This issue of Mineralogical Magazine – Journal of Mineral Sciences contains nine articles by distinguished mineralogists which were delivered as plenary lectures to the 18th General Meeting of the International Mineralogical Association held in Edinburgh in early September 2002. The IMA is the only truly international organization representing mineralogy, and is supported by subscriptions from 37 mineralogical societies world-wide. Its main activity is to hold a General Meeting every four years in one of the subscribing countries, and in 2002 the organization was handled jointly by a group based in Edinburgh and by the Mineralogical Society of Great Britain & Ireland. The organizers invited lecturers who we believed would provide an authoritative and exciting overview of the most vigorous areas of the science of mineralogy as we enter a new millennium. In this the lecturers have unquestionably succeeded and we are very grateful for the scholarship, time and effort that they have put into these presentations. Each article was checked by an expert, and I thank Ross Angel, Derek Bain, Bill Brown, Martin Lee, David Manning, Bryne Ngwenya and Alison Pawley for their help.

Slag dumps occur at several historical smelting sites in north Queensland, Australia. The microcrystalline slags contain primary slag phases, relict flux, ore and furnace materials and secondary weathering related minerals. Common primary slag phases are glass, Zn-rich fayalite (± Zn-rich kirschsteinite) and Zn-rich hedenbergite. Other minor minerals include wollastonite, Zn-rich melilite, Zn-rich iscorite (Fe7SiO10), magnetite as well as a number of sulphides (pyrrhotite, galena, bornite, sphalerite, wurtzite), metallic phases (Ag, Cu, Pb, Sb), alloys (Cu3Sn), and unknown metal compounds. The slag materials contain wt.% concentrations of Zn and elevated levels of Ag, As, Cd, Cu, Ni, Pb, Sb and W. Glass, hedenbergite and fayalite/kirschsteinite are the main repositories of Zn, whereas much of the Cu and Pb is hosted by glass, sulphides, Cu3Sn alloys, metallic Cu and Pb, and unknown CuSb, AsSnPb and FeAsCu compounds. The slags are undergoing contemporaneous reaction with air and rainwater. The weathering results in the release of metals and metalloids from primary slag phases, particularly from glass, and the partial immobilization of these metals in secondary soluble and insoluble minerals in the slag heaps. Zinc exhibits pronounced chemical mobility and reports together with elevated Ca and sulphate into surface seepages (up to 10.2 mg l−1 Zn at pH 6.97). The slag dumps represent long-term sources of metal pollutants, particularly of Zn, to local ground and surface waters.

The composition of magmas proposed as parental to the layered suite of the Bushveld Complex, and some models for the manner of their emplacement, are reviewed briefly. Included are some contributions published in South Africa, with which overseas readers might be less familiar. Emphasis is given to the broader features of the cumulates, and the contradictions raised by whole-rock compositional, Sr-isotopic, and trace-element data that cloud their correlation with proposed parental magmas. It is concluded that the Lower, Critical and Main Zones are the derivatives of only two primary magmatic lineages, while a third was added to residual liquids from which the layered rocks above the Pyroxenite Marker were formed. Excessive amounts of olivine and chromium in the cumulates of the Lower and Lower Critical Zones in the northern sector of the Western Limb can seemingly not be accounted for by the composition and volume of the putative magmas. This is attributed to (1) this sector being a proximal facies located close to the original feeder, and/or (2) crystal-charged magma batches, expelled from a lower magma chamber, being periodically injected into and dispersed within the liquids already in place in the Bushveld chamber. Thus, ongoing changes in the bulk composition of the liquids within the chamber would not be reflected in the rinds of earlier, chilled-facies rocks. The expulsion of significant volumes of liquid residua from the chamber during cumulate deposition is not ruled out.

