Showing papers in "Mineralogical Magazine in 1998"

Practical application of lead triple spiking for correction of instrumental mass discrimination

Abstract: The external precision of Pb isotope ratio measurements by TIMS is currently limited to ~0.1% due to variations in instrumental mass discrimination. It was shown by Dodson (1963) that the unknown mass discrimination factor could be obtained from a second measurement on an aliquot of a sample to which a double spike has been added. From the three isotope ratios measured on the two runs it is straightforward to solve for the three unknowns the respective fractionation factors of the two runs and the spike/sample ratio. Practical applications of the Pb double spike technique have used a mixed 2~176 tracer in the ratio ~9:1 (e.g. Hamelin et al. , 1985; Woodhead et al., 1995). These studies have demonstrated that a factor of up to -3 improvement in external precision can be obtained compared with good conventional, uncorrected measurements. Further, any systematic errors arising from matrix effects are effectively eliminated using the technique (Woodhead et al., 1995). Slightly more favourable results have been obtained by using a double spike composed of the non-natural isotopes 2~176 (Todt et al., 1996), which has the added advantage that only one mass spectrometric run is required; a disadvantage, ignoring the cost of these spikes, though, is that isobaric interferences on these masses must be continually monitored and corrected for during measurement. A systematic survey of stable isotope tracer compositions, axis combinations and mathematical solutions to this problem was recently undertaken by Galer (1997). From this study, there appears to be an optimal composition to use as a spike to correct samples of common Pb for instrumental mass fractionation. This spike composition (designated here as TS) is unusual in that it is a mixture of three stable isotopes of lead 2~176176 rather than just two, 2~176 used in previous studies. The error magnification factor 7 on the 2~176 ratio (i.e. the bias-corrected error relative to the measured, unspiked internal error) was estimated by Galer (1997) using a realistic error propagation model based upon the ion statistical uncertainties on the two runs and the TS composition. For the optimized TS composition, 7 lies at ~1.2 over a large range in spike/sample ratios, becoming larger than 2.0 only when the spike/sample ratio is less than 0.05 or greater than 3.8. This implies that when preparing the TS-sample mixture, the quantity of Pb in the sample need only be known to within a factor of ~40 either way to avoid an unfavourable 7 > 2. This behaviour is in sharp contrast to that of the 2~176 double spikes that have been used previously, which never have 7 < 2 for any spike/ sample ratio. Such double spikes as these, even in the most favourable cases, require internal precisions on the individual runs a factor of ~2 better to achieve the same final precision as the TS in effect, measurement times (or sample sizes) would need to be a factor of ~4 larger. In order to see how such an optimal triple spike would perform in practise, a tracer of TS composition was prepared and subjected to a number of tests to evaluate its performance. All Pb isotopic measurements were performed in static multi-collection mode on a Finnigan MAT-261. Inter-cup efficiency biases were determined by simplex minimization from measurements of the NBS-982 equal-atom Pb standard. The TS itself was calibrated by joint runs of the triple spike alone and mixtures in varying proportions with the NBS-982 standard the mass bias-corrected TS isotopic composition can then be obtained by solving the equations with the TS as the 'unknown' (cf. Hamelin et al. , 1985). Thus, any isotopic compositions determined using the TS are traceable to the NBS-982 Pb isotopic composition of Todt et al. (1996) which, in turn, was normalized to a 2~176 ratio of 1.00016. The first test of the triple spike was measurement of the NBS-981 common lead standard, whose isotopic composition has recently been redetermined by Todt et al. (1996). Samples of NBS-981 were mixed with TS in a large range of spike/sample ratios and combined with unspiked runs to yield fractionation-corrected ratios for NBS-981. To improve in-run

Nomenclature of the micas

Abstract: End-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials, and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

Trace element composition and cathodoluminescence properties of southern african kimberlitic zircons

Abstract: Zircon frequently occurs as a minor mineral in kimberlites, and is recognised as a member of a suite of mantle-derived megacryst minerals. Cathodoluminescence (CL) microscopy and laser ablation ICPMS analysis were used to study the internal structure and chemical composition of zircon crystals from southern African kimberlites. Zoning revealed by CL ranges from fine oscillatory to broad homogeneous cores and overgrowths. The ICPMS data show that kimberlite zircons have distinctive trace element contents, with well defined ranges for REE, Y, U, Th, P and some other trace elements. Both low REE contents (ΣREE < 50 ppm), and distinctive chondrite-normalised REE patterns with low and flat HREE are characteristic of kimberlite zircons. Samples or zones with yellow CL have higher Th, U, Y, and REE than those with blue-violet CL. Variations in the concentrations of a range of trace elements lead to different amounts of lattice defects, creating the possibility for different levels of direct excitation of luminescence centres, and therefore different CL colours. The distinctive CL and compositional features described here can rapidly identify kimberlite zircons in prospecting samples taken during exploration for kimberlite bodies.

