Examples of oscillatory zoning in metamorphic minerals, imaged using a Back-Scattered Electron Detector on the SEM, are described from a series of contrasting environments. These are a prehnite vein sampled by drilling in the Mirvalles geothermal field, Costa Rica, a pyroxene vein developed in a regional metamorphic shear zone in the Yilgarn block, Western Australia, and a bedded metasomatic diopside rock from regionally metamorphosed metasediments in Connemara, Ireland. In each case the formation of oscillatory zoning can be ascribed to mineral growth under supersaturated conditions due to fluid infiltration. Oscillations can be related in the first example to periodic episodes of pressure release and boiling in the geothermal field, but in the regional metamorphic examples actualistic models are harder to define. The development of oscillatory zoning is likely to be a characteristic feature of infiltration metasomatism and can be used as a criterion in the recognition of metasomatic mineral growth in metamorphic rocks outside the vein environment.

Kornerupine-rich layers up to several centimetres thick with minor sillimanite, spinel, Fe oxide and ilmenite occur in a diopsidite in sillimanite-cordierite gneiss south of Beraketa (24°27′S, 46°48′E), southern Madagascar. Kornerupine, sillimanite, spinel and hematite grains up to 1 mm across have mutual polygonal boundaries indicating textural equilibrium at their crystallisation. Kornerupine has XMg 0.67–0.80 and 0.9 to 2.6 wt.% B2O3. Sillimanite contains up to 2.0 wt.% Fe2O3. Spinel is essentially (Mg,Fe2+) Al2O4 with an XMg range of 0.29–0.40 and exsolution lamellae of Fe oxide. Textural relations demonstrate two limited reactions, each confined to areas less than 500 µm across: (1) Kornerupine and spinel reacted along grain contacts to form very fine-grained tourmaline, corundum and chlorite. The replacing phases are symmetrically zoned with a central tourmaline and hematite, bordered by an aggregate of chlorite, tourmaline and corundum, followed outwards by a rim of chlorite against the kornerupine and spinel. (2) Within kornerupine grains, zoned, round aggregates consist of very fine-grained chlorite, tourmaline and corundum of different composition than in (1). They define the terminal reaction of kornerupine breakdown.

Geothermobarometry indicates that the early kornerupine-bearing assemblage was stable at 7.0 kbar and 700 °C. This P-T point lies close to the retrograde, nearly isothermal trajectory defined independently by nearby sapphirine-bearing assemblages. The fine-grained aggregates formed most likely during further cooling, or by increasing water fugacity.

Experiments to explore the dissolution behaviour of Pb-rich orthoclase (1% PbO) and quartz have been carried out in the presence of pH buffered and unbuffered potassium acetate and lithium acetate solutions at 150°C and 50 MPa (500 bars). In pH-unbuffered potassium acetate solutions Pb and Na solubilities (and pH) increase with increasing fluid acetate content, reflecting increased bulk dissolution of the feldspar; silica solubility decreases despite an increase in measured pH from 7.5 to 8.9. Similarly, in experiments at pH 6 using a potassium acetate pH buffer, quartz solubility decreases with increasing acetate content. The use of lithium acetate pH buffers (pH 6 at 25°C) in experiments with orthoclase plus quartz results in the precipitation of the lithium chlorite cookeite, complicating interpretation of the fluid chemistry. It is also apparent that in the presence of orthoclase plus quartz (but not albite alone) acetate decarboxylation takes place at much higher rates than expected for the experimental configuration used. The observed effects are unlikely to be due to the presence of acetate alone; the influence of species produced by acetate decay (especially carbonate) must also be considered. This study provides little support for models which call upon acetate to enhance the solubility of aluminosilicate minerals, and suggests that acetate decarboxylation in nature may limit its involvement in dissolution processes. It emphasises the potential of feldspars as sources of elements for mineralisation, such as Pb.

Chromites from two horizons of the Sukinda area (India) marked as ‘grey ore’ and ‘brown ore’ zones have been studied by 57Fe Mössbauer spectroscopy, which revealed that both chromite types are oxidised and have a type of disordered spinel structure in which octahedral sites are occupied by Fe2+ ions.