Magnetite is present in most carbonatites, and in the most abundant and best-known form of carbonatite, coarse-grained intrusions, it typically falls in a narrow composition range close to Fe3O4. A fine-grained carbonatite from Zambia contains magnetites with an extraordinary array of compositions (from 18–1% TiO2, 10–2% Al2O3, and 16–4% MgO) outranging previously-reported examples. Zoning trends are from high TiO2 to high Al2O3 and MgO. No signs of exsolution are seen. Checks on similar rocks from Germany, Uganda and Tanzania reveal magnetites with comparable compositions, ranges, and zoning. Magnetites from alkaline and alkaline ultramafic silicate volcanic rocks cover only parts of this array. Magnetite analyses from some other fine-grained carbonatites, reported in the literature, fall in the same composition field, suggesting that this form of carbonatite may be distinctive. The chemistry and zoning would be consonant with rapid high-temperature crystallization in the carbonatite melts, with the lack of exsolution pointing to fast quenching: this contrasts with coarse-grained intrusive carbonatites, in which the magnetite compositions are attributed to slow cooling, with final equilibration at low temperature. In some complexes, both forms of carbonatite, with their different magnetite compositions, are represented.

The Pingtan and Tonglu igneous complexes in SE China are typical of the calc-alkaline series developed at active continental margins. These two complexes are dominated by felsic rocks, temporally and spatially associated with minor mafic rocks. Morphological and trace-element studies of zircon populations in rocks from each of these complexes show that the zircon populations may be divided into 3–4 distinct growth stages, characterized by different distributions of morphological indices (Ipr, Ipy and Iel), and different contents of the substituting elements (Hf, U, Th, Y and P). The four growth stages recognized in the zircons are believed to have formed successively in the magma chamber, during the emplacement, and in the early and later stages of magma consolidation, respectively. All four stages are recognized in the plutonic Pingtan complex, whereas the stages 3 and 4 are less developed in the volcanic/subvolcanic Tonglu complex. Based on the chemistry and morphology of the different zircon populations of the Pingtan and Tonglu complexes, it is suggested that basaltic magmas underplating at the boundary between crust and mantle caused partial melting of the mid–lower crust and produced granitoid magmas. Subsequently, mixing between magmas was important.

Trioctahedral ferromagnesium micas from the Archaean amphibolites of the Kola super-deep borehole (KSDB-3) complex have been compared with those from analogous Archaean surface rocks in an integrated study of their chemical compositions and structural states (using wet chemistry, microprobe, X-ray, Mössbauer and infrared methods). Reports on both the experimental procedure and the crystal chemistry of the trioctahedral micas are given. This has enabled the revision of the assignment of individual infrared (IR) absorption bands for the hydroxyl ion (vOH−), and demonstrated the possibility of determining the content of Mg, Fe2+, R3+; cations and vacancies in the octahedral sheet to within a few hundredths of atomic units on the basis of the infrared and Mössbauer data. Proof was found of the presence of molecular water replacing K+ in the interlayer, which results in anomalous variations in the cell parameters, as the mica structure expands in response to the ingress of H2O. Variations in the degree of non-stoichiometry of the micas is related to the presence of structural H2O and of octahedral M1 vacancies. We found that samples recovered from depth within the borehole display lower degrees of octahedral order than those found in the analogous surface rocks.

Ever since the landmark paper by England and Richardson (1977) it has been clear that metamorphic rocks are not merely passively advected towards the surface during unroofing, but respond in a dynamic and interactive way to the mechanisms and rates of exhumation. England and Richardson based their modelling upon erosion-driven exhumation dominated by fluvial processes in mid-latitude climates. Since then a variety of mechanisms has been identified, mostly emphasizing tectonic mechanisms including extensional tectonics (e.g. Platt, 1996), tectonic extrusion (Chemenda et al., 1996; Hynes et al., 1996; Thompson et al., 1997) and ‘gravity currents’ (Wernicke and Getty, 1997). It has also become apparent that exhumation itself may be strongly influenced by metamorphic processes. For example, changes in petrophysical properties such as density may dramatically affect the buoyancy of rock masses and hence alter the driving forces for exhumation (Bousquet et al., 1997). The evolution of fluids or melts may create weak zones in the crust and enhance ductile or brittle deformation and shear zone development (e.g. Austrheim, 1998).

