Weissite, Cu2–xTe (x ≈ 0.21), a very rare copper telluride, occurs in a sample from the Gambatesa mine, Val Graveglia, Liguria, Italy, where it occurs as purplish black anhedral grains up to 0.1 mm in length and shows a black streak. No cleavage is observed and the Vickers hardness (VHN100) is 142 kg/mm2. Weissite is dark bluish black, weakly pleochroic, and moderately anisotropic in bluish tints. Reflectance percentages in air for Rmin and Rmax are 37.0, 38.4 (471.1 nm), 33.2, 34.2 (548.3 nm), 31.2, 32.1 (586.6 nm), and 28.6, 31.0 (652.3 nm), respectively.

Weissite is trigonal and belongs to the space group P3m1 with the following unit-cell parameters: a = 8.3124(7) Å, c = 21.546(1) Å, V = 1289.3(2) Å3, and Z = 24. Electron microprobe analyses gave the chemical formula (Cu1.62Ag0.04Au0.04Fe0.04Sb0.04)Σ=1.78(Te0.96S0.02Se0.02). The crystal structure has been solved and refined to R = 1.95%. It consists of Cu and Te polyhedra forming complex crystal-chemical environments as is typical of many intermetallic compounds. The exceedingly short bond distances observed among the metals are discussed in relation to other copper tellurides and pure metals.

In specimens of Mn skarn from the type locality of Långban, hyalotekite, (Ba,Pb,K)4Ca2−(Si,B,Be)12O28F, occurs in a matrix consisting mostly of aegirine (⩽22 mol.% CaMnSi2O6), andradite (⩽27 mol.% Mn3Fe2Si3O12), hematite, pectolite, quartz, calcite, baryte, barylite, and hedyphane. Melanotekite, plumbian taramellite, ferrian K-feldspar (to 6.5 wt.% Fe2O3), rhodonite, a talc-like mineral, apophyllite, and several Pb-As-Sb-O minerals are found in trace amounts. In a single specimen of reedmergnerite-microcline pegmatite from Dara-i-Pioz, hyalotekite occurs in close association with microcline. Other accessory minerals are albite, aegirine, pyrochlore, eudialyte, and polylithionite. The optical constants for hyalotekite from Långban and Dara-i-Pioz are, respectively, α = 1.656, 1.646, β = 1.659–1.660, 1.649, γ = 1.670–1.671, 1.659 (all ± 0.002), 2Vγ (mcas) = 57.2–60.5 ± 0.5°, 57.0 ± 1.1°(λ = 589 nm). Cell parameters of the Dara-i-Pioz hyalotekite for a body-centred triclinic cell are: a = 11.284(2), b = 10.930(1), c = 10.272(8) Å, α = 90.35(2)°, β = 90.11(3)°, γ = 89.98(1)°. Electron and ion microprobe data show that Långban hyalotekite is heterogeneous even within a given sample; the most important substitutions are Pb = Ba, K and coupled B = Si and B = Be. Minor constituents include Mn in the Långban hyalotekite and Na in the Dara-i-Pioz hyalotekite, which also differs in its significantly higher Ba/Pb ratio. Conditions suggested for hyalotekite formation at Långban and Dara-i-Pioz are P ⩽ 4 kbar, T ⩾ 500°C, silica saturation, peralkalinity, and relatively high oxygen fugacities and low sulphur fugacities. These temperatures must have been sufficiently high to allow for miscibility of Pb with Ba (and K) despite the lone pair of electrons of Pb2+.

An unusual form of sector zoning has been recorded in some Lewisian zircons. The sectors lie in a plane cut perpendicular to the c-axis and are observed as variations in the intensity of cathodoluminescence, and in trace-element concentrations. Sectors radiating from the centre of growth out to the {100} faces exhibit low-intensity cathodoluminescence and are enriched in Y and depleted in Hf and Zr. Those radiating to the {110} faces are brightly luminescent, slightly enriched in Hf and depleted in Y. X-ray diffraction data, and comparison with previous studies of zircons, suggest that the sectors vary slightly in crystallographic orientation, such that the boundaries between sectors form discontinuities in the structure.

A garnetite that occurs locally in phyllitic schists exposed by the Santorini caldera consists mainly of euhedral garnet crystals showing alternating isotropic and birefringent lamellae. X-ray diffraction revealed two garnets with unit cell dimensions a of 12·052±0·002 and 11·984±0·005 Å, corresponding respectively to almost pure andradite and a granditic phase. Mean compositions determined by microprobe analysis are An97Sp1Alm2 for the isotropic and about An63Gr33Sp1Alm3 for the birefringent layers.

