Monte Carlo and molecular dynamics computer simulations are now able to provide detailed information concerning the structure, dynamics, and thermodynamics of pore fluids in 2:1 clays. This article will discuss interparticle interaction potentials currently available for atomistic simulations of clay-water systems, and will describe how computational techniques can be applied to modelling of clay systems. Some recent simulation studies of 2:1 clay hydration will then be reviewed. Comparison with experimental data promotes confidence in the molecular models and simulation techniques, and points to exciting future prospects.

The crystal chemistry of a natural, gem-quality, blue-grey Zn-rich spinel crystal from Jemaa, Kaduna State, Nigeria, was studied using electron microprobe, single-crystal X-ray diffraction, optical absorption and Raman spectroscopies. The composition of the crystal approaches the gahnite endmember (ZnAl2O4), ∼94 mol.%, with the remainder being dominated by a hercynite component (FeAl2O4). The unit-cell dimension is 8.0850(3) Å and the tetrahedral and octahedral bond distances are determined as T–O 1.9485(6) Å and M–O 1.9137(3) Å. Crystal chemical analysis resulted in the empirical structural formula T(Zn0.94Fe2+0.03Al0.03)M (Al1.96Fe2+0.03Fe3+0.01)O4, which shows Zn and Al almost fully ordered in the tetrahedrally and octahedrally coordinated T and M sites, respectively. Raman spectra obtained using the excitation of the blue 473.1 nm line of a Nd:YAG laser display three of the five Raman-active modes predicted for the general oxide spinel group of minerals. These are the Eg mode at 420.6 cm–1 and the T2g modes at 510 cm–1 and 661 cm–1, due to vibrations in the AlO6 octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 2000 29000 cm–1 show a dominant absorption band at ∼5000 cm–1 which is caused by spin-allowed electronic d–d transitions in Fe2+ located at the T sites. The blue-grey hue exhibited by the sample is mainly due to spin-forbidden electronic transitions in TFe2+ and to MFe2+ ↔MFe3+ intervalence charge transfer, and the poor saturation of the colour is due to the small concentration of Fe2+ and Fe3+.

An unusually diverse array of 25 secondary Te oxysalt minerals has been documented from Otto Mountain, California, and 18 of these from the Bird Nest drift sublocality. A paragenetic sequence for these minerals is proposed, using observed overgrowth relationships plus spatial association data and data from other localities. Apart from Te and O, the components Pb, Cu and H are essential in the majority of the minerals. The atomic Cu/Te ratio decreases through the paragenetic sequence. This, and the occurrence of minerals with additional components such as Cl–, CO32–, SO42– and Fe3+ at an intermediate stage, suggests nonmonotonic evolution of the parent fluids, reflecting differing access to or spatial distribution of various components. For the minerals with known crystal structures, two alternative 'structural units' were identified, one consisting only of the Te4+ or Te6+ oxyanion, while the other also included small, strongly-bound cations such as Cu2+. The degree of polymerization for the Te oxyanion correlated with the paragenetic sequence: the monomeric tellurate anions of early minerals were replaced progressively by dimers, chains and sheet structures, which may relate to a decreasing abundance of the 'network modifying' Cu2+ cation, analogous to Bowen's discontinuous reaction series in igneous rock-forming silicates. No relationship was seen between paragenetic order and the larger type of structural unit, or structural complexity as defined by information content. This contrasts with results in the literature for evaporite sulfates and pegmatite phosphates. While structure–paragenesis relationships may be widespread, the exact nature of such relationships may be different for different chemical systems and different paragenetic environments.

The first comprehensive assessment of the dissolution kinetics of simulant Magnox–ThORP blended UK high-level waste glass, obtained by performing a range of single-pass flow-through experiments, is reported here. Inherent forward rates of glass dissolution were determined over a temperature range of 23 to 70°C and an alkaline pH range of 8.0 to 12.0. Linear regression techniques were applied to the TST kinetic rate law to obtain fundamental parameters necessary to model the dissolution kinetics of UK high-level waste glass (the activation energy (Ea), pH power law coefficient (η) and the intrinsic rate constant (k0)), which is of importance to the post-closure safety case for the geological disposal of vitreous products. The activation energies based on B release ranged from 55 ± 3 to 83 ± 9 kJ mol–1, indicating that Magnox–THORP blend glass dissolution has a surface-controlled mechanism, similar to that of other high-level waste simulant glass compositions such as the French SON68 and LAW in the US. Forward dissolution rates, based on Si, B and Na release, suggested that the dissolution mechanism under dilute conditions, and pH and temperature ranges of this study, was not sensitive to composition as defined by HLW-incorporation rate.