Ree-sr-ba minerals from the khibina carbonatites, kola peninsula, russia :their mineralogy, paragenesis and evolution

Abstract: Carbonatites from the Khibina Alkaline Massif (360 › 380 Ma), Kola Peninsula, Russia, contain one of the most diverse assemblages of REE minerals described thus far from carbonatites and provide an excellent opportunity to track the evolution of late-stage carbonatites and their sub-solidus (secondary) changes. Twelve rare earth minerals have been analysed in detail and compared with literature analyses. These minerals include some common to carbonatites (e.g. Ca-rare-earth fluocarbonates and ancylite-(Ce)) plus burbankite and carbocernaite and some very rare Ba, REE fluocarbonates. Overall the REE patterns change from light rare earth-enriched in the earliest carbonatites to heavy rare earth-enriched in the late carbonate-zeolite veins, an evolution which is thought to reflect the increasing ‘carbohydrothermal’ nature of the rock-forming fluid. Many of the carbonatites have been subject to sub-solidus metasomatic processes whose products include hexagonal prismatic pseudomorphs of ancylite-(Ce) or synchysite-(Ce), strontianite and baryte after burbankite and carbocernaite. The metasomatic processes cause little change in the rare earth patterns and it is thought that they took place soon after emplacement.

Surface Area and Porosity of Primary Silicate Minerals

Abstract: Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite < oligoclase ≈ bytownite < hornblende ≈ diopside. Reproducibility of BET SSA values range from ±70% (SSA < 1000 cm/g) to ± 5% (SSA > 4000 cm/g) and values measured with N2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA, cm/g) = b + m log (d), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = –1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interfacelimited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory-ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratoryand field-based estimates of weathering rate. * E-mail: npm113@psu.edu BRANTLEY AND MELLOTT: SURFACE AREA AND POROSITY OF SILICATES 1768 weathered samples of plagioclase, K-spar, and hornblende based upon microscopic observations and variations in surface area measured by gas adsorption as a function of grain size for soils from Puerto Rico (Schulz et al. 1999) and Merced, California, (White et al. 1996). In addition, porosity in alkali feldspars has been documented by many workers (e.g., Worden et al. 1990; Walker et al. 1995; Lee and Parsons 1995). Pores have also been documented in laboratory-ground silicates: for example, Hodson and coworkers (Hodson et al. 1997; Hodson 1998) documented porosity in potassium feldspar and Brantley et al. (1999) documented mesoporosity in hornblende. A better understanding of the controls on mineral surface area and porosity in surficial environments is important for several reasons. For example, if internal surface area (e.g., Gregg and Sing 1982; Hochella and Banfield 1995) is present in primary silicate minerals, and if internal surface area is more or less reactive than external surface area, then quantification of the ratio of internal vs. external surface area could be important in the extrapolation of mineral-water kinetics from one system to another (see, for example, Lee and Parsons 1995; Hodson 1998). Whereas several authors have suggested that weathered and laboratory-ground samples differ with respect to the relative importance of internal and external area (e.g., Anbeek 1992a, 1992b, 1993; Anbeek et al. 1994), few have quantified the contribution from these two types of surface for minerals which are commonly used in comparisons of field and laboratory weathering rates (e.g., plagioclase, hornblende, olivine, diopside). Specifically, if the ratio of these two types of surface area differ between laboratory-ground and naturally weathered plagioclase, hornblende, olivine, and diopside, then perhaps these differences contribute to the large (up to five orders of magnitude) discrepancies between laboratoryand field-derived dissolution rates for these minerals (White and Brantley 1995). In addition, if porosity is present in silicate grains then the observed aging or sequestration of contaminants in soils and aquifers may be due to sequestration inside these pores (e.g., Farrell and Reinhard 1994; Werth and Reinhard 1997; Luthy et al. 1997). In a series of papers, Mayer (1994, 1999) also proposed that sequestration of organic matter into small pores may allow preservation of the organic matter on continental shelf sediments since hydrolytic enzymes should be excluded from such small pores. Again, however, little is known about mineral controls on the presence of porosity in the silicates making up the sediments. This paper addresses the following questions concerning SSA of primary silicates: What is the range of SSA measured using BET for laboratory-ground primary silicate powders? Can we infer the presence of porosity from adsorption data for laboratory-ground and weathered primary silicates? What do these observations imply for reaction kinetics in the laboratory and in the field? Our approach was to investigate relatively pristine mineral samples that have traditionally been used by geochemists to investigate mineral dissolution (samples largely derived from pegmatite deposits) using standard BET techniques for analysis of gas adsorption-desorption. We also used the same BET technique on seven naturally weathered samples to infer the presence or absence of porosity. BACKGROUND The specific geometric surface area, SSAgeo, can be expressed as a function of grain diameter d:

Microbial Destruction of Feldspars

Abstract: It now seems clear that feldspar weathering dynamics in natural systems is not well modelled by the typical inorganic and abiotic laboratory experiment. In particular, the role of micro-organisms in feldspar weathering has only recently been directly examined or documented. Mounting evidence, however suggests that bacteria attached to feldspar surfaces can greatly accelerate the rate of feldspar dissolution, but two important questions remain why, and how. Here we report the results of field experiments combined with parallel laboratory experiments that help constrain the question of how, and suggests a hypothesis for why; microorganisms may destroy feldspars to gain access to trace or limiting nutrients, primarily by the production of organic ligands.