The garnets are deduced to have been produced mainly by thermal metamorphism of the schists. Although cyclic variations in the composition of circulating fluids could have brought about the zoning, this may just as well be the result of temperature fluctuations, individual garnet compositions being indicative of the respective formation temperature.

The new mineral, zvěstovite-(Zn), ideally Ag6(Ag4Zn2)As4S13, was found in quartz–baryte gangue at the mine dump of the abandoned small deposit of Zvěstov, central Bohemia, Czech Republic. Zvěstovite-(Zn) is associated with tennantite-(Zn), tetrahedrite-(Zn), argentotennantite-(Zn), acanthite and supergene azurite and malachite. The new mineral occurs as rare relic anhedral grains rimmed by acanthite, up to 100 μm in size. Zvěstovite-(Zn) is grey, Mohs hardness is ca. 3½–4, in agreement with other members of the tetrahedrite group; the calculated density is 5.16 g.cm–3. In reflected light, zvěstovite-(Zn) is grey with a greenish tint, without bireflectance, pleochroism or anisotropy. Deep red internal reflections are ubiquitous. Reflectance values of zvěstovite-(Zn) in air (R%) are: 28.5 at 470 nm, 26.9 at 546 nm, 25.5 at 589 nm and 23.8 at 650 nm. The empirical formula for zvěstovite-(Zn), based on electron-microprobe analyses (n = 4), is Ag6.27[(Ag3.90Cu0.38)Σ4.28(Zn1.60Fe0.09Cd0.03)Σ1.72]Σ6.00(As2.26Sb1.48)Σ3.74S12.50. The ideal formula is Ag6(Ag4Zn2)As4S13, which requires (in wt.%) Ag 56.01, Zn 6.79, As 15.56 and S 21.64, total 100.00. Zvěstovite-(Zn) is cubic, I$\bar{4}$3m, with unit-cell parameters: a = 10.850(2) Å, V = 1277.3(8) Å3 and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern [d, Å (I) (hkl)] are: 3.1321(100) (222), 2.7125(21) (400), 1.9809(11) (521), 1.9180(31) (440) and 1.6357(15) (622). According to the single-crystal X-ray diffraction data (Robs = 0.051), the crystal structure of zvěstovite-(Zn) agrees with the general features of the members of the tetrahedrite group. Zvěstovite-(Zn) is named after its type locality, Zvěstov; the suffix indicates the dominant divalent C-constituent, according to the approved nomenclature of the tetrahedrite group. It is the As-isotype of rozhdestvenskayaite-(Zn). The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2020-061).

A new cobalt-iron mineral of composition close to CoFe has been found associated with awaruite (Ni3Fe) in the Red Hills serpentinites of the Wairau Valley, South Island, New Zealand. The name wairauite, after this locality, is proposed. Electron-probe analysis of the small grains so far discovered has shown that the nickel content is very low (0·5 ± 0·1 wt. %). Wairauite is optically very similar to awaruite but a tendency to form euhedral grains (octahedron and cube) and a slightly lower refiectivity are an aid to identification. No X-ray data are available at present, but it is suggested that the structure may correspond to that of the ordered CsCl type of the synthetic alloy of the same composition.

Ejected blocks of piutonic igneous facies have long been known in the trachytic and basaltic tufts of Green Mountain and Middleton Peak on Ascension Island, and a petrographic account of them has been recorded by Professor R. A. Daly in his memoir on the geology of Ascension Island.

Dokuchaevite, ideally Cu8O2(VO4)3Cl3, was found in the Yadovitaya fumarole of the Second scoria cone of the North Breach of the Great Tolbachik Fissure Eruption (1975–1976), Tolbachik volcano, Kamchatka Peninsula, Russia. Dokuchaevite occurs on the crusts of various copper sulfate exhalative minerals (such as kamchatkite and euchlorine) as individual prismatic crystals. Dokuchaevite is triclinic, P$\bar{1}$, a = 6.332(3), b = 8.204(4), c = 15.562(8) Å, α = 90.498(8), β = 97.173(7), γ = 90.896(13)°, V = 801.9(7) Å3 and R1 = 0.057. The eight strongest lines of the X-ray powder diffraction pattern are (d, Å (I)(hkl): (15.4396)(18)(00$\bar{1}$), (7.2762)(27)(0$\bar{1}$1), (5.5957)(43)(012), (4.8571)(33)($\bar{1}\bar{1}$1), (3.1929) (29)(023), (2.7915)(30)(202), (2.5645)(21)(032), (2.5220)(100)(1$\bar{3}$0), (2.4906)(18)(130) and (2.3267)(71)(2$\bar{2}$2). The chemical composition determined by electron-microprobe analysis is (wt.%): CuO 60.87, ZnO 0.50, FeO 0.36, V2O5 19.85, As2O5 6.96, SO3 0.44, MoO3 1.41, SiO2 0.20, P2O5 0.22, Cl 10.66, –O = Cl2 2.41, total 99.06. The empirical formula calculated on the basis of 17 anions per formula unit is (Cu7.72Zn0.06Fe0.05)Σ7.83(V2.20As0.61Mo0.10S0.06P0.03Si0.03)Σ3.03O13.96Cl3.04.