The Main Zone of the Bushveld Complex, which is ˜3–3.5 km thick, comprises a sequence of gabbronorites with minor anorthosites and pyroxenites. The Pyroxenite Marker (PM), a thin orthopyroxenite layer occurring towards the top of the Main Zone in the eastern Bushveld, marks the change from an inverted pigeonite-bearing microgabbronorite below, to a primary orthopyroxene-bearing porphyritic gabbronorite above. In the western Bushveld the PM has only been observed in core material although its surface position can be inferred from the mineralogical and textural changes. Whole-rock geochemistry of surface and core samples from the Brits and Marikana areas, together with mineral compositional data, have been integrated with published analyses to elucidate the magmatic processes that occurred during the addition of new magma into the chamber, just below the level of the PM. Modal and major-element data show that most lithologies lie close to the plagioclase-two pyroxene cotectic. However, there are three units below the PM in which distinct modal layering is developed. Changes in geochemical trends for trace-element abundances in both whole-rock and mineral-separate data occur at ˜150 m below the PM, below a layered package, the Hexrivier Unit. However, there is a displacement of at least 40 m between the beginning of the reversals in An content in plagioclase and in Mg# in pyroxene which occur at 124 m and 80 m below the PM respectively. In addition, a gradual change occurs in the mineral parameter Mg# in opx minus An in plagioclase, with the plagioclase becoming more primitive relative to the pyroxene. This decrease and the decoupling of geochemical trends has not been noted in the Main Zone of the Bushveld Complex before. The decoupling between plagioclase and pyroxene compositional reversals is not easily explained by modal effects, an influx of phenocrysts in the new magma, or by infiltration metasomatism. Instead we propose that new magma pulses first entered the chamber some 150 m below the PM and that this magma had a composition that crystallized more primitive plagioclase but similar pyroxene to the magma residing in the chamber. The chamber was intruded by progressively more magma that had a more primitive composition particularly in terms of pyroxene composition. This model can explain the decoupling between plagioclase and pyroxene compositional trends. Continued mixing between resident magma and the new influxes occurred over an interval of ˜150–200 m. Above the PM fractional crystallization processes dominated and continued into the Upper Zone.

Sustaining Earth, in the face of both technology thrusts and population dynamics, depends on our ability to maintain a delicate balance between human-promoted planetary modification and decline thresholds for land (soils), water, atmosphere, and biological systems. Mineralogy, as much as any other single science, will be central to this process. A set of links between Earth sustainability issues and the science of mineralogy are formulated and discussed in this discourse. The strongest ties exist in the areas of mineral-water and mineral-atmosphere interactions. Minerals are also particularly important in human disease generation. In addition, due to the role of minerals as invaluable economic resources, the environmental consequences of mining also come into play. New subdisciplines have recently emerged to bring mineralogy even closer to Earth sustainability issues, particularly mineral-microbe interaction science and nanomineralogy

Post-Variscan vein-type Co-Ni-Bi ores of the Bieber deposit, Spessart mountains, Germany, which are related to the Permian Kupferschiefer, have been investigated by ore microscopy, X-ray powder diffraction and electron-probe microanalysis. The samples contain a variety of ore minerals, notably skutterudite, native bismuth, cobaltite, alloclasite, niccolite, maucherite, gersdorffite, rammelsbergite/pararammelsbergite, safflorite, loellingite and emplectite. The ores display structures indicative of multiple brecciation and complex zoned arsenide assemblages. Three sequential stages of deposition are identified, which are (1) the Cu stage, (2) the main Co-Ni-Bi stage, and (3) the late stage. The arsenide minerals, notably skutterudite, diarsenides and sulpharsenides, show a large range of compositional variation in Co-Ni-Fe space. A relatively limited number of skutterudite and diarsenide compositions lie outside the compositional fields established in the literature. Skutterudite and diarsenides are characterized by a significant substitution of As by S up to 0.44 a.p.f.u. and 0.31 a.p.f.u., respectively, which is larger than the range previously reported for these minerals. Sulpharsenide compositions can be grouped into three populations, which conform to cobaltian arsenopyrite, cobaltite and gersdorffite. They display highly variable As/S ratios between 0.95:1.00 and 1.29:0.73, consistent with experimental data. Estimates of the formation temperatures, based on the presence of dendritic native bismuth and emplectite, are in the range 100–300°C, similar to different post-Variscan mineralization styles widespread in Central Europe. Comparison of the Co-Ni-Bi vein assemblage with the framework of available paragenetic information and radiometric age data for regional mineralization events indicates an age of mineralization of ~150–160 Ma for the Bieber deposit.

