Composite populations of monazite-group minerals of both metamorphic and metasomatic origin have been discovered in thin layers of granulite-facies metabasites interlayered with metapelites, located in the Val Strona di Omegna region of the Ivrea-Verbano Zone, Italy. In addition to monazite-(Ce), which is uncommonly poor in Th and is probably formed by incongruent dissolution of apatite, these populations include members of the monazite-huttonite series. The latter minerals contain between 13 and 30.1 mol.% ThSiO4 [= huttonitic monazite-(Ce)], and are known from only half a dozen other occurrences worldwide. We propose that breakdown of primary monazite-(Ce) in the metapelites during granulite-facies metamorphism mobilized Th and the REEs, which were then transported by high-grade metamorphic fluids into the metabasite layers to form the Th-rich minerals of the monazite-huttonite series.

Ag-Cu exchange equilibria between α-miargyrite AgSbS2 and pyrargyrite Ag3SbS3 and between chalcostibite CuSbS2 and both pyrargyrite and high-skinnerite Cu3SbS3 are reported for the temperature range 150–350°C. All of these features are constrained by Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) over the temperature range 120–400°C. Chalcostibite is found to take very little Ag into its structure. Utilizing Ag-Cu data from this study as well as the Ag-Cu solution models for pyrargyrite and high-skinnerite, a two-site Ag-Cu non-convergent ordered solution model for α-miargyrite () and a one-site Ag-Cu mixing model for chalcostibite ( = 25.0 kJ/gfw) are formulated. These results are constrained by a miscibility gap between α-miargyrite and chalcostibite from 225 to 325°C. The mixing model for α-miargyrite is expanded to include As for which α-miargyrite has a limited solubility along the Sb-As join with smithite AgAsS2.

Two Mg-Al spinels (FAS1 and FAS2) in high-grade hornfels from the Toal de Mason skarn (Predazzo Monzoni, NE Italy) were investigated by means of single crystal X-ray diffraction and electron microprobe analysis in order to evaluate the last intra-crystalline closure temperature.

The evidence provided by crystal chemical data from the investigated spinels, compared with that for two other spinels from a websterite dyke and a chlorite schist (TS2 and SP78a, respectively) with similar chemical characteristics, yielded a new empirical geothermometer. This allowed estimation of intra-crystalline temperatures as low as c. 400°C and indicates that the closure temperature of FAS1 and FAS2 is c. 500°C i.e. 150–200°C lower than the spinel crystallisation temperature.

In the Apuane Alps (AA) metamorphic core complex, syn-metamorphic mineral deposits are mainly restricted to extensional shear zones in the Lower Plate Palaeozoic basement. By contrast, the extension structures at upper levels, such as the detachment fault, that are typically the seat of fluid circulation and mineralization in other core complexes, are barren in the AA. Extension veins hosted by the Jurassic Carrara marbles are among the few examples of (minor) mineralization located in the upper levels of the AA core complex. Calcite–dolomite geothermometry and fluid inclusion data suggest that the mineralizing process in these veins began under pressure (P)-temperature (T) conditions close to the metamorphic peak (about 400°C, 3 kbar). Progressive cooling and mixing between metamorphic and late stage meteoric fluids were probably responsible for most of the mineral deposition. Batches of relatively saline fluids presumably resulted from interaction with evaporitic levels located along the detachment fault. In agreement with previous estimates, fluid inclusion constraints on the P—T synmetamorphic path of the AA suggest a relatively rapid cooling of the core complex as a result of uplift. However, the maximum estimated geothermal gradient (about 35°C/km) is considerably lower than in other core complexes, where large-scale hydrothermal circulation was associated with extension and uplift. Hence, in the AA, fluid circulation at shallow levels and mixing among fluids of different origin were not favoured, thus precluding the formation of mineral deposits along major extensional structures.

