Pyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.

Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

Microprobe analysis of marshite and miersite from Broken Hill, Australia, demonstrate extensive solid solution between the end-members CuI and AgI, indicating the possibility of a complete solid-solution series. Unit-cell parameters increase from 6.054 Å for marshite to 6.504 Å for miersite, closely following Vegard's Law. The Cu content of iodargyrite is generally below the limit of detection, but one zoned crystal contained 0.28 wt.% Cu. Crystallization of either miersite or iodargyrite at Broken Hill appears to be dependent upon the local availability and ratio of copper, silver and iodine ions.

The occurrence of MnO exsolution in periclase host crystals from Långban, Sweden, is described. The composition of the periclase is Mg0.942 Mn0.046Zn0.012O, and it coexists with exsolved manganosite crystals with a composition of Mn0.909Mg0.082Zn0.008O. Crystals of hausmannite are also present, indicating fO2 conditions approaching the MnO–Mn3O4 equilibrium.

Activity-composition data for the system MgO–MnO is critically reviewed. The data obtained from solid electrolyte measurements are rejected because of the unrealistic calculated equilibration temperatures and the large deviations from the gas equilibration Mn-Pt activity measurements. The large errors associated with the gas equilibration measurements, especially at large XMnO, also introduces uncertainties associated with the asymmetric interaction parameters. The non-parametric ‘jack-knife’ method was used to determine the asymmetric interaction parameters and their standard deviations from the data of Wood et al. (1994) and Tsai and Muan (1992). These were calculated as WMnO = 21.2 kJ/mol, σWMnO = 2.5 kJ/mol and WMgO = 8.2 kJ/mol, σWMgO = 3.3 kJ/mol.

Using the asymmetric interaction parameters, WMnO = 19.9 kJ/mol and WMgO = 13.7 kJ/mol for the system MgO–MnO as determined by Wood et al. (1994) and the equations for two-phase equilibrium as formulated by Thompson (1967), equilibration temperatures of 334°C and 466°C were calculated. The difference between the calculated temperatures is ascribed to inaccuracies in the experimentally determined interaction parameters.

The intracrystalline cation distributions in fourteen natural Zn-aluminate spinels were determined by means of X-ray single-crystal structural refinement, supported for some samples by Mössbauer spectroscopy.

Zinc substitutes for Mg and subordinately Fe2+ and its relevant changes in content, from 0.10 to 0.96 atoms per formula unit (apfu), are not related to variations of cell parameter. The latter is determined mainly by the substitution Fe3+ ⇌ Al. In agreement with data from synthetic samples, a small but definite amount of Zn (up to 0.06 apfu) is located in the octahedral M site. Fe2+, when present, shows a preference for tetrahedral coordination.

An improved value of the tetrahedral Zn(T)-O distance (1.960 Å) was obtained, integrating the set of interatomic distances used for the determination of cation distribution in spinels.

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.

In a granulite from the Kokchetav massif, a complex mineral assemblage and intricate textures have resulted from a combination of unusual rock composition and two–stage metamorphic history. The second, contact metamorphism produced mainly cordierite and anthophyllite, reflecting a bulk composition attributed to pre–metamorphic alteration of basic igneous rock. From the first, highpressure metamorphism, garnet relics persist while another mineral has been completely pseudomorphed. The garnet is partly replaced by a symplectite of three minerals: orthopyroxene vermicules in a coarser intergrowth of cordierite and calcic plagioclase. Despite variable proportions of cordierite and plagioclase, the Al:Si ratio of the symplectite is almost constant, because the proportion of orthopyroxene is smaller where the dominant aluminous mineral is cordierite (Al:Si ≈ 0.8) than where the even more aluminous plagioclase (Al:Si ≈ 0.89) is prominent. The bulk Al:Si ratio of this symplectite, approximately 0.69, is very close to that of reactant garnet (0.66), indicating that Al and Si have been retained almost completely during the local reaction, while other elements were more mobile. In the pseudomorphs, aluminous cores (with Al:Si ratios 1.61–1.93) indicate that the mineral which has been completely replaced was probably kyanite. These cores comprise plagioclase, zoisite, corundum and spinel, and are surrounded by layers of plagioclase and cordierite. Fe, Mg, and Ca have diffused to the core, through layers with low bulk concentrations of these elements, probably by grainboundary diffusion in the solid state.