The Iceland Plume in Space and Time: A Sr-Nd-Pb-Hf Study of the North Atlantic Rifted Margin

Abstract: The North Atlantic Igneous Province (NAIP) offers an unusual, if not unique, opportunity to investigate the composition, evolution and melting dynamics of a mantle plume. Unlike most plumes which remain trapped beneath a lithospheric cap, the interaction between the Iceland plume and the Mid-Atlantic Ridge have allowed the plume to decompress and melt at shallow levels. This is an important aspect since, crucial to existing models are (a) whether we can distinguish the various components in the plume and (b) whether we can distinguish a depleted component that is sourced in a shallow layer, or from another (deeper?) depleted source. In an effort to constrain the nature of the Iceland plume in time and space, we have undertaken a geochemical (major and trace elements) and isotopic (Sr-Nd-Pb-Hf) study of basalts from the North Atlantic Igneous Province. Included are samples ranging from some of the oldest stages of plume magmatism (i.e. ~60Ma, ODP Legs 152 and 163) to virtually zero-age rocks from the Iceland neovolcanic zone. Their spatial distribution is shown in Fig. 1. In this study, we will be considering only Iceland and the southern sites (i.e. ODP Legs 49, 81, 152, 163, and the Reykjanes Ridge). The northern localities (i.e. ODP Legs 38 and 104) will be discussed at a later date.

Polymorphic transformations between olivine, wadsleyite and ringwoodite: mechanisms of intracrystalline nucleation and the role of elastic strain

Abstract: Kinetic models and rate equations for polymorphic reconstructive phase transformations in polycrystalline aggregates are usually based on the assumptions that (a) the product phase nucleates on grain boundaries in the reactant phase and (b) growth rates of the product phase remain constant with time at fixed P-T. Recent observations of experimentally-induced transformations between (Mg,Fe)2SiO4 olivine (α) and its high pressure polymorphs, wadsleyite (β) and ringwoodite (γ), demonstrate that both these assumptions can be invalid, thus complicating the extrapolation of experimental kinetic data. Incoherent grain boundary nucleation appears to have dominated in most previous experimental studies of the α–β–γ transformations because of the use of starting materials with small (<10–20 µm) grain sizes. In contrast, when large (0.6 mm) olivine single crystals are reacted, intracrystalline nucleation of both β and γ becomes the dominant mechanism, particularly when the P-T conditions significantly overstep the equilibrium boundary. At pressures of 18–20 GPa intracrystalline nucleation involves (i) the formation of stacking faults in the olivine, (ii) coherent nucleation of γ-lamellae on these faults and (iii) nucleation of β on γ. In other experiments, intracrystalline nucleation is also observed during the β-γ transformation. In this case coherent nucleation of γ appears to occur at the intersections of dislocations with (010) stacking faults in β, which suggests that the nucleation rate is stress dependent. Reaction rims of β/γ form at the margins of the olivine single crystals by grain boundary nucleation. Measurements of growth distance as a function of time indicate that the growth rate of these rims decreases towards zero as transformation progresses. The growth rate slows because of the decrease in the magnitude of the Gibbs free energy (stored elastic strain energy) that develops as a consequence of the large volume change of transformation. On a longer time scale, growth kinetics may be controlled by viscoelastic relaxation.

The kinetics of dehydration in Ca-montmorillonite; an in situ X-ray diffraction study

Abstract: The thermal dehydration of naturally occurring Ca-montmorillonite has been studied by in situ X-ray diffraction at temperatures between 60› 1208C. The time-temperature-dependence of the position of the basal (001) reflection reveals that interlayer water loss on isothermal dehydration occurs in two stages. After an initial rapid decrease in interlayer spacing (on shock heating to an isothermal soak temperature) the reaction proceeds towards equilibrium more slowly. Furthermore, the width of the (001) reflection changes with time, reflecting transformation-dependent changes in homogeneity perpendicular to (001) with a maximum in peak width at the point where the rate of the reaction appears to change. This suggests that, as the interlayer spacing collapses, a local change is induced in the structure, affecting the means of movement of the water from the interlayer.

