bluuebunny/crossref_metadata_2025_split huggingface dataset:
Brinrobertsite: a new R1 interstratified pyrophyllite/smectite-like clay mineral: characterization and geological origin
Authors: Dong H. , Peacor D. R. , Merriman R. J. , Kemp S. J. 
Published: August 2004

AbstractBrinrobertsite, an ordered, mixed-layered, dioctahedral pyrophyllite-smectite (P/S), occurs in a metabentonite in the Ordovician Nant Ffrancon Formation near Bangor, N Wales. It comprises ~30% of the metabentonite, in association with quartz (~50%) and chlorite (clinochlore; 20%) which replaced glass shards and fine-grained glass matrix. Transmission electron microscopy (TEM) images show sequences of dominant ~24 Å 001 lattice fringes inferred to correspond to 2:1 layers with alternate pyrophyllite-like (low-charge) and smectite-like (higher-charge) interlayers (i.e. R1 ordering). The hk0 diffraction patterns are mostly hexanets with some spotty circles, implying that layers are largely coherently related, but with some turbostratic stacking. Collective data show that d100 = 5.2, b = 9.1, and d001 = 24–25 Å, assuming monoclinic or pseudomonoclinic symmetry. The composition, as determined by energy dispersive spectral analysis, is (Na0.22K0.07Ca0.06)(Al3.81Mg0.08Fe0.08)(Si7.84 Al0.16)O20(OH)4·3.54H2O, as consistent with the sum of the compositions of pyrophyllite-like and smectite-like units. Water content was determined by DTA/TGA analysis. The powder diffraction patterns have a principal peak with d001 = 24.4 Å. Patterns of air-dried and glycol-saturated brinrobertsite, including Na- and Ca-saturated and untreated samples, were modelled satisfactorily as R1-ordered P/S by the program NEWMOD-for-Windows. The unique composition of brinrobertsite relative to R1 IS, which is ubiquitous in metabentonites, was caused by leaching of alkalis and alkaline-earth elements by hydrothermal fluids associated perhaps with a nearby intrusion, as demonstrated by bulk-chemical analyses of the metabentonite. The crystal structure is modelled as having Al/Si distributions symmetrical by reflection across interlayers. This causes all 2:1 layers to be equivalent in having one tetrahedral sheet with little or no Al, and the other with significant Al substitution, giving rise to alternate high- and low-charge interlayers. Geological evidence suggests that brinrobertsite is a back-reacted product of hydrothermal alteration in the sequence: glass → pyrophyllite → brinrobertsite.
Thermodynamics of mixing and ordering in the diopside–jadeite system: I. A CVM model
Authors: Vinograd V. L. 
Published: August 2004

AbstractSolution calorimetry and phase equilibrium data for the diopside–jadeite system are assessed using a combination of the cluster variation method (CVM) and Redlich-Kister (RK) polynomial expansion. The CVM is used to model part of the free energy of mixing which depends on short-range order (SRO) and long-range order (LRO) effects. The SRO/LRO independent part of the free energy is modelled using an RK polynomial. The parameters of the RK and CVM models are obtained through the fit to the experimental data. The best-fit parameters are used to calculate activity-composition relations and a temperature-composition phase diagram of the diopside–jadeite system.
Thermodynamics of mixing and ordering in the diopside–jadeite system: II. A polynomial fit to the CVM results
Authors: Vinograd V. L. 
Published: August 2004

AbstractIn Part I (Vinograd, 2002) the relationship between temperature, composition and free energy of the diopside–jadeite solid solution was described using a CVM model. Here the same relationship is described using a combination of Redlich-Kister (RK) polynomials and Gaussian functions. The use of the Gaussians permits an accurate description of the free energy dip centred at Jd50, which is caused by the long-range ordering, while the effect of the short-range ordering can be easily modelled using the RK polynomial The simplified RK-Gaussian description is extended to the three-component diopside–jadeite–hedenbergite solid solution. The polynomial description permits an easy incorporation of the derived activity-composition relations into phase-equilibrium computation software.
Formation of coagulated colloidal silica in high-temperature mineralizing fluids
Authors: Williamson B. J. , Wilkinson J. J. , Luckham P. F. , Stanley C. J. 
Published: August 2004

