Showing papers in "Mineralogical Magazine in 1987"
TL;DR: In this article, a simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses, assuming that iron is the only element present with variable valency.
Abstract: A simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe 3 + ions per X oxygens in the mineral formula, F, is given by; F = 2X(1 - T/S) where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe 2 § Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing SP += 4H § substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency.
1,690 citations
TL;DR: A general solution to ECP interpretation is presented, involving the construction of reference ‘ECP-maps’ over the surface of a sphere, which depends on the crystal structure and the relationship between crystal and specimen coordinate systems.
Abstract: Backscattered electrons (BSE) are incident electrons reflected back from a target specimen and imaged with the scanning electron microscope (SEM) Three distinct BSE signals exist: atomic number or Z-contrast, in which composition determines image contrast; orientation contrast, in which specimen crystal structure determines image contrast; and electron channelling patterns (ECP), which are unique for a particular crystal orientation The origins of these three signals are described, with particular attention being given to the necessary SEM operational and specimen preparation requirements Z-contrast images are relatively simple to obtain and also have a familiar appearance such that their usage should become commonplace ECP in comparison require subsequent interpretation which depends on the crystal structure and the relationship between crystal and specimen coordinate systems A general solution to ECP interpretation is therefore presented, involving the construction of reference ‘ECP-maps’ over the surface of a sphere A brief summary of the applications and potential use of the three BSE signals in the geological sciences is also given
473 citations
TL;DR: In this article, a complete mechanism for the transformation of goethite to hematite based on the results of thermogravimetric, transmission electron microscope and X-ray diffraction investigations is presented.
Abstract: A complete mechanism for the transformation goethite to hematite based upon the results of thermogravimetric, transmission electron microscope and X-ray diffraction investigations is presented. A porous microstructure and hematite crystallites in twin orientation are found to develop during transformation. For the main part of the transformation, and at higher temperatures, the reaction is controlled by a two-dimensional phase boundary. Activation energies of 169 ± 8 kJ/mole (for an ore mineral) and 154 ± 15 kJ/mole (for a recent sedimentary goethite) were obtained for this part of the transformation. At early stages and lower temperatures, the mechanism is one of proton/iron transfer across the reaction interface. Important goethite characteristics are grain size, shape, crystallinity and excess water content. The activation energy is found to depend upon temperature and degree of dehydration.
145 citations
139 citations
TL;DR: In this article, major and trace element and isotopic data were presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja.
Abstract: Major 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 6180/a60, 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 6180 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.
104 citations
TL;DR: The hydrated copper oxalate, moolooite, CUC204. nH20 (n ~ 0.4-0.7) was identified by infrared spectroscopy and X-ray powder diffraction as discussed by the authors.
Abstract: Vivid blue inclusions in white whewellite or weddellite, occurring within the medulla of four lichen species growing on copper-bearing rocks, have been identified as the hydrated copper oxalate, moolooite, CUC204. nH20 (n ~ 0.4-0.7), by infrared spectroscopy and X-ray powder diffraction. The mineral is believed to have been formed by reaction between oxalic acid secreted by the lichen and ground or surface water containing copper.
100 citations
TL;DR: In this article, the authors used an approach based upon the atomistic or Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the infrared and Raman vibrational frequencies of forsterite.
Abstract: We use an approach based upon the atomistic or Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the infrared and Raman vibrational frequencies of forsterite. We investigate a variety of interatomic potentials, and find that although all the potentials used reproduce the structural and elastic behaviour of forsterite, only one potential (THB1) accurately predicts its lattice dynamics. This potential includes 'bond-bending' terms, that model the directionality of the Si-O bond, which we suggest plays a major role in determining the structural and physical properties of silicates. The potential was derived empirically from the structural and physical data of simple oxides, and its ability to model the lattice dynamics of forsterite is a significant advance over previous, force-constant models, which have been simply derived by fitting to the spectroscopic data that they aim to model. The success that we have had in predicting the lattice dynamics of forsterite indicates that the potential provides the previously elusive yet fundamental, quantitative link between the microscopic or atomistic behaviour of a mineral and its macroscopic or bulk thermodynamic properties.
77 citations
TL;DR: In this paper, the authors found that significant quantities of mono-and di-carboxylic acids are produced during hydrous pyrolysis and that the amounts and types of acid appear to vary as a function of the type of kerogen type, maturity and mineralogy.