Nucleation environment of diamonds from Yakutian kimberlites

Abstract: Abstract The micro-inclusions located in the genetic centre of Yakutian diamond monocrystals have been studied using optical (anomalous birefringence, photoluminescence, cathodoluminescence) and microanalytical (electron-microprobe, proton-microprobe, scanning electron microscope) methods. Most diamonds nucleated heterogeneously on mineral seeds, that lowered the energy barrier to nucleation. Nucleation of peridotitic diamonds occurred on a matrix of graphite+iron+wüstite, in an environment dominated by forsteritic olivine and Fe-Ni sulfide. Nucleation of eclogitic diamonds occurred on a matrix of sulfide ± iron in an environment dominated by Fe-sulfide and omphacite (±K-Na-Al-Si-melt). The mineral assemblages recorded in the central inclusions of Yakutian diamonds indicate that they grew in a reduced environment, with oxygen fugacity controlled by the iron-wüstite equilibrium. Nucleation of diamond occurred in the presence of a fluid, possibly a volatilerich silicate melt, highly enriched in LIL (K, Ba, Rb, Sr) and HFSE (Nb, Ti, Zr) elements. This fluid also carried immiscible Fe-Ni-sulfide melts, and possibly a carbonatitic component; the introduction of this fluid into a reduced refractory environment may have been accompanied by a thermal pulse, and may have created the conditions necessary for the nucleation and growth of diamond.

Lueshite, pyrochlore and monazite-(Ce) from apatite-dolomite carbonatite, Lesnaya Varaka complex, Kola Peninsula, Russia

Abstract: Apatite-dolomite carbonatite at Lesnaya Varaka, Kola Peninsula, Russia, hosts intricate mineral intergrowths composed of lueshite in the core and pyrochlore-group minerals in the rim. Lueshite is a primary Nb-bearing phase in the carbonatite and ranges in composition from cerian lueshite to almost pure NaNbO3. For comparison, the compositional variation of lueshite from the Kovdor and Sallanlatvi carbonatites is described. At Lesnaya Varaka, lueshite is replaced by nearly stoichiometric Na-Ca pyrochlore due to late-stage re-equilibration in the carbonatite system. X-ray powder diffraction data for both minerals are presented. Barian strontiopyrochlore, occurring as replacement mantles on Na-Ca pyrochlore, contains up to 43% Sr and 8-18% Ba at the A-site, and shows a high degree of hydration and strong ionic deficiency at the A- and Y-sites. This mineral is metamict and, upon heating, recrystallises to an aeschynite-type structure. Monazite-(Ce) found as minute crystals in fractures, represents the solid solution between monazite-(Ce) CePO4, brabantite CaTh(PO4)2 and SrTh(PO4)2. Our data indicate the high capacity of the monazite structure for Th and accompanying divalent cations at low temperatures and pressures that has a direct relevance to solving the problem of long-term conservation of radioactive wastes. Monazite-(Ce) and barian strontiopyrochlore are products of lowtemperature hydrothermal or secondary (hypergene) alteration of the primary mineral assemblage of the carbonatite.

Tungsten-bearing rutile from the Kori Kollo gold mine, Bolivia

Abstract: W-bearing rutile formed during alteration of jarosite by resurgent hydrothermal fluids in the oxide zone of the Kori Kollo gold deposit. The rutile shows sector zoning in basal sections and well developed multiple growth zones, both defined in backscatter electron images by variations in the W content. The maximum WO3 content is 5.3 wt.% and W substitutes for Ti with double substitution of Fe to maintain charge balance. The causes of multiple growth bands are considered to be changes in externally controlled variables occurring in a shallow hydrothermal system. Whereas Ti is probably leached from biotite in dacitic rocks, the W is introduced by hydrothermal fluids.

Structure and chemistry of phosphate minerals

Abstract: For complex rocks in which the structure of minerals, rather than their chemical composition, changes with progressive evolution of the system, it makes sense to try and monitor such an evolving system through the progressive change in the crystal structures of the constituent phases. In effect, the paragenetic sequences of minerals in such complex environments should be related to the crystal structures of the constituent minerals, in order to consider variations in structure topology, we need to organize crystal structures into hierarchical schemes, using the hypothesis that structures may be hierarchically ordered according to the polymerization of the coordination polyhedra with higher bondstrengths. Structural units are organized according to the mode of polymerization: unconnected polyhedra, clusters, chains, sheets and frameworks. The bond-valence structure of (OH) and (H20) shows that on one side, (OH) and H20 are strong Lewis bases; on the other side, they are weak Lewis acids. As a result, a very important role of both (OH) and (H20) is to prevent polymerization of the structural unit in specific directions. Thus, the dimensionatity of the structural unit is controlled primarily by the amount and role of hydrogen in the structure. The way in which we have formulated these ideas also allows development of a predictive framework within which specific aspects of the chemistry and structure of phosphates can be considered. This approach to mineral structure, applied via the idea of a structural unit, can play a major role in developing structural hierarchies in order to bring about some sort of order to the plethora of hydroxyhydrated-phosphate structures. Furthermore, by combining the idea of binary structural representation with bond-valence theory, we see the eventual possibility of predicting stoichiometry and structural characteristics of these minerals, particularly those in complex low-temperature hydrothermal environments.