Coexisting scapolite, biotite and hornblende in scapolite-biotite rocks from the Tongmugou Pb-Zn deposit, Qinling, northwestern China are characterized by high levels of chlorine. Scapolite composition varies from EqAn27 to EqAn47 with 47–80 wt.% Cl. The scapolite composition is a sensitive indicator of the NaCl activity in coexisting hydrothermal fluid. Biotite contains 0.3–1.2 wt. % Cl and also has high F contents (0.2–0.7 wt.%). The hornblende is a Cl-rich hastingsite with Cl>3.5 wt.% and high (Na2O + K2O) contents (3.2–3.9 wt. %), high Xk [= K/(K + Na)] values (0.45–0.55) and high XFc [= Fe/(Fe + Mg)] values (0.76–0.79). The chlorine within these minerals is thought to be derived from evolved seawater. The scapolite-biotite rocks are products of Cl-rich alteration of volcanoclastic sedimentary rocks during submarine hydrothermal processes. Multiple-stage hydrothermal activity culminated with the circulation of a high-NaCl fluid that was also responsible for the formation of the massive sulphide deposits.

Ion probe data for zircons from tuffs within the Llfynant flags (Arenig) and the Serw Formation (lower Llanvirn) of north Wales have been revised using better statistical methods for separating detrital ages, making allowance for recently-found variability in radiogenic 206Pb/238U in the reference zircon SL13, and testing the sensitivity of the ages to the secondary ion discrimination slope. The revised ages are options of 469.2 ± 2.1 (σ) or 472.9 ± 2.9 Ma for the Llfynant flags dependent on mixture modelling, and 465.3 ± 1.4 Ma for the Serw Formation. All ages are within error of previous SHRIMP results and the Serw age now has the same numerical value as a previous MSID age for the same sample. It is shown that an MSID age of 483 ± 0.5 Ma with interpreted Pb loss for a late Tremadoc bentonite is dependent on the correction for common Pb, and that a slightly more radiogenic choice for the common Pb composition places nearly all data on Concordia. The latter would indicate that the bentonite might contain two zircon populations: inherited grains at 482 Ma and tuff magmatic grains at 473 Ma, which is more compatible with the SHRIMP Arenig result. Interpretations of other MSID zircon ages from the Ordovician are also sensitive to choice of common Pb, and raise the likelihood that many multigrain ages might be too old owing to admixture with slightly older inherited zircon. A supposed 1–2% technical bias of SHRIMP 206Pb/238U ages relative to MSID is refuted.

Lithium cannot be determined by electron microprobe, but it may be an essential component in tourmalinesupergroup minerals. Therefore, its estimation is important for structural formula calculation and nomenclature. In this paper, we present a method to estimate Li content in tourmaline from microprobe data based on a multiple linear-regression model, which is not reliant on a particular normalization scheme. The results derived from this model are reasonably accurate, particularly for low-Mg tourmalines (<2 wt.% MgO) with Li2O contents higher than ∼0.3 wt.%. Furthermore, it provides a better fitness compared with estimations of Li assuming that Li fills any cation deficiency at the Y site.

During the study of hydrothermal alteration of cores recovered Dfrom holes drilled at Wairakei during the exploration for geothermal steam, an unusual zeolite mineral was found. Optical, chemical, and X-ray data indicate that this zeolite is a new mineral, the calcium analogue of analcime. The name wairakite is given to the new mineral after the locality Wairakei in New Zealand. Wairakei is situated in the central part of the North Island, approximately 4 miles north-north-east from Lake Taupo, in an active volcanic region. The material was submitted to Dr. D. S. Coombs, Department of Geology, University of Otago, Dunedin, for an investigation of its X-ray crystallography.

The Archean rocks of the Barrier Ranges, New South Wales, show zones of progressive regional metamorphism of which the highest is characterized by orthopyroxene-bearing basic granulites. Within this zone, further increase in metamorphic grade results in a detectable change in distribution of ferrous iron and magnesium between coexisting orthopyroxene and clinopyroxene, the latter becoming relatively richer in iron. Mutual solid solution between the two pyroxene phases also appears to increase. The value of coexisting pyroxenes as indicators of metamorphic grade is thus established. There are some points of disagreement between the Broken Hill results and previously advanced theories of cation distribution in pyroxenes.