Abstract: Concentrations of organic acids ranging up to several thousand parts per million have previously been found in oil-field waters. These acids are of interest because of their potential to enhance porosity by the dissolution of carbonates and aluminosilicates. They are believed to be generated from organic geopolymers (kerogen) in the late-diagenetic-early-catagenetic stage of thermal maturation. During the course of artificial maturation experiments in which kerogens of varying type were heated in the presence of water (so-called ‘hydrous pyrolysis’) and different minerals, the distribution and abundance of low molecular weight water-soluble acids were determined by gas chromatography and gas chromatography-mass spectrometry. Preliminary results suggest that significant quantities of mono- and di-carboxylic acids are produced during hydrous pyrolysis. The amounts and types of acid appear to vary as a function of kerogen type, maturity and mineralogy. Implications of these findings regarding the development of secondary porosity are discussed.
71 citations
TL;DR: In the Western Gneiss Region (WGR) of Norway as discussed by the authors, the authors found that high-P assemblages (phengite + kyanite, omphacite + quartz) are locally preserved within gneisses near the coast.
Abstract: The 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 Xcpx jd shows a decrease in T max, 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 Sunnmore and Nordmore. Maximum pressures were reached at temperatures 100–200° < T max. This P-T-t path is consistent with the preservation of jadeite-rich cpx (and possible coesite) in the coastal regions. Earlier overestimates of P max, 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 T max. 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.
66 citations
TL;DR: In this article, the authors present a series of examples illustrating the role of analytical SEM in the mineralogical investigation of archaeological problems, including the characterization and provenancing of geological raw materials, the elucidation of the processes used to transform those raw materials into useful objects and the recognition and characterization of changes which archaeological artefacts may have undergone during burial or during storage.
Abstract: The modern analytical SEM, which can provide high-quality imaging facilities together with quantitative elemental analysis using an energy-dispersive spectrometer, is finding wide application in the investigation of archaeological problems. Many of these investigations involve the study of silicate and carbonate-based artefacts which may be relatively unmodified from their original geological parent raw materials so that mineralogically based interpretations are often appropriate. In this paper we present a series of examples illustrating the role of the analytical SEM in the mineralogical investigation of archaeological problems, including the characterization and provenancing of geological raw materials, the elucidation of the processes used to transform those raw materials into useful objects and the recognition and characterization of changes which archaeological artefacts may have undergone during burial or during storage.
62 citations
TL;DR: In this paper, it was shown that re-equilibration may cause aqueous inclusions in carbonates to yield a poor record of their low-temperature history, hut a useful record of the maximum temperature experienced by the host rock.
Abstract: Diagenetic palaeotemperatures determined from aqueous fluid inclusions can be affected by re-equilibration during burial heating Calculations based on the observed behaviour of inclusions in fluorite under external confining pressure allows prediction of the temperatures and depths of burial necessary to initiate re-equilibration of aqueous inclusions in the common size range 40 to 4 txm Heating of 20 ~ to 60 ~ over the initial trapping temperature may cause errors of 10 ~ to 20 ~ in the homogenization temperature This suggests re-equilibration may cause aqueous inclusions in carbonates to yield a poor record of their low-temperature history, hut a useful record of the maximum temperature experienced by the host rock Previous work suggests inclusions containing petroleum fluids will be less susceptible to re-equilibration K E e w o R D S: palaeotemperatures, fluid inclusions, carbonates, fluorite
TL;DR: In this article, a simple formula based on a single elastic scattering event at the centre of the specimen is proposed to estimate the minimum detectable mass and the minimum mass fraction which can be measured using the technique.
Abstract: In a thin specimen X-ray absorption and fluorescence can, to a first approximation, be ignored and the observed X-ray intensity ratios, Ia/Ie, can be converted into weight fraction ratios, CA/CB, by multiplying by a constant kaB; CA/C B = kAB I A/IB. 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.
TL;DR: In this paper, the products of closed-vessel hydrous pyrolysis of known petroleum source rocks, and comparing the results with field observations, have been studied by analysing the products.
Abstract: Non-hydrocarbon gas species (COz, N2, 1-I2) are locally important in exploration for gas, and there is a growing body of evidence that acid water originating in shales materially affects the diagenesis of nearby sandstones. These gases have been studied by analysing the products of closed-vessel hydrous pyrolysis of known petroleum source rocks, and comparing the results with field observations. Alteration of petroleum source rocks at temperatures >250 ~ yields a significant amount of non-hydrocarbon components. Ethanoate and higher acid anions are liberated in substantial quantities; the yield appears to be related to the oxygen content of the sedimentary organic matter present. The non-hydrocarbon gases CO2, H2 and N 2 are frequently the dominant gaseous products from hydrous pyrolysis: in the natural environment the same rock sequences at a higher maturity preferentially generate hydrocarbon gases--mainly methane. This discrepancy may be attributed to reaction and phase thermodynamic effects between laboratory and natural systems, behaviour that has important implications in the prediction of gas generation and composition in nature by source rock pyrolysis in the laboratory.