AbstractRecent experimental studies have suggested that colloidal silica can form in high-T (300 to >700°C) hydrothermal fluids (Wilkinson et al., 1996). Natural evidence in support of this was found by Williamson et al. (1997) who proposed a colloidal (gel) silica origin for <50 μm irregularly-shaped inclusions of quartz contained in greisen topaz from southwest England. Confocal and microprobe studies, presented here, strengthen this argument although rather than forming a gel in the hydrothermal fluid, it is suggested that the colloidal silica aggregated as a viscous coagulated colloid, with much of its volume (<10 to 30 vol.%) consisting of metal (mainly Fe) -rich particles. This is evident from the largely solid nature of metal-rich shrinkage bubbles contained at the margins of the inclusions of quartz which shows that the material forming the inclusions contained much less liquid than would be expected in a silica gel. These findings may have important implications for models of ore formation since the precipitation of a coagulated colloid could inhibit hydrothermal fluid transport and cause co-deposition of silica and entrained ore-forming elements. The mode of formation of the colloidal silica and further implications of the study are discussed.
Chromite in greenstone lavas from the Kanakasu area, Nanjo Massif of the Mesozoic Mino terrane, central Japan
Authors: Agata T. , Hattori I. 
Published: August 2004

AbstractChromite occurs together with olivine as phenocrysts in basalts of the Kanakasu greenstone body. Chromite forms inclusions within olivine phenocrysts; it also constitutes discrete phenocrystic grains scattered in the groundmass. The Cr and Ni contents of chromite-bearing olivine basalts are unusually high relative to the MgO content. This is probably due to the presence of phenocrystic chromite and olivine. The mineralogy suggests that the groundmass of the basalts is hawaiitic in composition. Chromite, generally, is unlikely to crystallize from differentiated magma such as hawaiite melt. The chromite and associated olivine phenocrysts are probably xenocrysts. Discrete chromite commonly shows compositional zoning that resulted from reaction with host magma; some chromite evidently changed in composition. Chromite embedded in olivine was shielded from reaction with host magma, and has preserved the original chemical composition. The composition of embedded chromite ranges: Mg/(Mg+Fe2+) 0.37–0.58, Cr/(Cr+Al) 0.47–0.64, Fe3+ 0.16–0.47 p.f.u., and Ti 0.034–0.13 p.f.u. The relatively high Ti and Al contents suggest that chromite crystallized from an alkalic basalt magma. The Cr/(Cr+Al) ratio is relatively high when compared to those of chromite in mid-oceanic ridge and island-arc alkalic basalts; the Kanakasu embedded chromite is chemically identical to chromite from Hawaiian alkalic basalts. The Kanakasu chromite was probably formed in an intraplate oceanic island.
Oxidation-induced postmagmatic modifications of primary ilmenite, NYG-related aplite dyke, Tibchi complex, Kalato, Nigeria
Authors: Sakoma E. M. , Martin R. F. 
Published: August 2004

AbstractWe describe an ilmenite-bearing aplitic syenite dyke in the roof zone of the Tibchi granite, exposed at Kalato, in the Tibchi ring-complex, northern Nigeria. Inclusions of ferrocolumbite, rutile and ixiolite in the ilmenite are inferred to have been trapped at the magmatic stage. The main mafic mineral is annite. Compositionally, the ilmenite, rutile and ferrocolumbite have near-end-member compositions. A positive correlation between Sc and Ta/(Ta+Nb) indicates that Sc behaved incompatibly as ferrocolumbite grew. Such entrapped accessory minerals may well have formed by local saturation at the ilmenite-melt and annite-melt interface. During and after their crystallization, the melt reached saturation in H2O and degassed. A second generation of ilmenite enriched in Mn and Zn replaced the primary ilmenite along fractures and grain margins. As fO2 began to increase, composite blebs and rinds of ‘ferropseudobrookite’, rutile and hematite began to develop by oxidation-induced exsolution in the primary ilmenite. Incorporation of Nb, Ta, Sc and Si in the ‘ferropseudobrookite’ may well have stabilized it at Kalato. Ultimately, it is transformed to hematite + rutile. The IMA-sanctioned view that the solid solution between pseudobrookite and Ti3O5 is complete, and thus that ‘ferropseudobrookite’, as an intermediate member of the series, does not merit species status, needs to be re-evaluated.
Caveats in defining the magmas parental to the mafic rocks of the Bushveld Complex, and the manner of their emplacement: review and commentary
Authors: Eales H. V. 
Published: August 2004