The spectra of the grey ore sample can be fitted to three doublets corresponding to Fe2+ (A), Fe3+ (A) and Fe2+ (B) sites. This sample is less oxidised than the brown ore, in which progressive oxidation in the magmatic (?) stage led to the complete conversion of Fe2+ in A sites to Fe3+. The spectra of the brown ore are characterised by two doublets correpsonding to two tetrahedral (A) sites of Fe3+ with different next-nearest neighbour configurations and a third doublet for Fe2+ at the B site. The brown ores have higher chromium and Fe3+ content and have lesser amounts of Ni and Al in comparison to the grey ores. Megascopically, the former shows larger crystal sizes. The high Fe3+ content in the brown ore suggests that this type of chromite was formed in a region of high ƒO2 in the magmatic environment. This perhaps occurred at the part of the mantle where the temperature was higher and the rate of cooling was slower than that of the grey ores which crystallised in the magmatic melt.

Inclusions occurring in leucites from the Oscar and Zirkel Mesa leucite lamproites consist of high (>10 wt.%) and low (<4 wt.%) K2O discrete glass phases which are interpreted to have resulted from silicate liquid immiscibility. Both glasses are quartz-normative and rich in Ti, Fe, Mg, Ca and Ba and poor in Al. The high-K glass is strongly peralkaline. Neither of the glasses have compositions representative of any known lamproites. These inclusions represent the first described examples of silicate-silicate immiscibility shown by glass in alkaline rocks.

Calcite carbonatite (søvite), associated apatite cumulates and contact metasomatic phlogopite-rocks in the Fen complex (S. Norway) contain coexisting apatite and phlogopite. Two distinct compositional groups are recognized: fluorapatite (XF = 0.5–0.8) coexists with hydroxy-phlogopite with XF = 0.11–0.26, whereas hydroxyapatite (XF = 0.34–0.44) coexists with low-F hydroxy-phlogopite (XF ≈ 0.03). Apatite-biotite geothermometry suggests that the minerals equilibrated at igneous temperatures (less than 1200 to ca. 625 °C), and were not significantly disturbed by late low-temperature re-equilibration. This, combined with HF-barometry based on apatite-fluid and phlogopite-fluid F-OH exchange equilibria, makes it possible to recognize two crystallization trends at different levels of hydrogen fluoride fugacity. The existence of such trends reflects internal buffering of the HF fugacity by apatite and/or phlogopite. The recorded differences in HF fugacity suggest that two or more independent or semi-independent lines of magmatic descent gave rise to calcite carbonatite magma in the Fen complex. Combined apatite-phlogopite geothermometry and hydrogen fluoride barometry is a useful tool enabling us to see through post-magmatic alteration of carbonatites, and to establish primary magmatic controls even in cases where carbonate and iron-titanium oxide minerals have re-equilibrated at much lower temperatures.

Poikilitic eudialyte outcrops in a strongly peralkaline phonolithic cumulo-dome in Jabal Al Hasawinah, Libya. Textural relationships reveal that the eudialyte has formed at a very late stage of the rock petrogenesis. Electron microprobe analyses confirm its volatile (hydrous) nature and low REE content.

The low-grade contact metamorphic decomposition of antigorite serpentine to olivine and talc has been studied using TEM. The reaction was more complex than has been assumed by previous workers in that (i) FeO played a significant role in controlling the temperature of reaction, and (ii) minor amounts of an aluminous phase (chlorite) were probably produced. The reaction occurred over a range of temperatures within which the assemblage antigorite + talc + olivine was stable and the antigorite became progressively more Mg-rich as the reaction proceeded. Oriented nucleation of talc occurred within the antigorite. However, the olivine breakdown product did not require nucleation; rather it appears to have grown on pre-existing olivine grains. Communication between the sites of reaction was probably facilitated by an ubiquitous grain-boundary fluid.

Relict clinopyroxenes from metadolerites of the Early Permian Petroi Metabasalt formation, studied by electron microprobe, show a limited compositional range near the diopside-augite boundary in the pyroxene quadrilateral. Clinopyroxene analyses from three metadolerites, grouped in approximately equal Fs contents, define an overall smooth trend between Fs10 and Fs16. This is typical of clinopyroxenes from mildly alkaline basic magmas. Pyroxene stoichiometry suggests high Fe3+ contents (0.04 to 0.20 a.f.u.), which with high Al (0.10 to 0.31 a.f.u.) and Ti (0.03 to 0.08 a.f.u.) implies that CaTiAl2O6 and CaFe3+AlSiO6 are important ‘other components’. Relative Alz in CaFe3+AlSiO6 decreases and consequently CaTiAl2O6 increases with progressive fractionation. This, with the Fe2+:Fe3+ ratios in the Petroi clinopyroxenes, suggests falling ƒO2 in the magma with fractionation. The ƒO2 controlled entry of Alz, Tiy and Nax into the clinopyroxenes and hence the Petroi clinopyroxene trend.