Rare-earth element determination in minerals by electron-probe microanalysis; application of spectrum synthesis

Abstract: Electron-probe microanalysis (EPMA) is applicable to rat-e-earth elements (REE) in minerals with relatively high REE concentrations (e.g. hundreds of parts per million). However, given that each of the 14 REE has at least 12 X-ray lines in the L spectrum, finding peak-free regions for background measurement can be problematical. Also, measured peak intensities are liable to require correction for interferences. Hitherto, little attention has been paid to the optimisation of background offsets and the implications of the wide variation in REE distribution patterns in different minerals. The 'Virtual WDS' program, which enables complex multi-element spectra to be synthesised, has been used to refine the conditions used for different REE distributions. Choices include whether to use the LI31 rather than the L~I line, background offsets, and counting times for comparable relative precision. Correction factors for interferences affecting peak and background measurements have also been derived.

Mineralogy and 40Ar/39Ar geochronology of orangeites (Group II kimberlites) from the Damodar Valley, eastern India

Abstract: A suite of ultramafic-mafic alkaline igneous rocks in the Damodar Valley, eastern India, contains carbonate, phosphate and titanate minerals that are not characteristic or common in minettes or lamproites, but are typical of orangeites (Group II kimberlite) from southern Africa. Phlogopite grains from the Damodar alkaline rocks yield mean 40Ar/39Ar ages of 116.6±0.8 Ma, 113.5±0.5 Ma and 109.1±0.7 Ma (1σ errors) using laser dating techniques. These ages are similar to the Rb-Sr ages of African orangeites, which lie mostly in the range 121 to 114 Ma. Prior to this study, only one possible occurrence of orangeite (the ∼820 m.y.-old Aries pipe, Western Australia) was known outside the Kaapvaal craton and its environs. If the Damodar alkaline rocks are bona fide orangeites, it is likely that they were generated at depths of >150 km, within the stability field of diamond.

Differentiation of natrocarbonatite magma at Oldoinyo Lengai volcano, Tanzania

Abstract: Natrocarbonatite magma, erupted as lava flows in the Tanzanian volcano Oldoinyo Lengai in June and November of 1988, has evolved chemically since its formation. The June and November flows of 1988 display increasing Cl, F, Ba, K, Mg and Mn, concomitantly with Na, Ca and P depletion. Furthermore, the June magma, at the time of eruption, had higher Cl, F, Ba and K contents and lower Ca than the November magma and evolved to higher levels of Cl, F, Ba and K content and lower Ca, Na and P. The mineralogy of the lavas reflects these trends. Crystallization of fluorite and halite-sylvite solid solution, usually as a symplectic intergrowth, occurs when Cl and F concentrations reach the critical value necessary to stabilize both minerals and explains why neither occurs as a phenocryst phase. Natrocarbonatite magma has undergone considerable and rapid magmatic evolution, probably in small and separate magma chambers. Two minerals, nyerereite and gregoryite, have dominated the crystallization history of natrocarbonatite magma, and many lavas are phenocryst-rich. However, because most of the lavas are composed principally of these two minerals, crystal accumulation has not greatly changed their composition and, consequently we suggest that the bulk composition of the lavas closely approximates that of the parental magma.

Coronas, reaction rims, symplectites and emplacement depth of the Rymmen Gabbro, Transscandinavian igneous belt, southern Sweden

Abstract: Reactions between olivine and plagioclase form kelyphytic corona textures in the Rymmen gabbro, southern Sweden. The coronas surround olivine and consist of enstatite+ or -amphibole and phlogopite, combined with an outer rim of symplectic intergrowth of green spinel and amphibole. Corona reactions took place in fractures within olivine prior to the formation of chlorite, serpentine, and magnetite in the fractures at cooler conditions. Disequilibrium between olivine and plagioclase has been put forward as an explanation for the corona formation. However, inclusions of plagioclase in olivine show that there is no reaction between these minerals, since no new minerals formed between them. The conclusion drawn is that the most important factor for development of kelyphytic structures in the Rymmen gabbro is contact with a late deuteric fluid in an environment where olivine and plagioclase are close to each other. Temperature estimates for the formation of the kelyphytic coronas in the Rymmen gabbro yield consistent temperatures of around 800+ or -30 degrees C and pressures 6-8 kbar, which indicate emplacement at a depth of 20-30 km in the crust. Late deuteric fluids caused olivine replacement by intergrown orthopyroxene and magnetite, in Fe-Ti oxide-rich rocks only. In these, plagioclase is replaced by sympletctite when in contact with ilmenite and magnetite. It is noted that the symplectite minerals have low Ti contents. This suggests that the Fe-Ti oxide was a catalyst for the reaction between plagioclase and late-stage aqueous fluids rather than being a reactant, alternatively that the magnetite/ilmenite was a reactant and Ti was retained in the oxide.