Teruggite is the dominant phase in a soft, off-white, poorly-layered and weakly-cemented surface crust, 10–15 mm thick, occurring in the high-temperature El Tatio geothermal field of Chile. Other minerals present include halite, which is present throughout but also forms a thin (<0.5 mm), brittle, cratered surface to the deposit, nobleite, ulexite and opal-A, with possible traces of illite-smectite and at least one unidentified phase. With the exception of ulexite, none of the minerals associated with teruggite at El Tatio has been reported from other occurrences of this mineral, nor do they occur with nobleite in its sole other known occurrence in Death Valley. EDS and XPS analyses of the main mass of the deposit show the presence of Ca, As, B, Na, and Cl, consistent with the identified mineral assemblage, but with elevated concentrations in Ca and Cl that are presumably associated with a further phase. Little Mg is present and the El Tatio teruggite appears deficient in this element, with Ca presumably replacing Mg in the structure. Unlike earlier documented occurrences of teruggite, that at El Tatio is evaporitic, modern and surficial. It is located some 50 m from the nearest hot (~50°C) pool and there is no evidence of association with fluid discharge. As such, the deposit has presumably derived from a fluid moving in the uppermost levels of the El Tatio field; perhaps a heavily modified version of the brines found in the deep wells.

Edingtonite and yugawaralite showing sectoral textures were studied by polarized optical microscopy and X-ray analysis. In edingtonite, the m{110} sector (2Vα = 22°) is optically triclinic and the c{001} sector (2Vα = 52°) is orthorhombic. In yugawaralite, the k{011} sector is optically monoclinic, whereas the v{120} sector is triclinic. Their crystal structures were determined. The results of refinement showed that the space groups of the k{011} (Rw = 4.5%) and v{120} (Rw = 5.1%) growth sectors are monoclinic Pc and triclinic P1, respectively. In the v{120} sector, several interatomic distances, bond angles and site occupancies are different with respect to a symmetrical plane of the structure, and therefore the monoclinic c glide is extinct. Thus, the X-ray symmetry correlates with the optical one. From the relationship between the surface and internal texture, the symmetry and sector can be explained by cation (Al/Si) ordering during non-equilibrium crystal growth.

Most unroofing mechanisms invoked for the exhumation of blueschist-plus-eclogite terranes, including corner-flow and extensional collapse of the orogenic wedge, predict steep unroofing paths for the deeply-buried rocks and are applicable only to unroofing from depths within the crust. Many high-P and ultrahigh-P rocks of continental affinity are derived from greater depths than this. Their lack of warming during unroofing, together with indications that they may rest directly on less deeply buried equivalents, are suggestive of shallow unroofing paths similar to those for the subduction-channel model. They are interpreted to have been emplaced by the upward extrusion of coherent slices of continental crust, bounded below by thrust faults and above by normal faults, with unroofing paths essentially reversing the original burial paths.

Where continental crust has been subducted into the mantle, upward extrusion is probably driven largely by buoyancy forces, although examples of upward extrusion without subduction into the mantle indicate that buoyancy forces may not be essential. Two features in addition to buoyancy may promote upward extrusion. Slab breakoff may reduce the pull from the descending slab, and subduction-zone geometry may change as a continental margin is dragged into the subduction zone. Both features may promote the extrusion of continental crust at precisely the time at which it has been partially subducted.

