Linda Campbell

Fractionation of the REE during hydrothermal processes: Constraints from the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China

The composition of REE minerals from the Bayan Obo Fe-REE-Nb deposit, China, varies widely with paragenesis. The properties of fluid inclusions also vary with paragenesis, indicating changing fluid conditions during the deposits formation. In early disseminated monazite and in the main stage banded ores, the chondrite normalised La/Nd ratio varies from similar to 3 to 7, whilst vein-hosted bastnasite and monazite show ratios in the range similar to 4.8 to 5.8. The La-rich portion of this trend (La/Nd > 4) is inferred to correlate with high X(CO2) in the hydrothermal fluid. Minerals with lower La/Nd ratios are inferred to have formed from dominantly aqueous solutions. This correlation is interpreted as arising from the greater retention of neutral species in low dielectric constant, CO2-rich fluids, relative to the dominantly aqueous fluids. At high T, high ligand number complexes of La are predicted to be more strongly associated than for the other REE, leading to the observed fractionation. The La/Nd ratio of later stage REE-fluorocarbonate minerals ranges from similar to 0.6 to 2. This mineralisation took place at lower T (minimum estimates of 250-150 degrees C from fluid inclusions) from dominantly aqueous fluids. Under these conditions the association constants of REE complexes typically increase with atomic number, leading to greater M- to HREE solubilities in hydrothermal fluids. Late stage alteration of all the earlier paragenetic stages by Ba-bearing fluids lead to the release of La into solution. This process was probably controlled by the relative solubilities of end-member REE minerals as well as speciation in the hydrothermal fluid. The fractionation processes inferred from Bayan Obo are relevant to many other REE-rich hydrothermal systems. In particular La-Nd fractionations are common in REE mineralised carbonatites and could be fully accounted for by the models presented here. Copyright (C) 2000 Elsevier Science Ltd.

Environmental Mineralogy: Microbial Interactions, Anthropogenic Influences, Contaminated Land and Waste Management

The past 10 years or so have seen the emergence of a discipline known as ‘Environmental Mineralogy’. This should be regarded not as a new discipline per se, but as a new application of traditional mineralogy. Mineralogists have always sought to understand the chemical and physical environment under which a particular mineral forms and to determine the arrangement of atoms within that mineral. The field of Environmental Mineralogy asks the same questions in a different context. For example, can minerals assist in the remediation of contaminated soils and waters? Which minerals can potentially be deleterious to, inter alia, buildings, ecology and human health? Which minerals are suitable as containment for waste? How does the biota interact with minerals? Environmental Mineralogy is emerging as a field that seeks to define the roles of minerals in all environmental systems, and to work towards the preservation and restoration of such systems. Environmental Mineralogy is achieving prominence because of increasing concern regarding the environments in which we live. Mineralogists have perceived a gap in our understanding of how minerals behave in the surface environment and a need for innovative,‘green’ solutions to the problems of contamination and waste. However, the emergence of Environmental Mineralogy also owes much to modern analytical technology. Many minerals in the surface environment fall within the clay-grade range and therefore, demand high-resolution systems for analysis. Similarly, trace elements are now detectable at exceptionally low concentrations in a wide variety of matrices. Further, many mineral-environment interactions need to be examined at the atomic scale for a greater understanding of the interactive processes involved. This requires the application of the latest technologies such as X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and atomic force microscopy to name but a few. The aim of this monograph is to provide an up-to-date account of the state of this diverse subject area. With chapters containing a strong review element, it is hoped that this volume will appeal to both researchers and students alike. The volume is arranged in four sections: (1) mineral-microbe interactions; (2) anthropogenic influences on mineral interactions; (3) minerals in contaminated environments; and (4) minerals and waste management. These four sections by no means give exhaustive coverage of the subject area, but communicate some of the most important developments taking place at the present time.

Soil sorption of caesium modelled by the Langmuir and Freundlich isotherm equations

Abstract The Langmuir and Freundlich adsorption isotherm equations were used to model Cs sorption in 6 Welsh soils. The pH (in water), of the soils ranged from 3.5 to 6.5, and the estimated organic matter (determined by loss-on-ignition) ranged from 4.8% to 46%. According to the Langmuir adsorption isotherm, highest sorption maxima, b , were displayed by the loams at 102–204 mg kg −1 , compared with organic soils at 37–86 mg kg −1 . Subsoil horizons (15–30 cm depth) did not always display higher sorption maxima than their topsoil (0–15 cm depth) counterparts. Where an improved fit to the Freundlich model was found over the Langmuir model, it was assumed that either concurrent desorption of other species, or bond-energy heterogeneity in the substrate, were significant for these soils.