Precision serial lapping in combination with raster image acquisition is an efficient and versatile approach to 3-D microstructure reconstruction of geological materials. However, during associated sample preparation and data acquisition, geometric inaccuracies are introduced, which, if neglected, severely distort the desired 3-D models of minerals and rock fabrics. A novel method is presented to assess and correct the relative shifts and rotations between the images acquired at the individual erosion levels. The method relies on optical reflection prisms which provide a precise internal reference system that allows all images making up an erosion series to be registered on a common system of coordinates. Tested on a microanalyser with digital sample stage control, the method yielded position errors between successive images of approximately 1 µm. A thorough description of the mechanical design of the reference system and the coordinate transformation formulae are given.

Th-rich (up to 18.4 wt% ThO2) loparite occurs as an accessory phase in foyaite pegmatites at Mt. Eveslogchorr, Khibina complex, Russia. It is associated with aegirine, astrophyllite, eudialyte, lorenzenite, lamprophyllite, magnesio-arfvedsonite and gerasimovskite. Loparite crystals are zoned from niobian loparite (core) to niobian thorian and thorian niobian loparite (rim). Th-enrichment is accompanied by a decrease in Na, LREE, Sr and increase in A-site vacancies. The most Th-rich composition approaches (Na0.39LREE0.19Th0.12Ca0.05Sr0.02)Σ0.77(Ti0.76Nb0.27)Σ1.03O3. The mineral is partly or completely metamict and after annealing gives an X-ray diffraction powder pattern similar to that of synthetic NaLaTi2O6 and naturally occurring loparite of different composition. For the Th-rich rim sample, the five strongest diffraction lines (Å) are: 2.72 (100) 110, 1.575 (60) 211, 1.925 (40), 1.368 (30) 220, 1.222 (20) 310; a = 3.867(2) Å. The X-ray diffraction patterns do not exhibit peak splitting or other diffraction lines typical of low-symmetry and ordered perovskite-type structures. Composition determinations, infrared transmission spectroscopy and X-ray diffractometry show that thorian loparite is partly replaced by betafite with LREE and Th as dominant A-site cations (‘ceriobetafite’). Some loparite samples also exhibit thin replacement mantles of belyankinite with high LREE2O3 and ThO2 contents. Both ‘ceriobetafite’ and belyankinite were formed due to metasomatic alteration of loparite.

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 ΣREE + 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.

Structures of calcium-rare-earth (Ca-RE) fluorocarbonate minerals from southwest China have been investigated using selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). They are described as combinations of layers of bastnäsite-(Ce), CeFCO3 (B layers), and synchysite-(Ce), CeFCO3·CaCO3 (S layers) (Donnay and Donnay, 1953). We report here the discovery of six new, regular, mixed-layer structures in parisite-(Ce) observed using SAED and HRTEM. The symmetry, cell parameters, chemical formulae and stacking of the structural unit layers, etc., were determined for each. The regular, mixed-layer structures are formed by stacking unit layers of bastnäsite-(Ce) and synchysite-(Ce) in varying proportions along the c axis. HRTEM shows that there are different distribution modes for the Ce-F ion layers, the CO3 ion groups between the Ce-F ion layers and the CO3 ion groups between the Ce-F and Ca ion layers. The minerals may be regarded as polymorphs with the same chemical composition and the same spacing of the unit layers, but different arrangements of ion layers in the structural unit layers and different stacking sequences of the structural unit layers.

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.

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.

At any set of thermodynamic conditions a mineral will have some well defined equilibrium crystal structure. However, this structure can be locally disturbed by crystal defects, such as domain walls or solute atoms. This distorted structure will only affect a finite volume within the crystal, but the need to retain continuity within the crystal means that this volume must be non-zero. This means, for example, that the boundary between two domains will include a transition zone from one domain's crystal structure to that of the other domain. Thick twin domain walls can be studied quantitatively, by measuring the intensity of diffuse diffraction between pairs of twin-related Bragg peaks. In alkali feldspar (Or30) at room temperature, these walls are approximately 25 Å thick. Similarly, a single solute atom in a mineral will only affect a small region within a crystal. As a result, chemical mixing will only occur in a substitutional solid solution once there is significant overlap between the strain fields around individual solute atoms. This causes the ‘plateau effect’, where the properties of a phase transition are independent of composition. In alkali feldspar, this plateau extends from albite to 2% Or, which corresponds to a strain field radius of 10 Å.

These phenomena can be modelled using Ginzburg-Landau theory, which predicts that the range of these strain fields will increase as the temperature is raised to Tc. This has been confirmed by measuring the thickness of twin walls as a function of temperature.

