The processes by which a metal–oxygen bond dissociates in aqueous complexes are discussed and the reactions related to more complicated pathways of mineral dissolution. The dissolution of oxide minerals, and in fact many other classes of surface reactions, can be viewed as a ligand-exchange reaction because the bridging oxygens that link the metal to the mineral are progressively replaced by non-bridging functional groups. These ligand-exchange reactions are accelerated by protonations, hydroxylations and ligand substitutions that modify the lability of surface oxygens, but always at specific sites. Molecular information is important because reactions at some sites retard rates while reaction at other sites enhance them. Virtually all of the important variables that affect these reaction rates are local.

The thermodynamics and kinetics of CO2 speciation in silicate melts have been studied by measuring the concentration of CO2mol and carbonate in jadeite glass annealed at 575, 450 and 400°C. Assuming that the reaction is

1

where CO2mol..Obr represents a CO2 molecule weakly bonded to a bridging oxygen in the network and CO3 represents a bridging carbonate group with no net negative charge, ΔH for the reaction is –17 (+4/–8) kJ mol−1 and ΔS is –24 (+6/–9) J K−1 mol−1. The rate of equilibration of the species was measured at each temperature and the rate constants were deduced. The temperature dependence of the rate constants was used to determine the activation energy of the forward and reverse reactions which are 68 (+3/–31) kJ mol−1 and 86 (+1/–69) kJ mol−1 respectively. The data suggest that CO2mol may be much more abundant in silicate melts than previously assumed on the basis of studies of CO2-bearing glasses. Models of solubility, diffusion, and isotope fractionation should take this into account.

Moganite, monoclinic SiO2, is a component of microcrystalline, quartz-bearing, sinters of New Zealand derived from crystallization of non-crystalline and paracrystalline opaline silicas. It occurs at levels of <13 vol.% of the SiO2 phases present in sinters between 20,000 and 200,000 y old but is generally either absent or below the level of detection in Tertiary sinters. Unambiguous identification of moganite is most readily accomplished by laser Raman spectroscopy; the technique allows individual microtextural elements of a sinter's fabric to be analysed. Conventional scanning X-ray powder diffraction procedures are limited in their ability to discern the characteristic moganite diffraction lines from the very similar quartz pattern, especially in those samples where moganite is at low concentration and/or unanticipated. However, powder diffraction, using a position-sensitive detector system, allows not only the identification of the moganite pattern in the presence of a large proportion of quartz, but also semiquantitative estimates of the different silica phases present in bulk sinter samples of ~450 mg. Moganite is part of the sinter maturation sequence. It occurs as a metastable phase that will ultimately transform to quartz, given sufficient time or a change in ambient conditions.

Subsolidus phase relations in the plagioclase system are investigated using an Ising-type model. The model treats ordering of Al,Si and Na,Ca cations in three dimensions using a version of the cluster variation method. The interactions within the T (Al,Si) and M (Na,Ca) sublattices and the interactions between the T and M sublattices are constrained by the available experimental information on orderdisorder transitions in the system. The paper suggests a thermodynamic explanation for the existence of two distinct structural series (high and low) and gives constraints on the thermodynamics of low plagioclases.

Mineralogy has many applications in the business of waste and waste disposal:

1. • precipitation of minerals and adsorption of elements and compounds to mineral surfaces can serve to immobilize pollutants or concentrate them for recycling

2. • release of elements and compounds can occur by the converse processes of dissolution and desorption

3. • minerals can act as impermeable barriers between pollutants and the environment

4. • minerals can be used as analogues to determine the long-term stability of engineered materials used for the encapsulation of waste.

The Applied Mineralogy Group of the Mineralogical Society organized a meeting ‘The mineralogy of waste and waste disposal’ held at the British Geological Survey, Keyworth, Nottingham, on 25th September 2000. The meeting attracted members of both the academic (Earth Science, Chemistry and Material Science departments) and industrial communities. A wide variety of papers was presented dealing with such diverse issues as nuclear waste disposal and the recycling of phosphorus from wastewater. Seven of the papers are collected here.

A study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock.

Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.

Infrared powder-absorption spectra of ten synthetic P21/c pyroxenes with compositions in the clinoenstatite–clinoferrosilite system have been collected at room temperature in the range 50–1500 cm−1. Variations of peak positions as a function of Fe content have been obtained for those vibrational bands whose trend could be followed across the solid solution. Local elastic strains arising from cation substitution have been quantified by means of the autocorrelation function, which gives a measure (Δcorr) proportional to the line width of the IR bands. The structure of clinoenstatite appears to be very homogeneous, whereas Fe-rich pyroxenes have high degrees of local structural hetereogeneities. A positive deviation from linearity of the Δcorr values is observed in the spectral region between 100 and 250 cm−1 and may be indicative of a positive excess enthalpy of mixing of this solid solution. At higher energies the large changes in the phonon spectra as Fe content increases are associated with rotation and deformation of the tetrahedral chains for accommodating the larger cation. The results are compared with similar studies on other chain silicates.