AbstractThe composition of magmas proposed as parental to the layered suite of the Bushveld Complex, and some models for the manner of their emplacement, are reviewed briefly. Included are some contributions published in South Africa, with which overseas readers might be less familiar. Emphasis is given to the broader features of the cumulates, and the contradictions raised by whole-rock compositional, Sr-isotopic, and trace-element data that cloud their correlation with proposed parental magmas. It is concluded that the Lower, Critical and Main Zones are the derivatives of only two primary magmatic lineages, while a third was added to residual liquids from which the layered rocks above the Pyroxenite Marker were formed. Excessive amounts of olivine and chromium in the cumulates of the Lower and Lower Critical Zones in the northern sector of the Western Limb can seemingly not be accounted for by the composition and volume of the putative magmas. This is attributed to (1) this sector being a proximal facies located close to the original feeder, and/or (2) crystal-charged magma batches, expelled from a lower magma chamber, being periodically injected into and dispersed within the liquids already in place in the Bushveld chamber. Thus, ongoing changes in the bulk composition of the liquids within the chamber would not be reflected in the rinds of earlier, chilled-facies rocks. The expulsion of significant volumes of liquid residua from the chamber during cumulate deposition is not ruled out.
Geochemical effects of magma addition: compositional reversals and decoupling of trends in the Main Zone of the western Bushveld Complex
Authors: Nex P. A. M. , Cawthorn R. G. , Kinnaird J. A. 
Published: August 2004

AbstractThe Main Zone of the Bushveld Complex, which is ˜3–3.5 km thick, comprises a sequence of gabbronorites with minor anorthosites and pyroxenites. The Pyroxenite Marker (PM), a thin orthopyroxenite layer occurring towards the top of the Main Zone in the eastern Bushveld, marks the change from an inverted pigeonite-bearing microgabbronorite below, to a primary orthopyroxene-bearing porphyritic gabbronorite above. In the western Bushveld the PM has only been observed in core material although its surface position can be inferred from the mineralogical and textural changes. Whole-rock geochemistry of surface and core samples from the Brits and Marikana areas, together with mineral compositional data, have been integrated with published analyses to elucidate the magmatic processes that occurred during the addition of new magma into the chamber, just below the level of the PM. Modal and major-element data show that most lithologies lie close to the plagioclase-two pyroxene cotectic. However, there are three units below the PM in which distinct modal layering is developed. Changes in geochemical trends for trace-element abundances in both whole-rock and mineral-separate data occur at ˜150 m below the PM, below a layered package, the Hexrivier Unit. However, there is a displacement of at least 40 m between the beginning of the reversals in An content in plagioclase and in Mg# in pyroxene which occur at 124 m and 80 m below the PM respectively. In addition, a gradual change occurs in the mineral parameter Mg# in opx minus An in plagioclase, with the plagioclase becoming more primitive relative to the pyroxene. This decrease and the decoupling of geochemical trends has not been noted in the Main Zone of the Bushveld Complex before. The decoupling between plagioclase and pyroxene compositional reversals is not easily explained by modal effects, an influx of phenocrysts in the new magma, or by infiltration metasomatism. Instead we propose that new magma pulses first entered the chamber some 150 m below the PM and that this magma had a composition that crystallized more primitive plagioclase but similar pyroxene to the magma residing in the chamber. The chamber was intruded by progressively more magma that had a more primitive composition particularly in terms of pyroxene composition. This model can explain the decoupling between plagioclase and pyroxene compositional trends. Continued mixing between resident magma and the new influxes occurred over an interval of ˜150–200 m. Above the PM fractional crystallization processes dominated and continued into the Upper Zone.
Delayed accumulation of plagioclase in the Bushveld Complex
Authors: Cawthorn R. G. 
Published: August 2004