TL;DR: In this paper, ellipsoidal monazite nodules are found within Lower Palaeozoic sedimentary rocks in Wales, south-west England and Brittany are characterised by a pronounced zonation of light and heavy REE, an inclusion fabric of low grade metamorphic minerals indistinguishable from the host rock and a low Th content.
Abstract: Millimetric, ellipsoidal monazite nodules found within Lower Palaeozoic sedimentary rocks in Wales, south-west England and Brittany are characterised by a pronounced zonation of light and heavy REE, an inclusion fabric of low-grade metamorphic minerals indistinguishable from the host rock and a low Th content. They are interpreted as the product of in situ recrystallization of detrital monazites derived from pegmatitic or granitic source rocks and are potentially useful as indicators of Lower Palaeozoic sedimentary rock provenance.
TL;DR: In this article, a trial study in Devonian sandstones of Easter Ross successfully traced a hydrocarbon-bearing sandstone reservoir from an occurrence of uraniferous hydrocarbon nodules.
Abstract: Hydrocarbons (bitumens sensu lato) in sandstones have been recorded in several instances to have partially replaced their host rock, including quartz grains. Many replacive hydrocarbons are uranium-rich: associated non-uraniferous hydrocarbons are not replacive. Uranium is transported as carbonate complexes, which may be decomposed by organic acids to yield UO2 2+ ions and CO2. The UO2 2+ will be absorbed onto hydrocarbons before reduction to a mineral phase, generally uraninite; and the CO2 may be aggressive towards the silicate grains of the host sandstone. Accretionary nodules of replacive uraniferous hydrocarbon in red beds (e.g. at the cores of reduction spots) can provide valuable information about petroleum migration. They occur particularly in the vicinity of faults, and may record the interactions between metal-rich groundwaters and hydrocarbons leaking along a fault from an underlying reservoir. A trial study in Devonian sandstones of Easter Ross successfully traced a hydrocarbon-bearing sandstone reservoir from an occurrence of uraniferous hydrocarbon nodules.
TL;DR: In this paper, the authors show that the illite-smectite ordering reaction cannot be extrapolated between basins as an inorganic indicator of 'oil window' levels of maturity.
Abstract: Conversion of randomly ordered illite-smectite to ordered illite-smectite in the Upper Jurassic Kimmeridge Clay Formation from the North Sea has been recorded in the literature as occurring within the 'oil window' and has been suggested as an indicator of oil source rock maturity. Studies of authigenic clay minerals in the fine fraction (>0.5/xm) of the Kimmeridge Clay Formation mudstones from fourteen locations along the UK onshore outcrop between Dorset and North Yorkshire show that they comprise mainly ordered illite-smectites. The onshore Kimmeridge Clay section is organically immature, suggesting that the illite-smectite ordering reaction cannot be extrapolated between basins as an inorganic indicator of 'oil window' levels of maturity. These results also have important implications in source rock hydrocarbon expulsion and migration models which involve shale dewatering as a flushing agent. However, dewatering of shales may aid migration as it could cause fracturing of the shale bands separating the organic-rich layers within the source rock, prior to hydrocarbon generation. K E Y w 0 R D S" clay diagenesis, Kimmeridge Clay Formation, illite, smectite, organic maturation.
TL;DR: In this paper, a new experimental determination of the stability relationships for the dehydration of gypsum to the hemihydrate mineral bassanite at elevated temperature and pressure is described, based on the observation of very small changes in pressure on the onset of reaction due to the potential volume change in the reaction.
Abstract: A new experimental determination of the stability relationships for the dehydration of gypsum to the hemihydrate mineral bassanite at elevated temperature and pressure is described. The experimental method used depends on the observation of very small changes in pressure on the onset of reaction due to the potential volume change in the reaction. The technique yields P-T data of very high precision for this dehydration reaction, and the method is likely to be of use for other reactions. The experimental P-T results have been compared with those calculated from existing thermodynamic data for this reaction.
TL;DR: In this paper, the mechanism of natural biotite breakdown under pyrometamorphic disequilibrium conditions was investigated using transmission electron microscopy (TEM) and analytical electron microscope data.