Group I xenoliths, orthopyroxene-rich and orthopyroxene-free, contain Cr-spinel and clinopyroxene ± phlogopite, and occur together with Group II clinopyroxenites ± Ti-spinel ± phlogopite in K-mafic pyroclastics southeast of Gees. The petrography and clinopyroxene chemistry of orthopyroxene-rich (opx-rich sub-group) Group I xenoliths is consistent with an ‘original’ harzburgitic mantle that has been transformed to lherzolite by the addition of endiopside. In harzburgites, orthopyroxenes are reacting to diopside + olivine + alkali-silicate melt, and, by inference, the orthopyroxene-free (opx-free subgroup) Group I, dunite-wehrlite series can be linked to the opx-rich sub-group via this reaction. Progressive enrichment of dunitic material in endiopside-diopside has resulted in the formation of wehrlite. Phlogopite is titaniferous and occurs as a trace mineral in opx-rich, Group I xenoliths, whereas substantial phlogopite vein-networks are confined to the opx-free sub-group (dunite-wehrlite series). Interstitial, alkali-felsic glass occurs are veins within, and as extensions of, the phlogopite networks. Clinopyroxenes in phlogopite-veined xenoliths are decreased in Mg/(Mg + FeTotal) (mg) and Cr and increased in Ti, Al and Ca, compared with clinopyroxenes in xenoliths which have trace phlogopite. It is proposed that harzburgitic and dunitic mantle has been infiltrated by a Ca- and alkalirich, hydrous silicate melt rather than an ephemeral carbonatite melt. Dunite has been transformed to phlogopite wehrlite by the invasion of a Ca-, Al-, Ti- and K-rich, hydrous silicate melt. Ca-activity was high initially in the melt and was reduced by clinopyroxene precipitation. This resulted in enhanced K-activity which led to phlogopite veining of clinopyroxene-rich mantle. Group II phlogopite clinopyroxenites contain Ti-spinel and salites that are distinct in their Ti, Al and Cr contents from endiopsides and diopsides in Group I xenoliths. It is unlikely that these Group II xenoliths represent the culmination of the infiltration processes that have transformed dunite to wehrlite, nor can they be related to the host melt. These xenoliths may have crystallised from Ca- and K-bearing, hydrous silicate melts in mantle channelways buffered by previously precipitated clinopyroxene and phlogopite. Gees lherzolites contain pyroxenes and spinel with distinctly lower Al contents than these same minerals in lherzolites described previously from other West Eifel localities, which may reflect a distinctive lithology and/or processes of modification for the Gees mantle.

Apophyllite, analcite, chabazite, prehnite and stilbite are described from pectolite-, chlorite-, calcite- and quartz-bearing veins and joint coatings in the late Carboniferous quartz dolerite of the Whin Sill of the northern Pennines. These veins represent the final hydrothermal stages in the sill's emplacement and are not related to the widespread lead-zinc-barium-fluorite mineralization of the northern Pennine orefield. Thermogravimetric studies of apophyllites from the two Whin Sill occurrences reveal that at one locality the minreal is fluorapophyllite, at the other hydroxyapophyllite.

There is an association between the development of cleavage domes, a texture reflecting the displacement of insoluble matrix grains by porphyroblasts growing under a bulk hydrostatic stress, and textural sector-zoning. This has been found in garnet, staurolite, chiastolite, pyrite and possibly emerald porphyroblasts. Sector-zoned porphyroblasts form by lineage growth normal to the crystal faces. This causes several distinctive textures (type 1 inclusions and type 2 intergrowths, inclusion bands, growth prongs), all of which are directly or indirectly related to displacement growth. Graphite or other carbonaceous material is ubiquitous in samples showing textural sector-zoning.