Land south of Lille, contaminated by heavy metals from local metal refining, has become the subject of intensive research. Topsoil from this area is used in the current work to investigate the growth behaviour of Spring wheat (Triticum aestivum L., cv. Paragon) when amended with organo-zeolitic fertilizer. Research has shown that soil substrates amended with ammoniated zeolitic tuff promote large populations of nitrifying bacteria which, as the result of ensuing enzyme reactions, produce available nitrogen together with H+ ions. It appears that the proton activity promotes cation mobilization allowing plant uptake to reach levels which, to a large extent, satisfy the nutritional requirements of the plant; only in the case of Zn is the level far exceeded. Very large differences in plant morphology occur when plants are grown in the amended substrates and the resulting development of large dense root systems, which have the ability to bind soil particles, are particularly important in their ability to reduce surface erosion and pollution from run-off.

The Eonyang amethyst deposit is thought to be spatially and temporally associated with the biotite granite of the Cretaceous Kyongsang Basin, South Korea. The euhedral quartz crystals in cavities in the aplite which intrudes biotite granite are colour-zoned from white at the base to amethystine at the top. Fluid inclusions from rock-forming quartz in granitic rocks and euhedral quartz crystals in cavities were examined. Three types of primary inclusions were observed and three isochores for inclusions representing each type are constructed to constrain the trapping conditions and fluid evolution involved during the formation of the amethyst. The intersection of the isochore representing the early fluid inclusions with solidus temperature of the host granite indicates initial quartz formation at ~600°C and 1.0–1.5 kbar. Intermediate quartz formation, associated with the high-salinity inclusions, occurred at somewhat lower temperatures (400°C) and pressures of ~1 kbar. The amethystine quartz formed from H2O–CO2–NaCl fluids at temperatures between 280 and 400°C, and pressures of ~1 kbar. Based on the texture and mineralogy of host minerals and on the fluid inclusion characteristics, the euhedral quartz began growing at near solidus conditions of the granite and the pressure did not vary significantly until the end of crystallization of amethystine quartz crystals in cavities. Early quartz in cavities formed from moderately saline fluids that either exsolved from or were in equilibrium with the granite, whereas the amethystine quartz apparently grew from fluids of at least partial sedimentary origin. The granite crystallized at considerable depth under relatively low water pressures probably in the root zones of porphyry-type systems. Hydrothermal activities, fluid compositions and erosion factors combined to provide favourable conditions for the formation of the Eonyang amethyst deposit and its presence near the Earth's surface today.

New data on the Gongo Soco palladium-oxygenated compounds are presented for the first time since the work of Johnson and Lampadius (1837). A variety of PdO species is documented, always in association with Cu, Fe and Mn, and includes Pd(OH)2 or PdO.H2O, and phases with metal excess in relation to oxygen, interpreted as metastable. Their remarkable occurrence in ‘ouro preto’ (black gold) and jacutinga (auriferous itabirite/iron ore) is the result of mertieite-II alteration. It involves leaching of Sb and As, limited removal of Pd, and incorporation of Cu, Mn and Hg into PdO species. The locus of Hg-bearing PdO species in microfractures parallel to the enclosing gold-mineralized shear band suggests at least one low-temperature hydrothermal generation. The presence of a halite crystal attached to gold testifies to the involvement of Cl-saturated hydrothermal fluids in the oxidizing environment of jacutinga formation.

Lattice defects in lawsonite have been studied by transmission electron microscopy. It is proposed that twinning and easy glide systems are 1/2[110] {110}; the easy glide planes are coincident with twin planes. This mineral displays high sensitivity to the electron beam, even at low temperatures. In situ precipitates appear as a consequence of beam irradiation. The precipitation takes places first on dislocations, then on twin boundaries and then in the matrix, causing ‘coffee-bean’ contrast features typical of precipitates. The studies were performed at low temperature (∼110 K) in order to investigate the low temperature displacive transitions from space group Cmcm to Pmcn and P21cn and elucidate their microscopic character. No characteristic microstructural texture, such as antiphase domains associated with the transition, were observed, however. This is probably due to the high mobility of protons under the electron beam. The development of regularly spaced dislocations along twin planes is hypothesized as the only evidence that a phase transition takes place at a nanoscale.