Apatite paragenesis in the Bayan Obo REE-Nb-Fe ore deposit, inner Mongolia, China

Abstract Many studies of the Bayan Obo REE-Nb-Fe ore deposit have suggested that the REE mineralization was episodic. To investigate this, mineralized rocks from the main and east orebodies at Bayan Obo were selected for a paragenetic study of apatite. Apatite is abundant at Bayan Obo, displaying a diversity of textures and occurrences, and it is a mineral that is geochemically sensitive to REE environments, and hence, ideal for the purpose. The apatites were examined for their cathodoluminescent (CL) properties, and quantitative electron probe microanalyses were undertaken to determine their compositions. Four distinctive fluorapatite occurrences are recognized: (1) Discrete grains, with minimal trace element substitution, and displaying yellow/green CL. (2) Anhedral, REE-enriched apatites, displaying pink/cream/grey CL. (3) REE-bearing, high-Sr vein apatites with yellow/cream CL. (4) Fluorite-vein associated, large, euhedral, pure apatites, displaying cream/grey CL. Interpretation of the paragenetic sequence indicates a sedimentary host (apatite 1) for the multiple stages of ore deposition (apatites 2 and 3). The source of the hydrothermal/metasomatic ore-fluids is interpreted as carbonatitic, partly from the high Sr-Mn ratios (in apatites 2 and 3). A late event is recorded by type 4 apatites, but its relationship to the main era of REE-ore formation is not clear.

RARE EARTH CHEMISTRY OF PEROVSKITE GROUP MINERALS FROM THE GARDINER COMPLEX, EAST GREENLAND

Abstract Perovskite group minerals, general formula ABX3, from the intrusive ultramafic alkaline Gardiner Complex, East Greenland, range from almost pure CaTiO3 (perovskite, sensu stricto), to the rare earth element (REE) variety, loparite-(Ce). Chemical zonation in the perovskites (sensu lato), is described by the substitutions 2Ca2+ = (Na++ REE3+) on the A-site and 2Ti4+ = (Fe3+ + Nb5+) on the B-site. Other trace elements detected include Th, Sr, Al, Si, Zr, Ta and Sn. Excellent agreement was found between the determinations of the REE by electron microprobe and neutron activation analysis. Chondrite-normalized REE patterns display enrichment in the light rare earths for perovskite, loparite, apatite, melilite and diopside. Mean perovskite/apatite partition coefficients from four of the Gardiner rocks were calculated as La = 10.4, Ce = 13.8, Nd = 13.9, Sm= 9.9, Eu = 7.7, Gd = 5.2, Tb = 5.6, Tm = 5.5, Yb = 2.7 and Lu = 1.6, indicating that perovskite concentrates all REE to a much greater extent than apatite. Light-REE enrichment occurs in both perovskite and apatite.

EPITAXY OF RARE EARTH ELEMENT FLUORCARBONATE MINERALS FROM CARBONATITE DYKE AT BAYAN OBO, INNER MONGOLIA, CHINA

Rare earth element fluorocarbonate minerals such as bastnaesite and parisite are firstly discovered in carbonatite dykes from Bayan Obo, Inner Mongolia, China. Bastnaesite rarely occurs as homogeneously single crystal, but contains microfine phase of parisite. They form regular intergrowth along (0001) plane net, which is identified as epitaxy. This eptactic texture results from the variation of chemical composition of crystallizing agents of those minerals that crystallize directly from carbonatite magma.