High resolution transmission electron microscopy studies on the lillianite group minerals from the Ivigtut cryolite deposit, Ivigtut, South Greenland revealed the existence of disordered intergrowths of lillianite/gustavite-like blocks (N − 4) and heyrovskyite-like (N = 7) structural blocks. One disorder sequence is examined in detail, which gave an average homologue number N = 4.92 corresponding to a composition of Pb3.922xBi2·xAgxS6.92 with x ≈ 1.2. An Axial Next-Nearest Neighour Ising model was used to follow the fluctuations in the average homologue number N across the crystal. This yielded compositional fluctuations of the order of 70–170 Å over a 1800 Å region of the crystal, with a 202 Å lamella of ordered vikingite. Trends in the randomness of the gustavite-vikingite intergrowth were evaluated and the dominant slab sequence was found to be 4,4,4,7 and 4,4,7,7, suggesting that some longer period homologues may be stable. A number of defects were noted in which changes in slab widths were accommodated. The origin of these partially ordered/disordered lillianite homologues is discussed.

At Mamandur, southern India, zincian spinel is associated with Zn-Pb-Cu sulphide ores, metamorphosed to the granulite facies. The Zn-spinel (XZn = 0.44–0.82) occurs in assemblages of: (a) cordierite + biotite + sillimanite + sphalerite + pyrite and/or pyrrhotite; (b) garnet + biotite + sphalerite + pyrite and/or pyrrhotite; (c) hornblende + biotite + sphalerite + pyrite and/or pyrrhotite; and (d) it also occurs in highly sheared quartz veins with sphalerite + pyrite/pyrrhotite. Two texturally and compositionally distinct modes of occurrence of Zn-spinel have been recorded from the metamorphic assemblages: one shows a polygonal, granular (equilibrium) fabric with Zn-spinel grains in textural equilibrium with cordierite in the granulite facies assemblage; the other which is involved in the formation of coronas in garnet- and hornblende-bearing assemblage of a younger paragenesis (presumably belonging to a late retrograde stage), shows distinct compositional zoning with depletion of Zn and enrichment in Fe and Mg from the rim inwards. A third mode of occurrence is as perfectly euhedral grains embedded in a quartzose matrix in quartz veins, often with sphalerite moulded over the Zn-spinel grains.

From textural evidence and consideration of stoichiometric balance, Zn-spinel occurring in association with cordierite is suggested to have formed by the prograde reactions: 0.46 biotite (Mg-rich) + 1.80 sillimanite + 3.01 quartz + 0.11 sphalerite = cordierite + 0.20 Zn-spinel + 0.80 K-feldspar + 0.06 pyrite + 4(OH,F) or 0.48 biotite (Mg-rich) + 2.08 sillimanite + 2.72 quartz + 0.26 sphalerite = cordierite + 0.50 Zn-spinel + 0.83 K-feldspar + 0.13 S2 + 4(OH,F) and, the Zn-spinels occurring in association with garnet and hornblende by the retrograde reactions: garnet + 4.53 Al2SiO5 + 4.17 sphalerite = 5.38 Zn (-Fe)-spinel + 0.22 Ca-feldspar + 0.92 pyrite + 7.06 quartz + 1.23 S2 and hornblende + 26.93 Al2SiO5 + 20.17 sphalerite + 0.88 pyrite = 25.93 Zn-spinel + 2 Ca-feldspar + 56.81 quartz + 11.32 S2 + 2(OH,F) Zn-spinel in the sheared quartz vein may be of (meta-) hydrothermal origin. Formation of sphalerite through sulphurization of gahnite is ruled out on the evidence of the perfectly euhedral nature of the spinel and the absence of any breakdown product from the spinel.

Gold of magmatic origin has been discovered in the lamproitic dyke of El Tale (Fortuna, Province of Murcia), a small outcrop of ultrapotassic rocks of tile Tertiary magmatic province of southeastern Spain. It is the first finding of gold grains in lamproite magmas from southeastern Spain, and indeed worldwide. Pure gold occurs only in the glassy groundmass as very rare spherules and globular grains (up to 5 µm). Despite the occurrence of gold grains, the bulk rock contains not more than ∼19 ppb Au.