Recent advances in ion microprobe instrumentation and techniques have enabled the mapping of C isotope ratios across the whole of a polished plate of a natural diamond from Guaniamo, Venezuela. The resultant map of C isotope variation closely matches the cathodoluminescence image of the growth structure of the diamond and, therefore, indicates an extremely limited scale of diffusion of C atoms sincethetimeof diamond formation. This result is compatible with thelimite d mobility of N atoms shown by theIaAB aggregation stateof thediamond. Inclusions in thediamond aree clogitic, in common with many Guaniamo diamonds with temperatures of formation of around 1200ºC. At such temperature the IaAB aggregation state indicates a mantle residence time on the order of 1 Ga. Such temperatures of formation and mantle residence times are common to many natural diamonds; thus the extremely limited diffusion of C isotopes shown by the mapping indicates that many diamonds will retain the C isotope compositions of their initial formation.

The Vulsinian district is the largest and northernmost of the Roman Provinces. There is very little modern mineralogical data on the Italian Pliocene to Recent perpotassic alkaline volcanic rocks and this account deals with the compositions of the phenocrysts in the Vulsinian lavas.

The lavas comprise two suites: a leueite-bearing undersaturated series of leucitites, leueite tephrites, leucite phonolites, and trachytes; and a subordinate hy-normative series of mainly trachytes and latites. All lavas are porphyritic with mostly 1–15 vol. % phenoerysts. No cumulates were found. Major elements, and Cr and Ni were determined in the phenocrysts by microprobe analysis and more than 20 trace elements were determined on mineral separates by PIXE.

The undersaturated suite. Ubiquitous clinopyroxene phenoerysts belong to the Di-Hedss series and range from Di97 to Di46. Important amounts of Fe3+ are always present. In the mafic rocks the diopside is chromian, but Ti is low. AI mainly substitutes in the Z positions in all lavas. Only minor Na enrichment occurs with increasing total Fe (0–7 mole % acmite) and thus Ca ferri-Tschermak's component is important. In many of the maric lavas diopside mantles green cores of salite, which has a composition very like the salite of felsic lavas. Leucites contain 5–22 mole % orthoelase in solid solution, but show no systematic variation. Plagioclase, mostly An93-An72, occurs in the felsie lavas, and alkali feldspar only in some phonolites. They both have exceptionally high concentrations of Sr and Ba, with a maximum of 1.3 wt. % SrO and 5.6 wt. % BaO in hyalophanes. Olivine, Fo92-Fo66, occurs in some leucititic lavas in mostly accessory amounts. Phlognpite, magnetite and nepheline are accessory phases of the felsic lavas. Apatite only occur as micro-phenocrysts of the fclsic lavas. Haüyne in trace amounts is found in a few phonolites. Pargasitic amphibole microphenocrysts are found in one lava.

In most marie members diopside ±leuctie ±olivine were liquidus phases. This study does not confirm that these rocks are related by crystal fractionation. In more felsie lavas clinopyroxene (salite-ferrosalite) and leucite are joined by: plagioclase, magnetite ±phlogopite, and Ba-rich alkali feldspar ±haüyne. The felsic rocks are thought to be related by crystal fractionation.

Salitic green cores of phenocrystic pyroxene, mantled by diopside in rocks which also carry normally-zoned diopside, are relicts which provide evidence of either a relatively high PH2O, prior to the crystallization of diopside or magma mixing in the earlier life of these lavas. Pyroxene chemistry points towards low-pressure crystallization (2 kbar), generally in a dry environment.

The hy-normative suite. All lavas have phenocrysts of augite, sanidine, plagioclase, magnetite, biotite, and olivine. The pyroxene is less calcic and has less alumina, but is otherwise rather similar to the salites of the undersaturated suite. Compared to the undersaturated suite, feldspars do not have high Sr and Ba, magnetite has higher TiO2 and olivine crystallized from even the felsic lavas. The pyroxenes show the signs of low-pressure crystallization.

Apatite in most igneous intrusions has a high Cl/F ratio. However, chlor-apatite has been reported in the lower portions of the Bushveld and Stillwater Complexes. This has been used as evidence supporting the early separation of a Cl-rich discrete hydrous fluid in these intrusions. Mineralogical evidence is presented here to demonstrate that the Bushveld Complex, at least, formed from a nearly anhydrous magma, and did not release a hydrous fluid before apatite began to crystallize. It is suggested that apatite in the earliest cumulates equilibrated with trapped interstitial liquid, which converted it from the typical F-rich composition of cumulus apatite to a Cl-rich composition. This is an analogous process to that in which cumulus mafic minerals may become more Fe-rich on cooling and reaction with interstitial liquid.