Determination of zeolite-group mineral compositions by electron probe microanalysis

Abstract A new protocol for the quantitative determination of zeolite-group mineral compositions by electron probe microanalysis (wavelength-dispersive spectrometry) under ambient conditions, is presented. The method overcomes the most serious challenges for this mineral group, including new confidence in the fundamentally important Si-Al ratio. Development tests were undertaken on a set of natural zeolite candidate reference samples, representing the compositional extremes ofNa,K, Cs,Mg, Ca, Sr and Ba zeolites, to demonstrate and assess the extent of beam interaction effects on each oxide component for each mineral. These tests highlight the variability and impact of component mobility due to beam interaction, and show that it can be minimized with recommended operating conditions of 15 kV, 2 nA, a defocused, 20 μm spot size, and element prioritizing with the spectrometer configuration. The protocol represents a pragmatic solution that works, but provides scope for additional optimization where required. Vital to the determination of high-quality results is the attention to careful preparations and the employment of strict criteria for data reduction and quality control, including the monitoring and removal of non-zeolitic contaminants from the data (mainly Fe and clay phases). Essential quality criteria include the zeolite-specific parameters of R value (Si/(Si + Al + Fe3+), the ‘E%’ charge-balance calculation, and the weight percent of non-hydrous total oxides. When these criteria are applied in conjunction with the recommended analytical operating conditions, excellent inter-batch reproducibility is demonstrated. Application of the method to zeolites with complex solid-solution compositions is effective, enabling more precise geochemical discrimination for occurrence-composition studies. Phase validation for the reference setwas conducted satisfactorily with the use ofX-ray diffraction and laser-ablation inductively-coupled plasma mass spectroscopy.

Species sensitivity of zeolite minerals for uptake of mercury solutes

Abstract The uptake of inorganic Hg2+ and organometallic CH3Hg+ from aqueous solutions by 11 different natural zeolites has been investigated using a batch distribution coefficient (Kd) method and supported by a preliminary voltammetric study. The effect of mercury concentration on the Kd response is shown over an environmentally appropriate concentration range of 0.1-5 ppm inorganic and organometallic Hg using a batch factor of 100 ml g-1 and 20 h equilibration. Analcime and a Na-chabazite displayed the greatest methylmercury uptakes (Kd values at 1.5 ppm of 4023 and 3456, respectively), with mordenite as the smallest at 578. All uptake responses were greater for methylmercury than for the inorganic mercuric nitrate solutions, suggesting a distinctive sensitivity of zeolites to reaction with different types of solute species. It is likely that this sensitivity is attributable to the precise nature of the resultant Hg-zeolite bonds. Additionally, both the Si-Al ratio and the Na content of the initial natural zeolite samples are shown to influence the Kd responses, with positive correlations between Kd and Na content for all zeolites excluding mordenite.

The masquerade of alkaline–carbonatitic tuffs by zeolites: a new global pathfinder hypothesis

Rapid and progressive reaction of alkaline–carbonatitic tuffs with magmatic and crustal fluids disguises their initial character and origin. This is collectively indicated from (a) the extensive literature on zeolite formation from volcanic glass precursors and alkaline fluids, (b) mineralogical characteristics of specific zeolite species, (c) a comparative review of global distributions of alkaline–carbonatite suites and of zeolite minerals, and (d) new trace element data from zeolite samples. A unifying conceptual model based on tectonic and geological settings, hydrological regime and mineralogy is presented that helps to explain the global distributions and current understanding of occurrences. The model will assist in resource exploration by contributing deeper understanding of the economically important bedded zeolite deposits and further, serve as a guide to the discovery of new alkaline–carbonatitic suites, potentially of economic significance (metallic ores and rare earth elements). It follows that future testing of the hypothesis will impact on models of natural carbon cycling as volcanic contributions of CO2 are reviewed.

Discussion on Le Bas et al. (1997) 'Oxygen, carbon and strontium isotope study of the carbonatitic dolomite host of the Bayan Obo Fe-Nb-REE deposit, Inner Mongolia, China: Mineralogical Magazine, 61, 531-41

Abstract Some important and interesting data on the O, C and Sr isotopic compositions of rocks associated with the Bayan Obo Fe-Nb-REE deposit have been reported by Le Bas et al. (1997). However, a number of points relating to the interpretation of these data, and also to the terminology adopted, are worthy of further discussion. The essence of this discussion is that some of the geochemical and isotopic signatures found at Bayan Obo are not exclusively consistent with an igneous origin. A replacement hypothesis involving deep-sourced fluids could also explain the pattern of data, if minor carbonatitic intrusions into a sedimentary sequence are considered.