Dumortierite is described from several occurrences of cordierite-orthoamphibole-bearing rocks in the upper-amphibolite facies area of the Bamble Sector, south Norway. Dumortierite occurs with chlorite, muscovite and quartz in late M4 alteration zones or aggregates after M3, peak-metamorphic cordierite and garnet and early M4 vein-cordierite, and intergrown with or replacing orthoamphibole. Late M4P-T conditions, which are interpreted as conditions of dumortierite formation, are estimated from the associated late M4 kyanite-andalusite-chlorite-quartz assemblage and Mg-Fe exchange geothermometry to be 500 ± 50 °C and 3–4 kbar. Retrogression of M3 mineral assemblages is initiated by influx of fluids with XCO2 of 0.3–0.4 during early M4 followed by more water-rich fluids during late M4. Late M4 fluids may show local variations in alkalis and boron. The dumortierites are the most Mg-rich (2.23–3.42 wt. % MgO) ever reported, and contain 0.00–2.05 wt.% TiO2, 0.00–1.08 wt.% Fe2O3, 29.62–32.42 wt.% SiO2 and 55.20–59.71 wt.% Al2O3. Al is the most likely substituent for Si, which shows a minor deficiency at the tetrahedral sites in most dumortierites. The major variations in the mineral chemistry can be described by the coupled substitutions Mg + Ti = 2AlVI, 3Mg = 2AlVI and possibly Mg + H = AlVI. Favoured by low ƒO2 prevailing conditions a significant part of total iron in dumortierites at one locality is present as Fe2+ giving evidence for the Fe2+ + SiIV = AlIV + AlVI tschermakite substitution. FeMg−1 substitution is considered to be limited. Ti-rich dumortierites coexist with rutile, ilmenite or titaniferous magnetite. The development of dumortierite from orthoamphibole correlates with an observed decrease of Al, Mg and Na and increase of Si and Fe in orthoamphibole towards dumortierite.

A T.E.M. study of omphacites from the western Italian Alps (Sesia Lanzo Zone and Monviso eclogites) has revealed a bimodal size distribution of antiphase domains: (a) 250–350 Å, (b) ≥ 2500 Å. In addition observed dislocation substructures and ‘large-scale’ antiphase domains are intimately interconnected.

A model is presented that can explain modification of the antiphase domain sizes by the interplay of cooling/growth and dislocation glide and/or creep mechanisms. Subsequent coarsening of the modified antiphase domains is inferred to be the result of surface free-energy processes. The model clearly illustrates that only the ‘relatively undeformed’ areas containing the small-scale antiphase domains can be used for thermometric methods.

Ultramafic, troctolitic and gabbroic rocks at the northern end of the Belhelvie layered intrusion display progressive cryptic variation over a stratigraphic thickness of approximately 800 m in steeply-dipping cumulates, which young from W to E. This variation is shown by olivine (Fo87−77), orthopyroxene (En87−79), clinopyroxene (Ca45Mg48Fe7 to Ca44.5Mg45.5Fe10) and plagioclase (An81−75). The Belhelvie succession is believed to be equivalent to the poorly-exposed and structurally complex Insch Lower Zone. A laterally impersistent hypersthene-gabbro unit within the main sequence is re-interpreted as a downfaulted block of slightly more evolved cumulates. A repeat sequence of peridotites, troctolites and gabbros on the eastern side of the intrusion, and separated from the underlying main succession by a thin septum of country rock, is believed to represent a fresh influx of magma.

Gypsum evaporites are forming under desiccating conditions in relict cave passages in Mynydd Llangattwg, south Wales. Sulphur isotope compositions of these sulphates indicate that oxidation of diagenetic pyrite in the overlying Millstone Grit is the source of sulphate solutions, rather than dissolution of evaporitic sulphate in the limestones.

Optical microscopy of doubly polished thin sections of North Pennine sphalerite has revealed a range of previously unrecognised textures for the Alston Block mineralisation. Delicate growth zoning, interrupted by numerous solution disconformities, was seen in transmitted light. Two principal varieties of growth-banded sphalerite are recognised; the earlier (Type 1) is characterised by the development of thin opaque bands. Type 2 has colour bands between yellow and brown, correlated with iron content. In Type 1, iron levels (up to 3 wt.%) are not sufficient to account for the observed opacity. Ultra-violet and infra-red techniques failed to detect any organic inclusions. Electron microscopy revealed locally high concentrations of sub-micrometre inclusions, both beam-stable and beam-unstable, and a variety of growth-related crystal defects.

