these granitoids are associated with large negative Bouguer gravity anomalies. The distribution of such granitoids appears to be related to ‘megashears’, as in the case of the South Mountain batholith in Nova Scotia, or zones of tension. A belt of uranium enrichment, the Athabasca axis which is characterized by uraniferous granitoids with negative Bouguer gravity anomalies and associated tension faulting extends 2500 km northeast-ward from Edmonton, Alberta to the Melville Peninsula. This structure passes under the Athabasca basin which contains many large uranium deposits. Recent evidence that granitoids enriched in radioelements can provide low-grade heat sources over periods of hundreds of millions of years, capable, in favourable situations of maintaining low-temperature hydrothermal circulation is considered to account for uranium and other mineralization in the basin. It is suggested that the transfer of radioelements into the crust at the end of the Lower Proterozoic, was a factor in the stabilization of North American craton.
Uranium content, distribution, and migration in the Glendessarry syenite, Inverness-shire
Authors: Fowler M. B. 
Published: December 2006

AbstractThe distribution of uranium in a suite of variably deformed and metamorphosed rocks from the leucocratic member of the Glendessarry syenite has been determined using the fission track method. The uranium content of the magma increased during crystallization and uranium was concentrated in accessory minerals such as monazite, zircon, sphene, allanite, apatite, and microinclusions of a Zr- and Ti-rich phase. Contamination of the magma by pelitic metasediment enhanced the uranium content and monazite and zircon formed instead of sphene, allanite, and apatite.Evidence of subsolidus uranium mobility in late stage magmatic or metamorphic fluids is presented here and shows: (a) Intracrystalline redistribution of uranium, especially in grains of sphene. (b) Intergranular mobility in a fluid phase, which affected the uraniferous accessory minerals in several ways.
Radioelement geochemistry of Irish granites
Authors: O'Connor Patrick J. 
Published: December 2006

AbstractData for U, Th, and K, determined by neutron activation analysis, in 128 rock samples from 14 Irish Caledonian and Tertiary granite plutons are sum-marized. Irish Newer Caledonian granites (ninety-four samples) have values of radioelements (average 3.6 ppm U, 12.1 ppm Th, and 3.4% K) consistent with derivation from a source of broadly uniform composition. U mineralization occurs in the voluminous and forcefully emplaced Main Donegal and Leinster plutons whereas smaller passively emplaced granites (Galway, Barnesmore, Rosses) lack mineralization. Irish Tertiary granites (thirty-four samples), which represent small batches of highly fractionated acid magmas, show moderate or high radioelement contents (11.5 ppm U, 39.9 ppm Th, 4.5% K) but are not mineralized. It is concluded that the use of elevated radioelement contents as a criterion of granite ‘fertility’ may be misleading in the British Isles Caledonian Province. The radioelement chemistry of granites may reflect their source composition more closely than expected for differentiated and/or contaminated acid melts.
Calculation of the temperature of crystallization of silicates from basaltic melts
Authors: French W. J. , Cameron E. P. 
Published: December 2006

AbstractThis paper discusses the relationship between the chemical composition of basic melts and the temperatures at which olivine, clinopyroxene, and plagioclase begin to crystallize at one atmosphere. Diagrams are given which show the correlation between crystallization temperature and melt composition and which allow some of the temperatures to be estimated. Because the relationship between melt composition and crystallization temperature is virtually linear over short compositional ranges, the data available can be subdivided and examined by linear multivariate statistical techniques. The result is a set of equations which permit the crystallization temperatures to be calculated with an average error of less than 6 °C and a maximum error of 27 °C. These equations have been tested by experimental determination of crystallization temperatures for a range of rocks from the Marquesas Islands.
Terminations of multiple-chain lamellae in grunerite asbestos
Authors: Whittaker E. J. W. , Cressey B. A. , Hutchison J. L. 
Published: December 2006