Tsugaruite, Pb4As2S7, is a new Pb-As sulphosalt from the Yunosawa mine, Aomori Prefecture, northern Japan. X-ray studies indicated it to be orthorhombic Pnn2 or Pnnm, a 15.179(1), b 38.195(1), c 8.0745(1) Å, Z = 16 [Pb4As2S7] with a subcell with c′ = 1/2c. The six strongest diffractions in the X-ray powder pattern [d (in Å), (I), (hkl)] are: 4.47 (30) (340), 3.705 (34) (1.10.0), 3.395 (100) (450), 2.870 (34) (192), 2.819 (53) (550), and 2.739 (48) (560, 2.13.0). The average of seven electron microprobe analyses gave Pb 68.70, Tl 0.13, As 12.45, S 18.64, total 99.92 wt.%. The empirical formula calculated on the basis of total atoms = 13 is Pb3.99Tl0.01As2.00S7.00, corresponding to the ideal formula Pb4As2S7. The calculated density is 6.83 g/cm3 for the ideal formula.

Tsugaruite is opaque with a metallic lustre and lead-grey streak. No cleavage was observed. It is brittle with uneven fracture. VHN25 is 75.4–94.9 kg/mm2, mean 86.4 (n) = 4), corresponding to 2.5–3 in Mohs' hardness scale. In reflected plane-polarised light in air, it is white with a greenish tint, more greenish than jordanite. Bireflectance and pleochroism are weak, weaker than those of jordanite; and anisotropism is weak to moderate, weaker than that of jordanite. No internal reflections. The reflectance spectra are tabulated in the text.

Tsugaruite was found in a thin baryte veinlet exposed in the wall of a small open cut of the Yunosawa mine. It forms radiating groups of long tabular tapering crystals elongated along c and tabular to b, up to 0.04 mm wide and 2 mm long. The associated minerals are jordanite, which formed earlier than tsugaruite, and galena. The chemical similarity of tsugaruite and jordanite and their intergrown character indicates that earlier jordanite analyses may also include analyses of tsugarite.

The Fe 2p X-ray absorption spectra of single crystal gillespite, BaFeSi4O10, show a strong linear dichroism, i.e. a large difference in the absorption when measured with the polarization of the X-rays either parallel or perpendicular to the plane of the FeO4 group. The isotropic spectrum, obtained from measurement at the ‘magic angle’, and the polarization dependent spectra have been compared to atomic multiplet calculations and show an excellent agreement with theory. Analysis of the branching ratio, the linear dichroism, and the detailed peak structure confirms that the 5A1 level is the ground state at room temperature and pressure. The 5B2 level is sufficiently low in energy that a distortion of the electronic charge density, induced by increased pressure, may result in a 5B2 ground state.

We present a study on major and trace element geochemistry of some intermediate lithologies from the predominantly basaltic Tibbit Hill volcanic suite in the Humber Zone of the Quebec Appalachians. The intermediate rocks probably formed as lava flows in the volcanic sequence. Their presence shows that this rift-related, c. 554 Ma volcanic sequence is not bimodal (basaltic-comenditic) as previously thought, but consists of a spectrum of compositions ranging from mafic through intermediate to felsic lithologies. The entire volcanic sequence is poly-deformed and generally metamorphosed to greenschist facies conditions.

The intermediate rocks of the Tibbit Hill Formation are trachyandesitic, trachytic and comenditic in composition, and exhibit a wide range of SiO2 content (52 to 68 wt.%). Mg is highly depleted in most samples. Variations of silica versus the alkalis show that most of the samples are alkaline in nature. The rocks display a tholeiitic trend on a standard AFM diagram.

In general, the examined rocks also exhibit a wide range of Sr (15 to 174 ppm), Rb (0 to 156 ppm), Zr (155 to 899 ppm), Nb (18 to 123 ppm), and Y (18 to 94 ppm). The concentration of Hf and Ta are generally low (6.6–14.8 ppm, and 3.3–6.6 ppm, respectively), compared to those of Zr and Nb. Nevertheless, these rocks contain relatively high concentrations of the HFS elements, thus reflecting an enriched source. The suite is also relatively enriched in the rare earth elements (REE), and exhibits fractionated, subparallel REE pattems; the latter are generally uniform and conformable.