Fluid inclusion thermometry in both sphalerite varieties and the accompanying quartz gangue implies a saline mineralising fluid (20–25 wt.% equiv. NaCl) at a relatively low temperature (100° to 140°C). Tubular inclusions are conspicuous. A deformation-induced lamelliform optical anisotropy is superimposed on a growth-related grid-iron anisotropy. Growth band offset is apparent where the deformation fabric cross-cuts the growth banding. Deformation on {111} twin and slip planes was indicated by electron microscopy.

Barstowite, ideally 3PbCl2.PbCO3.H2O is a new mineral from a lead-antimony vein at the south-western end of Bounds Cliff, St Endellion, Cornwall. It occurs as very small (0.2–0.5 mm) subparallel elongate intergrown crystals which form aggregates (up to 3 mm) containing minor inclusions of galena and jamesonite. Other associated minerals are quartz, dolomite, phosgenite, cerussite, sphalerite, pyrite and chalcopyrite. Barstowite is colourless to white and is transparent with an adamantine lustre and white streak. It is brittle, has an imperfect prismatic cleavage, and an uneven fracture. Low reflecting, barstowite has a moderate bireflectance from grey to dark grey in hue and weak to moderate anisotropy. Internal reflections (colourless) are abundant. Measured reflectance values in air and oil are tabulated. Colour values relative to the CIE illuminant C for R1 and R2 in air respectively are: Y% 12.9, 13.3; λd 477, 476; Pe% 3.5, 3.7. VHN100 111 (range 108–117); calculated Mohs hardness is 3. Infrared spectra of barstowite, phosgenite and cerussite are compared.

X-ray studies show that barstowite is monoclinic with space group P21/m and a 4.218(2), b 9.180(2), c 16.673(4) Å, β 91.49(3)°. It has a cell volume of 645.38 Å3 with Z = 2. Dcalc. is 5.76 g/cm3, Dmeas. is 5.71 g/cm3. The strongest six lines of the X-ray powder pattern are [d in Å (I) (hkl)] 4.02 (10) (022); 2.296(8) (040; ; 106); 2.377(6) (007; 026); 4.16(5) (004); 2.108(4) (200); 3.79(3) (014). The name is for the late Richard William Barstow, the Cornish mineral dealer.

The Penouta deposit is associated with a small Hercynian apogranite stock that intrudes Precambrian-Cambrian gneisses of the Ollo de Sapo Formation. Tin ore occurs as disseminations of cassiterite in the apogranite and as greisenized zones and quartz veins which traverse both the alkaline leucogranite and the surrounding metamorphic country rocks.

A fluid-inclusion study, utilizing microthermometric, crushing tests and Raman spectroscopic techniques on quartz from an intragranitic vein and a greisen of the host rock, indicates that the evolution of fluids was similar in both samples and occurred in the three main stages: The first stage is characterized by complex CO2 (CO2-N2-CH4-H2S) and complex CO2 aqueous (H2O-NaCl-CO2-N2-CH4-H2S) fluids of low salinity (Tm ice > −6°C), homogenization temperatures between 250 and 410°C homogenization pressures below 900 bars, and thermobarometric trapping conditions with temperatures below 700°C and pressures below 3250 bars. These fluids were probably responsible for the greisenization of the apogranite and wall rocks, and the precipitation of cassiterite. The second stage is represented by low-salinity aqueous solutions (H2O-NaCl) with Tm ice ⩾ −4.5°C, trapped at homogenization temperatures between 110 and 300 °C and homogenization pressures below 100 bars. This stage can be correlated with kaolinization. The third stage is characterized by higher salinity aqueous fluids (Tm ice ⩾ −16.5°C) containing Na+ and other cations, trapped at homogenization temperatures between 100 and 130°C and homogenization pressures below 5 bars. These fluids can be associated with the epigenetic or supergene phases of the orebody.

A new method for measuring the crystallinity index of quartz is presented. It is based on the measurement of the intensity of the first derivative of the infrared absorption spectrum at the shoulder at c. 1145 cm−1. The results correlate well with X-ray and D.T.A. crystallinity measurements.