AbstractGrunerite asbestos (var. amosite) from Penge, Transvaal, contains lamellae consisting of multiple-chain I-beams which frequently terminate within the structure. Such terminations may be either coherent or incoherent, the latter involving disturbances of the structure akin to dislocations. These may be of the nature of edge dislocations or of dislocations having both edge and screw components.Models are presented which have been used to elucidate the structural disturbances associated with the termination and side-stepping of multiple-chain lamellae. Odd-multiple-chains are shown to be of particular significance in giving rise to dislocations with a screw component along the c-axis.
The thermal expansion of staurolite, Fe4Al18Si8O44(OH)4
Authors: Gibbons K. , Dempsey M. J. , Henderson C. M. B. 
Published: December 2006

AbstractThe thermal expansion of iron end-member staurolite has been studied by high-temperature powder X-ray diffraction methods and by modelling with the Distance Least Squares (DLS) computer program. The X-ray approach was complicated by dehydroxylation of the staurolite. Mean linear expansion coefficients for the a, b, and c cell edges of dehydroxylated staurolite determined by the X-ray method are (× 10−6 °C−1); 20–500 °C, 8.93, 8.23, and 7.95, respectively, and 20–800 °C, 7.85, 9.43, and 9.13, respectively. Expansion coefficients of a, b, and c calculated for hydroxylated staurolite using the DLS program over the same temperature ranges are (7.86, 7.18, and 7.55 × 10−6 °C−1) and (7.87, 7.17, and 7.57 × 10−6 °C−1). The good agreement between the results from the two methods supports the use of computer modelling in estimating the thermal expansion behaviour of complex structures. The latter approach could be preferable for studying hydrated minerals.
Compositional variation in the pyroxenes and amphiboles of the Kangerdlugssuaq intrusion, East Greenland: further evidence for the crustal contamination of syenite magma
Authors: Brooks C. K. , Gill R. C. O. 
Published: December 2006

AbstractPyroxenes and amphiboles from the under-saturated to over-saturated syenites of the Kangerdlugssuaq intrusion have been examined to see what light they throw on the two contrasting petrogenetic models for the intrusion. Pyroxene crystals are strongly zoned outwards from augitic to more acmitic compositions, with the most calcic cores present in the foyaite, contrary to the expected pattern if the foyaite is the most evolved rock type as previously postulated. Amphiboles, which are absent in the foyaites, show an exceptionally wide compositional field varying from actinolite through richterite and katophorite to arfvedsonite. Many are manganoan and potassian varieties. However, there is no consistent variation throughout the intrusion as previous work has suggested. These results are not favourable to the idea of a crystal fractionation model for the intrusion and we suggest that the foyaite is closest to the original magma which has hybridized with the enclosing gneisses and basalts to produce the over-saturated rocks. Such a model is consistent with the existing isotopic data.
Exsolution structures in calcic pyroxenes from the Bjerkreim-Sokndal lopolith, SW Norway
Authors: Rietmeijer F. J. M. , Champness P. E. 
Published: December 2006

AbstractIron-rich (Fs:En ∼0.8) calcic pyroxenes that have been subjected to granulite-facies metamorphism contain up to seven generations of exsolution lamellae. They can be grouped into four stages. In stage 1 pigeonite exsolved parallel to ‘001’ and ‘100’ (where ‘hkl’ signifies ∼ (hkl)) and mostly inverted later to orthopyroxene. During stage 2 orthopyroxene exsolved parallel to (100), while during stage 3 orthopyroxene was quickly followed by metastable ‘001’ pigeonite. The stage 3 precipitates clearly grew and thickened together for some time. During stage 4 a ‘100’ pigeonite was exsolved. The stage 3 and 4 precipitates show evidence of reheating, dissolution and later, renewed growth. Sometimes orthopyroxenes of stage 3 have crossed a ‘001’ pigeonite lamella and caused it to invert by a shear mechanism.Chemical analysis shows no rotation of the tie lines between Ca-rich and Ca-poor phases, in contrast to previous studies of Skaergaard and Bushveld pyroxenes. The geothermometers of Wood and Banno (1973) and Wells (1977) indicate solidus temperatures of about 850°C and 900°C respectively, but the geothermometers were found to be unsuitable for subsolidus conditions. We estimate the pressure to have been about 9 kbar during solidification. Estimates of nucleation temperatures obtained from the orientations of the exsolved lamellae (Robinson et al., 1977) were 850–700°C for stage 1, and 600–400°C for stage 3. We believe this geothermometer to be unreliable for the low temperatures involved in stage 4.