The crystal structure of dokuchaevite represents a new structure type with eight Cu sites, which demonstrate the remarkable diversity of Cu2+ mixed-ligand coordination environments. The crystal structure of dokuchaevite is based on OCu4 tetrahedra that share common corners thus forming [O2Cu6]8+ single chains. Two of the eight symmetrically independent copper atoms do not form Cu–O bonds with additional oxygen atoms, and thus are not part of the OCu4 tetrahedra, but provide the three-dimensional integrity of the [O2Cu6]8+ chains into a framework. TO4 mixed tetrahedral groups are located within the cavities of the framework. The structural formula of dokuchaevite can be represented as Cu2[Cu6O2](VO4)3Cl3.

New analyses are given of granites from 23 intrusions in Scotland, Ireland, and the Isle of Man. Together with existing analyses, these were used to measure the regional variation in composition of Caledonian granites by means of trend surface analysis. An explanation of the variation is presented that involves the production of magma by melting in the crust, at a depth determined by the geothermal gradient.

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.

A nepheline mugearite dyke in the Walcha area, New South Wales, contains xenoliths of Cr-diopside lherzolite, garnet- and spinel websterites, and two- pyroxene-garnet granulites. The Cr-diopside lherzolite xenoliths are regarded as refractory upper mantle which finally equilibrated at c. 950–980°C. The garnet websterites are considered to represent spinel websterite segregations from an Mg-rich olivine tholeiite magma at 14–16 kbar which on isobaric cooling to 1000–1050°C passed into the garnet pyroxenite stability field. Spinel-amphibole websterites which probably last equilibrated at c. 950°C and pressures of 9–12 kbar have dose mineralogical affinites with the Cr-diopside lherzolite xenoliths. The mafic granulites may represent metamorphosed crystal segregates resulting from relatively large degrees of crystallization of tholeiitic basaltic magma which finally equilibrated at c. 14 kbar and 1000°C Subcalcic clinopyroxene and spinel megacrysts crystallized from the host nepheline mugearite at c. 13–20 kbar.

Au-Cu mineralization at Palai-Islica occurs as disseminations in massive silicified volcanic rocks and, more abundantly, in sulphide-bearing quartz veins. The major ore minerals in the deposit are pyrite ± chalcopyrite, sphalerite and galena and there is a great variety of accessory minerals, including Au-Ag alloys and native gold. Pyrite, the most abundant sulphide, is closely associated with gold. Seven different types of pyrite have been distinguished with a variable concentration of different trace elements. Among these, the only one free of trace elements (type IV) is related to Au-Ag alloys. Pyrite associated with these Au-Ag alloys has cubic and pentagonal dodecahedral habits, whereas pyrite with pentagonal dodecahedral habit only is from barren zones. In addition, there is no significant invisible gold in the pyrite, but there is a relatively large amount of Ag in collomorphic pyrite (up to 0.20 wt.%) or type III pyrite (up to 1.47 wt.%). Arsenic is the most abundant trace element in pyrite (up to 6.11 wt.%), present as a metastable solid solution or as a non-stoichiometric element. A variety of marcasite related to the gold levels also has a considerable amount of trace elements (As up to 1.15 wt.%, Sb up to 0.40 wt.%).