Lattice parameters of synthetic titanite powder, CaTiOSiO4, have been determined between room temperature and 1023 K. Only the e11 and e13 components contribute significantly to the strain tensor associated with the antiferroelectric-paraelectric phase transition at Tc = 487 K. A finite strain component e13 is observed in the paraelectric phase for 487 K < T < 825 K. The disappearance of this shear strain marks the isosymmetric transition near 825 K. The temperature evolution of the volume strain and of e11 is proportional to the squared order parameter observed in single-crystal diffraction experiments. The magnitude of the volume strain is sufficiently large to relate the observed near tricritical behaviour of the antiferroelectric-paraelectric phase transition to strain coupling.

Total neutron scattering measurements, analysed using a modification of the reverse Monte Carlo modelling method to account for long-range crystallographic order, have been used to describe the temperature-dependent behaviour of the structure of quartz. Two key observations are reported. First, the symmetry change associated with the displacive α–β phase transition is observed in both the long-range and short-range structural correlations. Secondly, some aspects of the structure, such as the Si–O bond length and the thermally-induced dynamic disorder, the latter of which sets in significantly below the transition, are relatively insensitive to the phase transition. These results are used to show that the α-domain model of the β-phase disorder is inappropriate and that the classical soft-mode picture of the phase transition is too simplistic. Instead, it is argued that the structural behaviour is best described in terms of its ability to respond to low-frequency, high-amplitude vibrational modes. This view is supported by additional single-crystal diffuse neutron scattering measurements.

The crystal structure of ulrichite, CaCu2+(UO2)(PO4)2·4H2O (space group P21/c, a = 12.784(3), b = 6.996(1), c = 13.007(3)Å, β = 91.92(1)°, V = 1162.7(4)Å3, Z = 4) was redetermined using X-ray diffraction data measured from a twinned crystal with Mo-Kα radiation and a CCD area detector (2510 unique reflections with Fo > 4σ(Fo), R1 = 8.8%). Ulrichite crystallizes in space group P21/c rather than C2/m reported previously. The newly determined atomic positions give reasonable coordination polyhedra. One unique Ca atom is irregularly coordinated by eight O atoms (<Ca–O> = 2.46 Å). One unique U atom shows a {2+5} coordination with characteristic bond angles and lengths (1.806(11)Å, 1.842(12)Å and five bonds between 2.252(15) and 2.441(11)Å). Furthermore, the structure contains groups in which strongly elongated CuO6 ‘octahedra’ (also describable as CuO4 squares) are corner-linked to two PO4 tetrahedra via two opposite, equatorial O atoms. Edge- and corner-sharing UO7, CaO8 and PO4 polyhedra form heteropolyhedral sheets parallel to (001) that are linked to adjacent sheets via the CuO6 ‘octahedra’ and hydrogen bonds.

Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi2O6.H2O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite.

Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 90°C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12–13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 70°C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi2O6.

Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25–70°C and with pH at each temperature. Absolute rates of dissolution ranged from 10−10 mol m−2 s−1 at pH 9.5 at 25°C to 10−7 mol m−2 s−1 a pH 12 at 70 and 90°C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (aH+)n, where k is the rate constant and n is −0.3 at 25°C, −0.4 at 50°C, −0.6 at 70°C and −0.7 at 90°C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 90°C were unsuccessful, although a limiting rate at 70°C, pH 10 was calculated to be 4 × 10−11 mol m−2 s−1, roughly 100× less than the rate of dissolution under the same conditions.

These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K+ in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids.

Trace mineral assemblages in the bornite- and chalcopyrite-rich Cu-Fe zone of the Ocna de Fier-Dognecea skarn deposits, Banat, SW Romania provide additional constraints on the genesis of this classic zoned skarn system. Observed assemblages substantiate a model, in which the Cu-Fe zone forms the proximal fluid-plume root of the system. Observed trace mineral assemblages in the magnesian forsterite-bearing skarns crystallized at ~650°C in a volatile-rich environment, evidenced by widespread phlogopite, ludwigite, valleriite and apatite. The entire assemblage thus belongs to the initial stage of skarn formation. Prolonged cooling led to sequential exsolution of trace mineral phases from bornite and chalcopyrite during the retrograde stage, although still at temperatures in excess of 500°C. Bornite is typified by the abundance of exsolved phases along cleavage planes and along crystal margins, notably chalcopyrite and pyrrhotite, but also cobalt pentlandite, carrollite, wittichenite, galena, mawsonite, silver and electrum. Chalcopyrite hosts cobalt pentlandite, carrollite, wittichenite, galena and a sequence of Se- and Te-bearing minerals (kawazulite, bohdanowiczite, hessite, volynskite), along, although not restricted to, grain margins. The assemblage bornite-chalcopyrite-magnetite, with the trace phases, cobalt pentlandite, carrollite, wittichenite and various Se- and Te-bearing minerals represents a characteristic assemblage common to a disparate range of deposits formed at temperatures in excess of 500°C in the presence of volatiles and typified by relatively low fS2 fluids.