AbstractThe Upper Critical Zone of the Bushveld Complex consists of several cyclic units, each ranging from ultramafic to leucocratic. One view is that they were initiated by addition of, and crystallization from, relatively magnesian magma. Experimental studies on plausible parental magma compositions to this sequence show that plagioclase joins orthopyroxene in the crystallization sequence once the Mg# of the orthopyroxene has decreased below 83. Some norites from the Upper Critical Zone contain orthopyroxene with this Mg#, supporting the validity of the experimental studies. There are no orthopyroxene compositions with higher Mg# than 83 in the entire Upper Critical Zone in either the eastern or western limbs of the Bushveld Complex. This observation suggests that all these rocks formed from plagioclase-saturated magmas. If this interpretation is correct, pyroxenites in this succession are not the result of crystallization from a magnesian magma. Instead, they result from the mechanical separation of plagioclase and orthopyroxene, probably due to crystal sorting during settling, from a magma lying at the cotectic for these two minerals. Rocks exist in the Upper Critical Zone that contain non-cotectic proportions of cumulus plagioclase and orthopyroxene, again supporting models of crystal sorting during settling. In such a model, anorthosites result from the delayed accumulation of plagioclase relative to pyroxene, and not to formation from a magma saturated only in plagioclase.When traced from northwest to southeast in the western limb, there is a change in the relative proportions of plagioclase to orthopyroxene in the Upper Group 2 chromitite cyclic unit. In the northwest the unit is dominated by ultramafic rocks. In the southeast the plagioclase to pyroxene ratio exceeds that of the cotectic proportion, relations that may result from the lateral transport of suspended plagioclase grains to the southeast.
‘Decompressional’ reaction textures formed by isobaric heating: an example from the thermal aureole of the Taylor Brook Gabbro Complex, western Newfoundland
Authors: Ings S. J. , Owen J. V. 
Published: August 2004

AbstractReaction textures including corona structures in granulites from the Proterozoic Long Range Inlier of western Newfoundland are spatially associated with a Silurian (0.34 Ga) mafic intrusion, the Taylor Brook Gabbro Complex. They comprise, in metabasites and tonalitic gneiss, coronal orthopyroxene and plagioclase on garnet and, in metapelites, cordierite and spinel formed at the expense of sillimanite, garnet and quartz. Although generally interpreted to indicate near-isothermal decompression (ITD) following regional metamorphism, which in the inlier occurred at ˜1.10–1.03 Ga, these features appear to be absent elsewhere. Therefore they are interpreted to be products of contact metamorphism (near-isobaric heating – IBH) within the thermal aureole of the gabbro. Thus, there is a ˜0.7 Ga difference (i.e. mid-Proterozoic vs. mid-Silurian) between the age of the regional metamorphic mineral assemblages and the contact aureole assemblages. The observation that classic ITD features occur in this aureole environment underscores the fact that P-sensitive reactions can progress during IBH as well as by pressure release.
Crystal chemistry of clinopyroxenes from Linosa Volcano, Sicily Channel, Italy: implications for modelling the magmatic plumbing system
Authors: Bindi L. , Tasselli F. , Olmi F. , Peccerillo A. , Menchetti S. 
Published: August 2004

AbstractThe Island of Linosa is a small part of the large submarine volcanic complex which is locatedat the SW edge of the Linosa Graben, Sicily Channel. The island was formed between 1.06±0.10 and 0.53±0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera. Major and trace element data show that the compositional variability of the three activity stages is limited, with most of the rocks showing basaltic to hawaiitic composition. Evolved benmoreites and trachytes are foundas lithics in some pyroclastic units of Paleolinosa. The mafic rocks of the three stages show porphyritic texture, with phenocryst assemblages characterized by olivine, clinopyroxene and plagioclase. The volume ratio of olivine vs. clinopyroxene decreases from early to late stages of activity in mafic rocks with comparable major element composition. Clinopyroxene phenocrysts from mafic rocks of the three stages have poorly variable composition, clustering in the augite field. Phenocrysts from the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 (total) with the increasing SiO2 content of the host rocks. Crystals from the second and the third stage (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with very high MgO, Al2O3 and TiO2 contents. Crystal chemical investigation of clinopyroxenes from rocks of the three stages with comparable degrees of evolution, revealed significant variation of structural parameters, in particular VM1 and Vcell. These show a consistent decrease, passing from clinopyroxenes of the early stage to crystals extracted from the mafic lavas of stages 2 and 3. Given the similar compositional ranges of the host rocks, structural variations of clinopyroxenes are interpreted to reveal modifications of crystallization pressure, which increased, passing from Paleo-Linosa to the Arena Bianca and Monte Bandiera stages. Given this information, the observed crystal-chemical variations provide information on the depth of magma reservoirs and on the evolution of the plumbing system of Linosa volcano.
Silicate melt inclusions in the cumulate minerals of gabbroic nodules from Stromboli Volcano (Aeolian Islands, Italy): main components of the fluid phase and crystallization temperatures
Authors: Salvioli-Mariani E. , Mattioli M. , Renzulli A. , Serri G. 
Published: August 2004