The magmatic origin of gold is inferred from the heterogeneous distribution of the grains in the rock, the shape of the grains (which is comparable to that of gold grains crystallized experimentally from mafic liquids) and from the As-Sb-S-Cl-poor composition of the bulk rock. The Pd/Ir and Au/Pd ratios, little affected by the early segregation of olivine (∼10 vol.%), support an Au- and PGE-rich composition of the mantle source. The low f O2 (≈IW) and the complexing with F and alkalies probably favoured partitioning of Au and PGE into the lamproite magma.

Titanite occurs in the groundmass of a holocrystalline volcanic ejectum collected in the pyroclastic rocks of the Vico Volcanic complex at Tre Croci, near Viterbo, Italy. The host rock is composed of abundant K-feldspar and minor plagioclase, biotite, clinopyroxene and a feldspathoid (sodalite). Titanite is typically associated with zirconolite, biotite and Fe-oxides. It has a medium Al content (Al2O3 + Fe2O3 = 4–6 wt.%) and contains significant amounts of Zr and LREE, with a chondrite- normalised REE pattern similar to those of titanites from other alkaline rocks. Titanite has been corroded by fluids probably rich in dissolved F and P during a late alteration stage, with evidence for some remobilization and redistribution of the REE and actinide elements.

Sideronatrite [Na2Fe(SO4)2(OH)·3H2O] occurs as yellow botryoidal encrustations on low cliffs of weathered pyrite-bearing mudstones at Barton-on-Sea, Hampshire, England. Extensive areas of the cliffs, up to ∼100 m2, are coated with sideronatrite and its low solubility in cold water secures its longevity. Dry samples of sideronatrite convert readily to metasideronatrite [Na2Fe(SO4)2(OH)·H2O], the reaction to sideronatrite being reversible. Sideronatrite, it is suggested, forms as a result of weathering of pyrite that is present in the argillaceous sediments and reaction with Na from the sea-salt spray.

Detailed analysis of the crystal structures of minerals and inorganic compounds containing oxocentred [OCu4] tetrahedra reveals the following principle for relationships between oxocentred tetrahedra and tetrahedral oxyanions (Tn+O4)−(8−n) with T = S6+ or V5+, or triangular pyramidal oxyanions (SeO3)2− (which may be considered as (SeO3E)2− tetrahedra assuming the lone pair E of selenium to be a fourth ligand): (Tn+O4)−(8−n) tetrahedra are ‘attached’ to the oxocentred tetrahedra so that their grounds (O-O-O and Cu-Cu-Cu) are parallel to each other and the mutual orientation is regular (O corners are exactly under Cu corners). We propose to describe these relationships as ‘face-to-face’, meaning that the oxocentred and cation-centred tetrahedra ‘meet’ along their ‘whole’ triangular faces. Geometric characteristics of ‘face-to-face’ relationships are discussed.

Alunogen and meta-alunogen are the dominant phases present in transient sulphate efflorescences that are the latest products of the alteration of ignimbrite country rocks in the long-lived Te Kopia geothermal field. Meta-alunogen pseudomorphs alunogen and both species occur as white, fibrous, tangled masses, as prismatic, parallel growths, and as thin, platy, crystals, 8–15 μm across, that coalesce in an open cellular network. Small (<2 mm diam.) spherical aggregates of radiating, acicular halotrichite (Fe0.51Mg0.49Al2(SO4)4.22H2O), potash alum, mirabilite, melanterite and tschermigite are present locally. The cations needed to form these minerals derive from the host rocks with the exception of sulphur and ammonia that come from H2S and NH3 gases ascending with steam. The particular efflorescence assemblage reflects the prevailing conditions and ionic activities of a local micro-environment. Kaolinite formed by acid sulphate alteration is now being altered by steam to yield alunogen. In turn, alunogen can react with silica, or co-dissociate with silicic acid, to form kaolinite. The alternating dissolution and reprecipitation of kaolinite and alunogen moves aluminium in and through the surficial environment at Te Kopia.