Abstract: Transmission electron microscopy has been used to investigate the mechanism of natural biotite breakdown under pyrometamorphic disequilibrium conditions. Biotite in a xenolith of pelitic gneiss collected from a Tertiary dolerite sill, Isle of Mull, Scotland, shows evidence of an incipient reaction, characterised by a darkening in colour and the appearance of areas of fine-grained reaction products. TEM and analytical electron microscope data show that the reaction can be described as: Fe-A1 biotite --* Mg-A1 biotite + magnetite + hercynitic spinel + K-feldspar/melt + vapour. The orientations of the product phase are controlled by the crystallography of the reacting biotite, demonstrating that the transformation proceeds by a topotactic mechanism. An empirical method, based on the Mg/(Fe 2+ + Fe 3 +) ratios of coexisting spinel and biotite from experimental data, is used to deduce that the reaction occurred above ~ 770 ~ A comparison of the natural reaction microstructures with those produced experimentally suggest that the xenolith was probably above 800 ~ for less than 48 hours and cooled to temperatures of 770 ~ after ~ 150-200 hours. K E Y W O R D S : transmission electron microscopy, biotite, pyrometamorphism.
TL;DR: Chlorine-rich potassium hastingsite occurs in a calcareous pegmatite, a replacement zone and an amphibolite lens within hornblende gneiss on West Ongul Island, Lfitzow-Holm Bay, East Antarctica as discussed by the authors.
Abstract: Chlorine-rich potassium hastingsite occurs in a calcareous pegmatite, a replacement zone and an amphibolite lens within hornblende gneiss on West Ongul Island, Lfitzow-Holm Bay, East Antarctica. The amphibolite lens and hornblende gneiss were metamorphosed to the kyanite-sillimanite grade of the granulite facies during Proterozoic metamorphism. Chemical analysis (3.27 wt. % C1), unit cell parameters and optical properties of the Cl-rich potassium hastingsite are given. Cl-rich ( > 3 wt. %) calcic amphiboles reported from various rock types are mostly more than 0.9 in (Na+ K) content, more than 0.4 in K/(Na + K) ratio, more than 0.75 in Fe 2 +/(Fe 2 § + Mg + Mn) ratio and more than 1.9 in A1 iv content (total iron as FeO and O = 23). The unit cell volume of Cl-rich hastingsite is distinctly larger than that of Cl-poor hastingsite.
TL;DR: In this paper, five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; this zoning is a hydration effect.
Abstract: Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims. Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastn/isite. These are briefly described and microprobe analyses presented.
TL;DR: The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivines basalt magma at ~ 12 kbar followed by isobaric cooling (to approximately 900-1000 ~) and subsolidus reequilibration as discussed by the authors.
Abstract: Xenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ~ 12 kbar followed by isobaric cooling (to approximately 900-1000 ~ and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8-12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H20- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increased PH20- Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevatedfo2 and fso2 conditions.
TL;DR: In this article, a preliminary geochemical study was undertaken to constrain local tectonic models, and due to the variable altered nature of the volcanics, determine the lithological composition and magma type.
Abstract: The volcanic rocks of the ~arkl~la area in northeastern central Anatolia are associated with volcaniclastics, turbiditic limestones and pelagic-hemipelagic shales of Upper Cretaceous-Palaeocene age. A preliminary geochemical study was undertaken to constrain local tectonic models, and due to the variable altered nature of the volcanics, determine the lithological composition and magma type. Chemically the volcanics are an andesite-dominated suite of calc-alkali lavas, probably developed adjacent to an active continental margin in a local (ensialic back-arc?) basinal area. The volcanic activity was probably related to a postulated magmatic arc just south of the area during the early Tertiary.
TL;DR: Okhotskite, an Mn3+dominant pumpellyite-group mineral, is monoclinic, A2/m,a = 8.887, b = 6.000, c: 1S55A as mentioned in this paper.
Abstract: Okhotskite, an Mn3+-dominant pumpellyite-group mineral, is monoclinic, A2/m,a = 8.887, b = 6.000, c: 1S55A
TL;DR: Phengite and chlorite have undergone decomposition during pyrometamorphism caused by the intrusion of a dolerite feeder pipe into Dalradian greenschists in Argyllshire, Scotland.