A close spatial relationship between a lateral ramp and a lobate zone of extruded high-P rocks in the Mesoproterozoic Grenvillian orogen indicates that lateral ramps may be important in localizing extrusion. Lateral ramps disturb the two-dimensional flow, with channelling of material into the region of the lateral ramp as it is extruded. Many exhumed ultrahigh-P terrains are associated with jogs in the trends of orogenic fronts that may reflect the presence of lateral ramps at depth. Ultrahigh-P rocks may be expected to be concentrated at such jogs, and may record the channelling in their deformation history.

Aspects of the (bio)geochemical cycling of metals (including Fe, Cu, Pb, Zn, Hg, As, Sb, U, Tc, Np) at or near the Earth's surface are discussed with reference to the recent work of the authors. Key stages of the breakdown of metalliferous minerals, transport of metals as solution complexes or colloidal precipitates, and interaction of metals in solution with the surfaces of minerals are considered. Emphasis is on molecular-scale observations using techniques such as scanning probe microscopy, photoelectron and (synchrotron) X-ray spectroscopies. The importance of the biological/mineralogical interface is also emphasized with reference to the bacterial colonization of mineral surfaces and formation of biofilms, and their influence on mineral surface reactivity and flow of fluids through rocks and sediments. Also noted is the importance of relating molecular and micro-scale observations to macroscopic phenomena. Molecular-scale understanding is central to attempts to model many processes of relevance in mineral exploration and exploitation, and in the containment of hazardous wastes and remediation of polluted areas. Mineralogists have a central role to play in the relevant environmental sciences and technologies.

Sectored vesuvianite showing optically triclinic properties was studied by X-ray and P-FTIR analyses, and the origins of the internal optical texture are discussed. A monoclinic refinement (space group P2/n) suggests that site occupancies are slightly different among the Al(2) series, though the OH– dipole is randomly oriented in all sectors. A relationship between the surface and internal texture suggests that these sectoral structures were produced during crystal growth, not by phase transitions.

Spinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite.

The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.

Solution calorimetry and phase equilibrium data for the diopside–jadeite system are assessed using a combination of the cluster variation method (CVM) and Redlich-Kister (RK) polynomial expansion. The CVM is used to model part of the free energy of mixing which depends on short-range order (SRO) and long-range order (LRO) effects. The SRO/LRO independent part of the free energy is modelled using an RK polynomial. The parameters of the RK and CVM models are obtained through the fit to the experimental data. The best-fit parameters are used to calculate activity-composition relations and a temperature-composition phase diagram of the diopside–jadeite system.

Ilmenites from the least-altered rocks of the Beja-Acebuches Ophiolite Complex (SE Portugal), with low Ti values and excess Fe, despite rare optical evidence of hematite exsolution, were studied by 57Fe Mössbauer spectroscopy and X-ray diffraction. According to single-crystal XRD the sequence of alternate layers characteristic of the ideal ilmenite structure is preserved, the excess Fe being accommodated in the Ti layers. No superparamagnetic oxides were detected by 57Fe Mössbauer spectroscopy. The typical spectra of bulk αFe2O3 and of Fe3+-containing ilmenite, in the paramagnetic state above 49 K and magnetically ordered at 6 K, are observed. The average degree of oxidation of the ilmenites, estimated from the chemical analysis assuming ideally stoichiometric full cation site occupancies, is also confirmed by 57Fe Mössbauer data. Since our crystal chemistry study gave no evidence of crypto-exsolution textures within the ilmenite with the observed compositions, fast cooling from magmatic temperatures and decomposition of ilmenite in supergene conditions is suggested.

Clays are the most complicated objects in quantitative mineral analysis of sedimentary rocks. Complex quantitative analysis of clays comprises four major steps: measuring bulk quantities, quantifying the mixed layering, determining the three-dimensional organization, and measuring the particle size. Computerization has resulted in major progress in all four areas during the last decade. X-ray diffraction remains the major tool of the quantitative studies of clays, supported by Fourier Transform Infrared Spectroscopy (bulk quantities), chemical analysis (bulk quantities) and electron microscopy (particle size). This contribution reviews recent developments in the techniques used for quantifying clays and their properties, and looks at the use of these quantification techniques in K-Ar dating of geological processes.