Ordovician K-bentonites of Eastern North America

Abstract: whole of the orogenic belt and the structural geology of the UHPM rocks, the latter including both the nature of the outermost boundaries of the UHPM units and the deformation within them. Major and trace element geochemistry are treated by M. Zhai and B. Cong (Chap. 5) whilst S. Li deals with isotopic geochronology (Chap. 6). These chemical chapters surprisingly separate the key petrographical data by R. Zhang et al. (Chap. 4) from the key mineralogical data also by R. Zhang et al. (Chap. 7). All of the preceding data are integrated into a petrological discussion of the metamorphic evolution of the UHPM rocks by B. Cong et al. (Chap. 8) and into a geodynamical discussion by Q. Wang et al. (Chap. 9); in the latter chapter tectonic models for UHPM genesis, and especially for the difficult problem of exhumation of these rock units, are first presented in a systematic and comprehensive way and then dissected one by one. In all chapters 2 8 many similarities with UHPM rocks elsewhere in the world can be found, but although this is usually mentioned in the introductory sections, profitable comparisons are unfortunately rather rare in the detailed sections. Previously I have referred to the first three books concerned exclusively with eclogites and UHPM as 'Book 1' (ed. Smith, 1988); 'Book 2' (ed. Carswell, 1990) and 'Book 3' (eds. Wang and Coleman, 1995) (Mineral. Mag., 55, 490-2, 1991, and 61, 324-5, 1997). They were all multiauthored and international and they presented new data incorporated into reviews of older data, and I have recommended their acquisition by librararies and all research schools in metamorphism and geodynamics. I hesitate to call this new book 'Book 4' on the same level as it contains a lesser proportion of new material, it concerns only one region, comparisons with UHPM rocks elsewhere in the world are rarely detailed, and all the authors are Chinese or expatriate Chinese (but not all the different Chinese eclogite research schools are included as co-authors). Although the overall presentation resembles the three earlier books, one feels that it is written as an intemal report for a Chinese government ministry. This segregation is rubbed in by the unconventional but consistent use of capital letters for all Chinese authors and small letters for all non-Chinese authors in every bibliographic citation, whether within the text or in the final cumulative reference list!!! The book is entirely written in English whose quality is generally good, but some parts are frustrating. The Foreword is often unintelligible and it is astonishing that Kluwer agreed to publish it without a proper translation. Fortunately the Preface is intelligible and appropriate. Most of the scientific content of the book, which is indeed presented in a modem 'Westem' format with good illustrations and tables, has been published before in an article in one or other journal. A problem for researchers is that the Dabieshan-Sulu region has produced an extraordinary quantity of published papers since the discovery of UHPM in 1989, far greater than those on Norway or Italy where UHPM was discovered many years earlier. Although in part this is due to an influx of motivated foreign researchers from many 'Western' countries, it is also due to uncomfortable Chinese tendencies (a) to publish a new paper for each new data point, and (b) for different research teams to publish very similar data collected on the same locality. In this sense the book is extremely useful in that one can learn much about the important Dabieshan-Sulu region in a single volume without having to re-read for the nth time the same information in yet another article. The scientific end result of this book is about the same as that of the state of research in the other UHPM terrains in the world: that the rocks are fascinating by the peculiarity of their mineral assemblages and textures, and that although we now have good constraints on their chemistry, deformation, age and P T history, still no one really knows where they were formed nor how they got to their present locations at the Earth's surface. At the end of a millenium, this can be considered as one of the most pressing unresolved problems of our 'dynamic Earth' and amply justifies further research funding, and, from time to time, publication of a review book like this one. In conclusion, despite the few above-mentioned drawbacks, as well as the relatively high cost, the acquisition of this book is considered necessary for the specialist and useful for the generalist such that it merits the rating as 'Book 4' in the unofficial series of books devoted to HPM and/or UHPM. D.C. SM~TU

Chemistry of columbite-tantalite minerals in rare metal granitoids, eastern Desert, Egypt