Chemical features of these volcanic rocks are typical of those of anorogenic A1 type suites, related to hotspots, mantle plumes, or continental rift zones. This is consistent with earlier interpretation of volcanism associated with an Iapetan RRR triple junction, occurring shortly before the onset of seafloor spreading. At that stage of crustal evolution, alkaline to transitional basaltic magma pierced into the crust, and experienced fractionation to produce the liquids of intermediate composition. Rare earth element geochemical modelling supports the hypothesis that the most evolved composition for which REE data are available (comendite; 67.9 wt.% SiO2) was produced by 20% fractional crystallization of the least evolved trachyandesite (56.7 wt.% SiO2) of this intermediate volcanic assemblage.

The rare mineral otavite CdCO3 was found at the abandoned Pb-Zn mine of Montevecchio (SW Sardinia, Italy). Otavite occurs in the oxidised gossan, in association with aurichalcite. SEM imaging shows that otavite is composed of lenticular hollow structures containing aggregates of tiny rhombohedral crystals. Oxidation of primary Cd-bearing sulphides (sphalerite and greenockite) releases up to 24 ppm of Cd to drainage waters of the Montevecchio area. These waters are generally saturated with respect to otavite. Therefore, this mineral might play an important role in controlling Cd mobility in the environment.

Hyalotekite, a framework silicate of composition (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2 (Si,B)28F, is found in relatively high-temperature (⩾ 500°C) Mn skarns at Långban, Sweden, and peralkaline pegmatites at Dara-i-Pioz, Tajikistan. A new paragenesis at Dara-i-Pioz is pegmatite consisting of the Ba borosilicates leucosphenite and tienshanite, as well as caesium kupletskite, aegirine, pyrochlore, microcline and quartz. Hyalotekite has been partially replaced by barylite and danburite. This hyalotekite contains 1.29–1.78 wt.% Y2O3, equivalent to 0.172–0.238 Y pfu or 8–11% Y on the Ca site; its Pb/(Pb+Ba) ratio ranges 0.36–0.44. Electron microprobe F contents of Långban and Dara-i-Pioz hyalotekite range 1.04–1.45 wt.%, consistent with full occupancy of the F site. A new refinement of the structure factor data used in the original structural determination of a Långban hyalotekite resulted in a structural formula, (Pbl.96Bal.86K0.18)Ca2(B1.76Be0.24)(Sil.56B0.44)Si8O28F, consistent with chemical data and all cations with positive-definite thermal parameters, although with a slight excess of positive charge (+57.14 as opposed to the ideal +57.00). An unusual feature of the hyalotekite framework is that 4 of 28 oxygens are non-bridging; by merging these 4 oxygens into two, the framework topology of scapolite is obtained. The triclinic symmetry of hyalotekite observed at room temperature is obtained from a hypothetical tetragonal parent structure via a sequence of displacive phase transitions. Some of these transitions are associated with cation ordering, either Pb–Ba ordering in the large cation sites, or B–Be and Si–B ordering on tetrahedral sites. Others are largely displacive but affect the coordination of the large cations (Pb, Ba, K, Ca). High-resolution electron microscopy suggests that the undulatory extinction characteristic of hyalotekite is due to a fine mosaic microstructure. This suggests that at least one of these transitions occurs in nature during cooling, and that it is first order with a large volume change. A diffuse superstructure observed by electron diffraction implies the existence of a further stage of short-range cation ordering which probably involves both (Pb,K)–Ba and (BeSi,BB)–BSi.

The thermal dehydration of naturally occurring Ca-montmorillonite has been studied by in situ X-ray diffraction at temperatures between 60–120°C. 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.

New occurrences of hibbingite, γ-Fe2(OH)3Cl, have been found associated with platinum-group minerals in the Noril'sk Complex, and with the Korshunovskoye iron ores of the southern Siberian platform. The Norils'k grains, which are up to 0. 6 mm in diameter, are associated with the platinumgroup minerals froodite, cabriite, urvantsevite and with native silver in massive pentlandite–cubanite– chalcopyrite ore. The Korshunovskoye iron ore sample in which hibbingite was found is composed of fine-grained magnetite ore associated with halite. Hibbingite, hematite and silver grains are found in cavities in halite; the reddish-brown hibbingite grains usually occur as encrustations in the cavities. The size of hibbingite and hematite grains is up to 100 µm.

Hibbingite from the Noril'sk Complex contains a significant kempite (Mn2(OH)3Cl) component; in some cases it contains over 50 mol. % Mn. These data suggest that at least a partial solid solution series exists between hibbingite and kempite. All known occurrences of hibbingite represent paragenetically late mineral assemblages. In the case of the Korshunovskoye deposits, the occurrences are associated with highly concentrated hydrothermal brines derived from the Lower Paleozoic saline sediments of the Siberian Platform cover.