Structural refinements of lawsonite have been obtained at pressures up to 16.5 GPa using angle-dispersive powder diffraction with synchrotron radiation on a natural sample contained in a diamond anvil cell. Lawsonite compresses smoothly and relatively isotropically up to 10 GPa. Its bulk modulus is 126.1(6) GPa (for K’ = 4), consistent with previous results. A trend of decreasing Si–O–Si angle indicates that compression is accommodated partly through the narrowing of the cavities containing Ca and H2O in the [001]ortho direction. At 10–11 GPa there is a phase transition from Cmcm to P21/m symmetry. The occurrence of a mixed-phase region, spanning >1 GPa, indicates that the transition is first order in character. The phase transition occurs through a shearing of (010)ortho sheets containing AlO6 octahedral chains in the [100]ortho direction, which causes an increase in βmono. Across the transition, the number of oxygens coordinated to Ca increases from 8 to 9, causing an increase in the average Ca–O bond length. The compressibility of P21/m lawsonite could not be determined due to solidification of the methanol/ethanol pressure-transmitting medium. On the basis of an experiment in which the P21/m lawsonite structure was heated to 200°C at 12.0 GPa, we predict a shallow positive P-T slope for the phase transition, and therefore no stability field for P21/m lawsonite in the Earth.

An investigation into the relative stabilities and equations of state of stoichiometric FeS was conducted using first-principles pseudopotential calculations. These calculations were based on density functional theory and performed using ultrasoft Vanderbilt pseudopotentials within the generalized gradient approximation. We have identified four stable polymorphs of FeS along the 0 K isotherm as a function of pressure: troilite, an orthorhombic MnP-type structure, a monoclinic structure, and a CsCl-type structure. The calculated internal energy as a function of volume for each polymorph was fitted to 4th order logarithmic and 3rd order Birch-Murnaghan equations of state, yielding values for the bulk modulus, K, and its first and second derivatives with respect to pressure, K′ and K″. These equations of state may be used to characterize models of planetary cores.

This paper provides a brief perspective on synthetic, phosphate-based waste forms for high level radioactive waste (HLW). Evidence in support of their long-term stability is then discussed by reference to the degradation of natural monazites with emphasis on the fate of released uranium, thorium and the rare earths (REE). It is apparent that the REE can be mobilized and fractionated at temperatures anticipated in a HLW repository (∼200°C). This provides an indication of the likely fate of the trivalent actinides (Am(III), Cm(III)) if incorporated in similar matrices. Thorium, though released on alteration of monazite, tends to re-concentrate locally in secondary, microcrystalline phases. In relative terms, U is readily removed from monazites. Although it can be re-concentrated in alteration products, the potential exists for substantial loss of U to groundwater. The findings of this research have important implications for the performance of radioactive waste disposal systems where there is a clear need for improved chemical data to describe the precipitation-dissolution of phosphate phases. It is concluded that monazite-like ceramics designed for the containment of HLW will retain tetravalent actinides but may release uranium in response to natural degradative processes.

Stromatolitic carbonate in moat-fill of a 1.48 Ga caldera was converted to mineralized calc-silicate skarn by interaction with magma-derived brine at Ketcherside Gap in southeastern Missouri. The skarnrecords early low fO2 conditions similar to those of reduced W and Au skarns. Initial skarn-forming conditions were: PL ≤ 221 bar (22.1 MPa), XCO2 ≤ 0.10, T = 450–;400°C, log fO2 ≈ –30, and log fS2 ≤ –13. Late skarn records combined effects of T↓, fO2↑,fS2↑, XCO2↓, and appearance of an immiscible CO2-rich vapour. The absence of a contact aureole indicates that skarn reactions were driven by advective heat transfer from an infiltrating fluid. Elsewhere in the region, hypersaline, synvolcanic fluids produced oxidized endoskarn in rhyolite. Development of carbonate-hosted, reduced skarn in caldera moat sediments is attributed to: (1) the nature of the host rock and reaction along a lengthy flow path; (2) early, but temporary, dilution of brine influx by CO2-producing reactions; and (3) cooling, possibly accompanied by increased brine influx with time.