Abstract: Phengite and chlorite have undergone decomposition during pyrometamorphism caused by the intrusion of a dolerite feeder pipe into Dalradian greenschists in Argyllshire, Scotland. All reaction products are extremely fine grained. Transmission electron microscopy has revealed that phengite pseudomorphs consist of biotite, spinel, mullite, sanidine and phengite, and that chlorite pseudomorphs consist of combinations of chlorite, spinel, orthopyroxene, magnetite, cordierite and biotite. Although the reactions were short-lived and did not go to completion, microprobe analysis and phase diagram analysis have revealed that there has been significant chemical interaction between the phyllosilicates and the surrounding rock. Numerous orientation relationships exist between the original minerals and their reaction products; the close-packed planes in the precursor phyllosilicates were inherited by their reaction products.
TL;DR: At least six separate rare earth and uranium-bearing daughter crystals occur in fluid inclusions hosted by andraditic garnet from the Mary Kathleen REE-U ore skarn, Queensland, Australia as discussed by the authors.
Abstract: At least six separate rare earth and uranium-bearing daughter crystals occur in fluid inclusions hosted by andraditic garnet from the Mary Kathleen REE-U ore skarn, Queensland, Australia. The daughter minerals are particularly high in La, Nd and Ce which reflects the relatively high concentration of these in the bulk ore (La20 a = 33.5%, Nd20 3 = 9.1% and Ce20 2 = 51.5% of the 2.6 wt. % REE common in the ore). The host garnets themselves contain up to 7600 ppm REE and 5 to 2700 ppm U. The energy-dispersive spectra (EDS) are consistent with the following minerals: a (Y, Ce, U, Ca, Fe, Nb, Ta) mineral; a (Ca, Fe, Ce) carbonate(?) mineral; a (Fe, Ca, Y, Ce, Nb, Ta) mineral; a possible carbonate of La, Mn and Nd; a chlorite of Mn and La as well as a possible chloride or oxychloride of K, Mg, Mn and La. Their occurrence infers that relatively high concentrations of REE and U prevailed in the original, oxidized, concentrated (30-70 wt. % total dissolved salts), high-temperature (550-670 ~ ore solutions. Their presence as daughter crystals may be due to the fact that CaC12 is a dominant salt in the solutions and that the latter's solubility was sufficiently high to 'salt out' the less soluble REE-U components.
TL;DR: The Hatapang pluton as discussed by the authors was discovered during geological mapping and mineral exploration in northern Sumatra by the British Geological Survey in conjunction with the Indonesian Directorate of Mineral Resources and showed significant greisenization and veining around its margins associated with Sn and W mineralization.
Abstract: The Hatapang granite was discovered during geological mapping and mineral exploration in northern Sumatra by the British Geological Survey in conjunction with the Indonesian Directorate of Mineral Resources. The pluton comprises a two-mica granite which shows significant greisenization and veining around its margins associated with Sn and W mineralization. An Rb-Sr isochron derived for the pluton indicates an age of 80 Ma and an initial 87Sr/86Sr ratio of 0.7151. This together with major and trace element data show the Hatapang to be of clear S-type affinity. The greisens are quartz-mica-topaz rocks and are almost totally deficient in Na. Trioctahedral mica compositions progress from biotite through siderophyllite to zinnwaldite during final differentiation and greisenization of the granite. Accompanying dioctahedral micas are phengitic. Associated with late-stage differentiation of the granite is the precipitation of tourmaline and various Nb-Ta oxides. Sn and W mineralization is manifested as cassiterite in the greisens, while wolframite tends to be related to quartz veining. A later and lower temperature sulphide event produced a suite of base metal sulphides and Ag-Bi-Pb sulphosalts. The identification of a Sn-W granite of Cretaceous age in northern Sumatra provides a link with occurrences of economically important Late Cretaceous Sn-W granites in Thailand and Burma and increases the potential of an area which until recently was thought to lie outside the SE Asian tin belt.
TL;DR: A small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, has unusual bulk compositions and mineral assemblages as discussed by the authors.
Abstract: Specimens collected from a small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, have unusual bulk compositions and mineral assemblages. One sample consists of phlogopite enclosing blue spinel (rag 96) with minute granules of corundum and sapphirine at the margins; a second of phlogopite enclosing porphyroblasts of corundum and peraluminous sapphirine. In the first the sapphirine is close to the 7 : 9 : 3 composition; in the other it is markedly peratuminous, e.g. (Mgl.628Fe0.028)A14.714Si0.6a6010, intermediate between the 7 : 9 : 3 and 3 : 5 : 1 members. The texture suggests that this sapphirine is a stable phase in equilibrium with eastonitic phtogopite and corundum. The very potassic, very magnesian bulk composition of the rocks is attributed to potassium metasomatism of a protolith consisting of magnesian chlorite and quartz.