AbstractThe studied gabbroic nodules occurring in the Petrazza pyroclastic rocks consist mainly of plagioclase (An95–87), olivine (Fo83-73) and clinopyroxene (Mg# 90–77), with subordinate opaques (Ti-magnetite) and amphibole (Mg-hastingsite), which constitute the cumulate minerals. Interstitial material has a relatively high, but variable, degree of vesicularity and consists of variable amounts of glass and quenched crystals of plagioclase (An71–55), amphibole, clinopyroxene and rare biotite, olivine and opaques. Silicate melt inclusions are abundant in the cumulate minerals, but complete homogenization to melt has been observed only in the inclusions occurring in clinopyroxene, where the temperatures of homogenization vary from 1134 to 1190°C. Microthermometric investigations of fluid inclusions and of the shrinkage bubble of the melt inclusions suggest that the magma contained CO2. The apparent scarcity of H2O indicates that this component was strongly partitioned into the magma at the time of crystallization of the investigated minerals; this is further supported by the occurrence of (1) daughter biotite- and amphibole-bearing inclusions which show that the H2O activity in the magma was sufficiently high to allow their crystallization, and (2) calcic plagioclase (An95–87) which can be crystallized from a high-alumina basaltic magma at pressure ≤2 kbar, temperatures in the range 1050–1100°C and in the presence of 3–4 wt.% of water (MELTS software simulations).The composition of the melt inclusions suggests that the hosting plagioclase, olivine and clinopyroxene crystallized from slightly different batches of magma. The S content in the melt inclusions of clinopyroxene and olivine is high (up to 0.41 wt.%). The presence of Fe-Cu(-Ni)-rich blebs of sulphide in plagioclase, olivine, amphibole, and locally in the melt inclusions too, further supports the important role of sulphur in the primitive magmas of the investigated gabbros. Small differences in redox conditions or in the Fe content of the melts favoured S mobilization as sulphide.
Chrysoberyl, beryl and zincian spinel mineralization in granulite-facies Archaean rocks at Dowerin, Western Australia
Authors: Downes P. J. , Bevan A. W. R. 
Published: August 2004

AbstractA deposit of chrysoberyl (BeAl2O4), including the variety alexandrite, occurs near Dowerin, in the southwestern region of the Archaean Yilgarn Craton, Western Australia. The deposit is situated in the northern part of the Lake Grace Terrain, a crustal component of the southwestern Yilgarn Craton, in granulite-facies gneisses (2640–2649 Ma; T = 700°C, P <6 kbar) adjacent to the margin of the Kellerberrin Batholith (2587±25 Ma). Beryllium mineralization at Dowerin occurs in plagioclase-quartz-biotite-garnet gneiss and cross-cutting tourmaline-plagioclase veins situated adjacent to lenses of actinolite-cummingtonite-phlogopite schist. Crystals of chrysoberyl (0.15–1.74 wt.% Cr2O3; 2.25–3.23 wt.% FeO; trace–0.13 wt.% ZnO; SiO2 <0.05 wt.%) are found embedded in almandine or plagioclase, and closely intergrown with biotite and/or zincian hercynite in the host-rock gneiss. Minor Cr and Fe in the alexandrite variety of chrysoberyl were possibly derived from associated zincian hercynite and/or almandine. Trace beryl (0.04–0.20 wt.% Cr2O3; 0.54–0.71 wt.% FeO; trace– 0.22 wt.% Na2O; 0.1–0.71 wt.% MgO) occurs as anhedral interstital grains between crystals of chrysoberyl, plagioclase and biotite, and as rare inclusions in chrysoberyl. Textural and mineral chemical evidence suggests that chrysoberyl and zincian spinels (chromite to hercynite containing from 2–8 wt.% ZnO) formed during granulite-facies regional metamorphism and probably pre-dated the formation of metamorphic tourmaline-plagioclase veins during the same metamorphic episode. The Be, B and Zn required to form chrysoberyl, beryl, tourmaline and zincian spinels may have been released by metamorphic reactions in host-rock metapelites during prograde granulite-facies metamorphism.