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

The ignimbrites of the northeast Niğde area, which are subdivided into the Lower, Middle and Upper ignimbrites on the basis of their compositional and stratigraphical characteristics, display textural variations from the base to the top. A large proportion of these ignimbrites have been altered and recrystallized by the processes of compaction, welding and devitrification, and as a result of volatile movement. The alteration and recrystallization processes include four stages: hydration, glassy and spherulitic stages accompanied by vapour-phase crystallisation. In the early phases of devitrification, detailed shard shapes are easily recognisable with the scanning electron microscope, but as alteration proceeds clarity of detail disappears because compaction results in collapse of the structure of the rock. Spherulitic and vapour-phase crystallisation usually involves the growth of alkali feldspar and cristobalite. These later stages are more common in the Upper Ignimbrite than the Middle and Lower ignimbrites.

End-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials, and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

The names ‘coutinite’ [= lanthanite-(Nd)], ‘coutinhite’ [= lanthanite-(La)], and ‘neodymite' [= lanthanite-(Nd) or lanthanite-(La)], do not follow the rules of nomenclature for rare earth minerals. These names were introduced without International Mineralogical Association (IMA) approval and are now formally discarded (Nomenclature Proposal 98-B) by the Commission on New Minerals and Mineral Names (CNMMN) of the IMA.

New data on sulphur valence and magmatic oxidation state for Central Andean volcanic rocks, in combination with published data for experimental and natural samples, allow derivation of a simple relationship between magma oxidation state and sulphur speciation. For a number of highly oxidized Central Andean volcanic rocks fO2 has been calculated using magnetite-ilmenite or olivine-spinel pairs and the sulphur valence in glasses has been measured using the peak shift of S-Kα radiation relative to a pyrite standard. Previously published experimental and natural data have been incorporated with a wider range in fO2 and S valence. The variation in sulphur speciation (as S2- or SO42-) as a function of log fO2 is described by an empirical polynomial fit which reproduces the data to within ±0.5 log units and allows use of electron microprobe measurements of the S-Kα wavelength shift for estimation of magmatic oxygen fugacities. This approach is applicable for fO2 between FMQ-2 and FMQ+6, encompassing most terrestrial magmas. The method has been used to calculate the in fO2 conditions under which melt inclusions were trapped in andesitic magmas before magma mixing in two Central Andean volcanoes, and to calculate the oxygen fugacity of a slowly-cooled pyroclastic flow in which the Fe-Ti oxide phases have subsequently re-equilibrated. In combination with Fe-Ti oxide data, two distinct trends emerge for Lascar volcano. Basaltic andesite-andesitic magma chambers follow T-fO2 trends which parallel the FMQ buffer curve, indicating ferrous-ferric iron buffering of oxygen fugacity. Dacitic anhydrite-bearing magmas with admixed basaltic andesite and andesite follow trends of increasing fO2 with decreasing temperature, indicative of buffering of fO2 by SO2-H2S in a co-magmatic gas phase. This trend continues into the metamorphic aureole of the magma chamber, resulting in highly oxidized (close to magnetite-hematite) conditions.

Over the last 50 years many definitions of element substitution series have been proposed for describing natural Fe-Li micas with complex chemical compositions. In order to compare these definitions and ascertain their reliability, a new geometric frame composed of ideal mica points, mica joins (segments), substitution vectors and substitution planes is constructed to express the entire substitution system of ideal Fe-Al-Li micas. The frame is built in composition space with the coordinate system (0; Si, AlVI AlIV, Fe2+, Li, □VI, K), but has a 3-dimensional analogue, i.e. a visual image.

Using this frame, it has been proved that there are only five possible types of replacement for definitions of Fe-Li micas, and that all possible types have been suggested except the AlIV-constant. Furthermore, it has been proved that the main point in different definitions is that the replacement of Li by Fe2+ needs to be balanced with AlIV, AlVI, or □VI. In order to solve this problem, a set of formulae determining spatial relations between geometric elements in the frame is suggested. With these formulae, the abstract frame is suggested to be a datum system used to ascertain quantitatively the reliability of definition of natural Fe-Li micas.