Abstract: Paragenetic, textural, and chemical characteristics of columbite-tantalite minerals are examined as steps towards identifying the metallogenetic processes of their host granitoids. Columbite-tantalite-bearing granitoids of the Eastern Desert province of Egypt can be categorized into: (i) metaluminous alkali granites: (ii) peraluminous Li-albite granites: and (iii) metasomatized biotite and/or muscovite granite (i.e. apogranites). Columbite of the alkali granite is of FeNb 2 O 6 composition and associated with annite. The low F and Li contents of the associated mica precludes the important role of these volatile elements during the late stage of evolution of the alkali granites, thus delaying fractionation of Mn over Fe and Ta over Nb. Compositionally, columbite-tantalite of the Li-albite granites is constrained between MnNb 2 O 6 and MnTa 2 O 6 (the Ta/(Nb-Ta) atom . ratio ranges between 0.10 and 0.80). This low to high ratio and the association of columbite-tantalite with topaz, fluorite and lithian micas (in the series zinnwaldite-white mica) indicate a higher solubility for Ta-fluoride complex compounds and their more stabilized state at lower temperatures in Li- and F-rich sodic melts. The columbite-tantalite commonly exhibits a mottled or patchy zoned texture with the rims consistently higher in Ta than the cores, reflecting the later effect of a corrosive supercritical vapour phase. The columbites of metasomatized granites range in composition between FeNb 2 O 6 and MnNb 2 O 6 . They are characterized by high Ti and U, and low Ta contents (the Ta/(Nb+Ta) atom . ranges between 0.01 and 0.15), indicating deposition from alkaline (K (super +) , Na (super +) -rich), and relatively high-temperature interacting fluids. However, the Mn-enriched columbites are commonly encountered in the apical parts of the apogranites and formed in response to high mu KF and mu LiF required for stabilizing the associated Li-siderophyllite, or zinnwaldite. Columbites of the apogranites commonly exhibit progressive (either normal or reverse) zoning which can be attributed to the disequilibrium conditions (e.g. sudden change in the pH) between the growing crystal and the solutions.

Magma replenishment, and the significance of poikilitic textures, in the lower main zone of the western bushveld complex, south africa

Abstract: Petrographic and compositional variations in the Lower Main Zone (LMZ) of the western Bushveld Complex indicate changing regimes of magma replenishment. The lowermost unit of the LMZ, designated N-I, is an enigmatic sequence of leuconoritic cumulates, characterized primarily by up-sequence increases in both orthopyroxene Mg# and whole-rock Sr isotope initial ratio. The Sr isotope profile of N-I is ascribed to injection and progressive integration of small influxes of fresh magma with high (Main Zone-type) Sr isotope initial ratios. The basal Fe-enrichment in N-I, on the other hand, is ascribed to a separate, later mechanism involving the downward migration of late-stage Fe-rich liquids. The overlying two units, N-II and G-I, delineated chiefly in terms of basal Mg-enrichment of orthopyroxene, are ascribed to injections of fresh magma into the chamber. Poikilitic orthopyroxene grains in the basal parts of both N-II and G-I suggest entrainment and partial resorption of plagioclase grains from the semi-crystalline resident material into which the fresh magma was intruded.

Phosphoran olivine from Pine Canyon, Piute Co., Utah

Abstract: Olivines containing up to 6.1 wt.% P2O5 occur in association with apatite and an unnamed NaCaMg phosphate phase. Phosphoran olivine has been recorded only in pallasite meteorites and from one other terrestrial occurrence. The rarity of palpable phosphorus substitution for silicon may be related to the fact that in most terrestrial rocks, phosphorus is incorporated initially, and remains as, apatite; thus phosphorus excess in an undersilicated environment must be uncommon. However the highly capricious distribution of phosphorus in individual olivines from this and the meteoritic occurences suggests disequilibrium, and questions whether phosphoran olivine is ever stable relative to combinations of P-free olivine and phosphate/phosphide minerals.

A new mineral, chrisstanleyite, Ag2Pd3Se4, from Hope´s Nose, Torquay, Devon, England

Abstract: Chrisstanleyite, Ag2Pd3Se4, is a new mineral from gold-bearing carbonate veins in Middle Devonian limestones at Hope's Nose, Torquay, Devon, England. It is associated with palladian and argentian gold, fischesserite, clausthalite, eucairite, tiemannite, umangite, a Pd arsenide-antimonide (possibly mertieite II), cerussite, calcite and bromian chlorargyrite. Also present in the assemblage is a phase similar to oosterboschite, and two unknown minerals with the compositions, PdSe2 and HgPd2Se3. Chrisstanleyite occurs as composite grains of anhedral crystals ranging from a few lam to several hundred gm in size. It is opaque, has a metallic lustre and a black streak, VHNloo ranges from 371-421, mean 395 kp/mm 2 (15 indentations), roughly approximating to a Mohs hardness of 5. Dcalo = 8.308 g/cm 3 for the ideal formula with Z = 2. In plane-polarised reflected light, the mineral is very slightly pleochroic from very light buff to slightly grey-green buff, is weakly bireflectant and has no internal reflections. Bireflectance is weak to moderate (higher in oil). Anisotropy is moderate and rotation tints vary from rose-brown to grey-green to pale bluish grey to dark steel-blue. Polysynthetic twinning is characteristic of the mineral. Reflectance spectra and colour values are tabulated. Very little variation was noted in eleven electron-microprobe analyses on five grains, the mean is: Ag 25.3, Cu 0.17, Pd 37.5, Se 36.4, total 99.37 wt.%. The empirical formula (on the basis of ZM + Se = 9) is (Ag2.01Cuo.o2)z2.o3 Pd3~02Se3.95 , ideally Ag2Pd3Se 4. Chrisstanleyite is monoclinic, a 6.350(6), b 10.387(4), c 5.683(3) A, /~ 114.90(5) ~ space group P21/m (11) or P21(4). The five strongest X-ray powder-diffraction lines [d in A (1)(hkl)] are: 2.742 (100) (-121), 2.688 (80) (-221), 2.367 (50) (140), 1.956 (100) (-321,150) and 1.829 (30) (-321,042). The name is in honour of Dr Chris J. Stanley of The Natural History Museum in London. The mineral and its name have been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.