This paper presents the results of microprobe investigations of the Platinum-Group Elements (PGE) of the Selukwe Subchamber, Great Dyke, Zimbabwe. The PGE are associated with base metal sulphides in the uppermost pyroxenites of the Ultramafic Sequence of the Great Dyke. The following minerals have been indentified: bismuthotellurides (moncheite, maslovite, michenerite, kotulskite and polarite); arsenides (sperrylite); and sulphides and sulpharsenides (cooperite, laurite, braggite and hollingworthite). Platinum Group Minerals (PGM) occur in three distinct textural environments: (1) at the boundary of sulphides and silicates/hydrosilicates, (2) entirely enclosed within sulphides, and (3) entirely enclosed within silicate or hydrosilicate minerals. The stratigraphic distribution, environments and textures of the PGM have important genetic implications, and cannot be explained by a single process. A multi-process model for the petrogenesis of the PGE mineralisation in terms of complexation and intermediate compound formation is proposed. The primary mineralising events were due to orthomagmatic processes, but the observed textures are the result of microscale remobilisation of PGM components by trapped interstitial fluids (bydromagmatic processes).

The [AlO4/M+]0 (where M = H, Li, Na and K) defects in α-quartz have been investigated by ab initio calculations at the density functional theory (DFT) level, using the CRYSTAL06 code, 72-atom supercells, and all-electron basis sets. Our DFT calculations yielded substantially improved results than previous cluster calculations with minimal basis sets. For example, the [AlO4/M+(a<)]0 defects with M = H, Li and Na have been shown to be more stable than their [AlO4/M+(a>)]0 structural analogues (where a> and a< denote the location of the charge-compensating ion on the long-bond and short-bond side respectively), correctly predicting the common occurrence of paramagnetic [AlO4/M+(a>)]+ centres. In addition, the [AlO4/K+]0 defects have been investigated for the first time and are shown to be stable in quartz. Moreover, our calculations confirm previous suggestions that incorporation of the [AlO4/M+]0 defects results in significant structural relaxations that extend at least to the nearest Si atoms and give Li—O and Na—O bond distances in better agreement with the experimentally obtained values. The present theoretical results on the [AlO4/M+]0 defects provide a more complete picture for the coupled Al3+—M+ substitutions and hence new insights into crystal-chemical controls on the uptake of Al in quartz.

New mineral data for kyanite-bearing eclogites supplement existing data for eclogite nodule suites from the Roberts Victor and Bellsbank kimberlite intrusions, South Africa. Calculation of equilibration temperatures using the refined KD calibration of Ellis and Green (1979) indicates that the majority of these eclogites equilibrated over a considerably narrower range (940–1185 °C) than implied by earlier estimates which did not take account of the influence of the Ca-component of garnet on KD. Most of the Roberts Victor diamondiferous eclogites appear to have equilibrated in the lower part of this range, whilst the equilibration temperatures for the Roberts Victor diamond-free eclogites extend to higher values than those from Bellsbank. The petrogenetic implications of the (calculated) temperature and pressure equilibration conditions for these eclogite suites are discussed with particular reference to possible uppermantle geotherms and available data on the graphitediamond transition boundary.

A detailed geochemical study is presented of the uppermost Critical Zone, especially of the footwall and hanging wall to the Merensky Reef, at Impala Platinum Mines in the Bushveld Complex. The approximately 100 m-thick sequence below the Merensky Reef consists of 13 distinct layers which have sharp boundaries. They are adcumulates with varying proportions of cumulus plagioclase, orthopyroxene and chromite.

Experimental studies on the composition of coexisting orthopyroxene liquid indicate that the magma which produced this sequence contained between 4 and 6% MgO. The magma from which the Merensky Reef formed was more evolved than the footwall magma.

Significant variations exist for both the En content of orthopyroxene and mg# number of whole-rock analyses in short vertical sections. Pyroxenite and norite always have higher values than anorthosite. Extremely sharp breaks in these values correlate with changes in modal proportions, and argue against both significant fractionation within the studied interval, and infiltration metasomatism. Quantitative modelling shows that the entire footwall section could have contained pyroxene with a uniform primary composition of En82, and that all the variation now observed reflects the effect of reaction with trapped magma.