Structure and Stability of Aluminum-Silica Complexes in Neutral to Basic Solutions. Experimental Study and Molecular Orbital Calculations

Abstract: The stability of A1H3SiO 2+, the aqueous complex formed in acid solutions between AI 3+ and H4SiO4~ has been recently quantified in the temperature range 25-300~ at Psat (Pokrovski et al., 1996; Salvi et al., 1998). It was found that the stability of A1H3SiO] + strongly increases with temperature and this complex can dominate A1 speciation in strongly acidic high temperature fluids. To assess the possible effect of aluminum-silica aqueous complexing at the neutral to basic conditions typical of most crustal fluids an experimental study including potentiometric titrations, 27A1 NMR measurements, and boehmite solubility determinations was combined with molecular orbital calculations. The results reported in this sudy allow accurate determination of the thermodynamic properties of aluminumsilica complexes formed in neutral to basic solutions, thus providing new insights on A1 transport and wateraluminosilicate interactions in natural systems.

Mineral chemistry of a peralkaline combeite-lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania

Abstract: A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.

Phenocrystic fluorite in peralkaline rhyolites, Olkaria, Kenya Rift valley

Abstract: The first occurrence of phenocrystic fluorite in a peralkaline rhyolite is reported from Quaternary lavas (agpaitic index 1.11-1.37) erupted from the Greater Olkaria Volcanic Complex, Kenya. Fluorite compositions are almost stoichiometric, with 0.65-1.49 REE + Y oxides. Relative to coexisting glass (melt), the fluorites are enriched in REE, Y and Sr, are strongly depleted in all other trace elements and concentrate MREE relative to LREE and HREE. Y partitions into fluorite more strongly than the HREE. Fractionation of fluorite in its modal abundance would not significantly affect the REE abundances in the residual liquids, nor result in stronger Sr depletion. The P, T and composition conditions which stabilize fluorite in rhyolitic magmas are still obscure.

A reexamination of the turquoise group; the mineral aheylite, planerite (redefined), turquoise and coeruleolactite

Abstract: The turquoise group has the general fonnula: AO-1B6(P04)4_xCP030HMOH)s-4H20, where x = 0-2, and consists of six members: planerite, turquoise, faustite, aheylite, chalcosiderite and an unnamed Fe2+-Fe3+ analogue. The existence of 'coeruleolactite' is doubtful. Planerite is revalidated as a species and is characterized by a dominant A-site vacancy. Aheylite is established as a new member of the group, and is characterized by having Fe2+ dominant in the A-site. Chemical analyses of 15 pure samples of microcrystalline planerite, turquoise, and aheylite show that a maximum of two of the (P04) groups are protonated (P030H) in planerite. Complete solid solution exists between planerite and turquoise. Other members of the group show variable A-site vacancy as well. Most samples of 'turquoise' are cation-deficient or are planerite. Direct detennination of water indicates that there are 4 molecules of water. Planerite, ideally DAI6(P04MP030HMOHhAH20, is white, pale blue or pale green, and occurs as mamillary, botryoidal crusts as much as several mm thick; may also be massive; microcrystalline, crystals typically 2-4 micrometres, luster chalky to earthy, H. 5, somewhat brittle, no cleavage observed, splintery fracture, Dm2.68(2), Dc 2.71, not magnetic, not fluorescent, mean RI about 1.60. a 7.505(2), b 9.723(3), c 7.814(2) ft., a. 111.43°, ~ 115.56°, Y 68.69', V 464.2(1) ft.3, Z = 1. Aheylite, ideally Fe2+ AI6(P04MOHhAH20, is pale blue or green, and occurs as isolated and aggregate clumps of hemispherical or spherical, radiating to interlocked masses of crystals that average 3 micrometres in maximum dimension; porcelaneous-subvitreous luster, moderate to brittle tenacity, no cleavage observed, hackly to splintery fracture, not magnetic, not fluorescent, biax. (+), mean RI is about 1.63, DJIl2.84(2), Dc 2.90. a 7.400(1), b 9.896(1), c 7.627(1) ft., a. 110.87°, ~ 115.000, Y 69.96°, V 460.62(9) A3, Z = 1.