Two independent methods for determining the proportion of trapped liquid are presented, based on mg# number and incompatible element abundances. Both yield a uniform proportion in all samples of approximately 10%. Immiscible sulphide liquid from the Merensky Reef can be shown to have infiltrated downwards for <5 m, despite its high density contrast with silicate magma, very low viscosity and low crystallization temperature. Residual silicate magma would have had even more restricted mobility. The migration of residual liquid or fluid through pothole structures in the floor of the Merensky Reef is not supported by the present data.

The behaviour of 25 major and trace elements, together with mineral changes, has been examined in a deep weathering profile on the Carnmenellis granite, Cornwall. The bulk chemical and mineralogical composition of the regolith shows only minor changes from the parent rock indicating a low degree of chemical weathering. However, within the regolith marked differences occur between the fine fractions from the various horizons. Chlorite and degraded muscovite characterize the fines of horizons A+B and C whereas gibbsite and kaolinite characterize horizon D. Secondary minerals becoming base deficient with depth is the reverse of normal behaviour and two possible theories are advanced to explain this anomaly. Most of the trace elements are concentrated in the < 10 µm fraction of the various horizons, in part by an association with degraded muscovite. Tin and Zr occur in resistates and Be, Sr, and Co are mainly lost during weathering.

The mineral component (at least 95 wt. %) of dental enamel is hydroxyapatite (hydroxylapatite) with multiple substitutions. The biogenic origin of enamel is reflected in the unusual ribbon-like morphology of the crystals, which are extremely elongated in the c-axis direction, and their organized arrangement within the tissue. The study of enamel dissolution has been driven by the very high prevalence of dental caries. In enamel caries, the initial demineralization results in subsurface dissolution of mineral. While the surface remains intact, reversal of the lesion by remineralization is possible. Problems of understanding the physico-chemical processes in enamel demineralization include the general problems concerning the structure and chemistry of apatites formed in aqueous media. Added to these are the general problem of dissolution in an inhomogeneous porous medium and the complication that enamel apatite has a naturally variable composition which changes during demineralization. The use of model systems in caries research is illustrated by reference to X-ray absorption studies of enamel and synthetic analogues.

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 bastnäsite. These are briefly described and microprobe analyses presented.

Two new barium-bearing minerals: gurimite, Ba3(VO4)2 (IMA2013-032) and hexacelsian, BaAl2Si2O8 (IMA2015-045) were discovered in veins of paralava cutting gehlenite-flamite hornfels located in the Gurim Anticline in the Negev Desert, Israel. Gurimite and hexacelsian occur in oval polymineralic inclusions in paralava and are associated with gehlenite, pseudowollastonite or wollastonite, rankinite, flamite, larnite, schorlomite, andradite, fluorapatite, fluorellestadite, kalsilite, cuspidine, aradite, zadovite and khesinite. Gurimite and hexacelsian form elongate crystals <10 μm thick. The minerals are colourless and transparent with a white streak and vitreous lustre, and have (0001) cleavage, respectively good in gurimite and very good in hexacelsian. Fracture is irregular. Density calculated using empirical formulas gave 5.044 g cm–3 for gurimite and 3.305 g cm–3 for hexacelsian. Mean refractive indexes, 1.945 and 1.561, respectively, were also calculated using the empirical formulas and the Gladstone-Dale relationship. The minerals are uniaxial and nonpleochroic. The following empirical crystal chemical formulae were assigned to holotype gurimite: (Ba2.794K0.092Ca0.084Na0.033Sr0.017)∑3.020(V1.8275+S0.0916+P0.0515+Al0.040Si0.005Fe0.0053+)∑2.017O8,and holotype hexacelsian: (Ba0.911K0.059Ca0.042Na0.010)∑1.022Al1.891Fe0.0723+Si2.034O8. The Raman spectrum of hexacelsian is similar to the one of the synthetic disordered β-BaAl2Si2O8. The Raman spectrum of gurimite is identical to that of synthetic Ba3(VO4)2. The electron back-scattered diffraction (EBSD) pattern of gurimite was fitted to the structure of its synthetic analogue with the cell parameters of R3m, a = 5.784(1),c = 21.132(1) Å, V = 612.2(2) Å3, Z = 3, giving a mean angular deviation = 0.43° (good fit). The Raman spectra of hexacelsian and its EBSD pattern suggest that natural hexacelsian corresponds to disordered synthetic β-hexacelsian P63/mcm, a = 5.2920(4) Å, c = 15.557(2) Å, α = β = 90°, γ = 120°. We suggest that after relatively fast crystallization of the main constituents of the paralava, gurimite, hexacelsian and also other Ba-bearing phases crystallized from residual melt enriched in incompatible elements that filled interstices between crystals of the main constituents.