The Little Avon river rises on the scarp slope of the Cotswolds near Hawkesbury, 14 miles north-east of Bristol, and flows in a westerly direction over Mesozoic strata, until half a mile south-east of Wickwar it cuts through the Triassic mantle and exposes to view three inliers of Wenlock Limestone (Whittard & Smith, 1944, p. 65 and map). The West Gloucestershire Water Company was interested in the valley as a possible site for an impounded reservoir, and as part of a preliminary survey a trial-shaft was sunk into the Wenlock Limestone of the southernmost inlier situated near Sturt Bridge. The shaft penetrated 28 feet of dolomitized siltstones and crinoidal limestones, and at a depth of 4 feet it entered a band of calcareous chamositic siltstone averaging 9 inches in thickness. The shaft had been completed and the sides boarded before we heard it had been sunk; the hand-specimens were therefore obtained from the tip-heaps, but there is no doubt that all the material originated from the same horizon.

Instrumental methods of chemical analysis, including electron and ion-probes, X-ray fluorescence, atomic absorption, and plasma-source spectrometry, have largely replaced classical wet-chemical analysis except for ‘referee’ analyses, but have not so far proved able to determine the net state of oxidation of a rock or mineral with reasonable accuracy. Mössbauer spectroscopy can indeed give fairly accurate Fe2+ and Fe3+ estimates in favourable circumstances, but fails when the Fe2+: Fe3+ ratio is very small or very large or when the iron is present in several different lattice positions with different surroundings; and it cannot arrive at the net state of oxidation when other elements of variable valency are present. The term ‘net state of oxidation’ calls for some comment, and is dealt with in the full paper; for the moment we note as an example that Mössbauer spectroscopy shows that much of the iron in ludlockite is ferric, whereas a ‘ferrous iron’ determination corresponds to an arsenate of lead and ferrous iron, from which it follows that part of the arsenic must be trivalent, a result that could not have been arrived at from either the ‘ferrous iron’ or the Mössbauer data along. Thus in the present state of the art a wet-chemical method for net state of oxidation is a necessity.

Using type material from the Österby pegmatite in Dalarna, Sweden, the chemical composition and structural parameters of thalénite-(Y) [ideally Y3Si3O10(OH)] were examined by wavelength dispersive spectroscopy electron microprobe (WDS EMP) analysis and single-crystal X-ray diffraction. High contrast back-scatter electron images of the Österby material show at least two generations of thalénite-(Y). The formula of the primary thalénite-(Y) normalized to 11 anions is (Y2.58Dy0.11Yb0.09Gd0.06Er0.06Ho0.02Sm0.02Tb0.02Lu0.02Nd0.01Tm0.01)Σ3.00Si3.01O10F0.97OH0.03. The secondary thalénite-(Y), replacing the primary material, is weakly enhanced in Y and depleted in the lightest and the heaviest rare-earth elements, yielding the formula (Y2.63Dy0.12Yb0.06Gd0.06Er0.05Ho0.02Sm0.02Tb0.02Tm0.01Nd0.01Lu0.01)Σ3.00Si3.01O10F0.98OH0.02. Structural data for thalénite-(Y) from Österby clearly indicate the monoclinic space group P21/n, with a = 7.3464(4), b = 11.1726(5), c = 10.4180(5) Å, β = 97.318(4)°, V = 848.13(7) Å3, Z = 4, which is consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R1 = 0.0371 for 1503 unique observed reflections, and the final chemical composition obtained from the refinement, (Y2.64Dy0.36)Σ3.00F0.987[Si3O10], Z = 4, is in good agreement with the empirical formula resulting from electron microprobe (EMP) analysis. Both techniques reveal a strong dominance of F over OH, which means that the type material actually corresponds to the fluorine analogue. Moreover, new EMP analyses of samples of thalénite-(Y) from an additional seven localities (Åskagen and Reunavare in Sweden; White Cloud and Snow Flake in Colorado, USA; the Guy Hazel claim in Arizona, USA; Suishoyama and Souri in Japan) clearly show the prevalence of F over OH as well. Based on these observations, the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association has recommended a redefinition of the chemical composition of thalénite-(Y) to represent the F-dominant species with the ideal formula Y3Si3O10F, as it has historical priority. Consequently, the later described fluorthalénite-(Y) has to be discredited.

The Kef Lakhal amphibolitcs and associated amphibolitc and olivine-rich enclaves are dcscribcd and their major and trace element chemistry indicates that both amphibolites were evolved medium to high alumina tholeiitic basalts with talc-alkaline affinities probably formed within plate settings. The olivine-rich enclaves are disrupted periodotites of the type lherzolite-harzburgite and probably represent mantle residua after melting.

In this paper the Late Pleistocene carbonatite-melilitite association (CMA) of Italy is evaluated in terms of the Plio-Quaternary extensional structures in the crust and mantle lithosphere. In particular, we briefly discuss the geometry and stress field of the host geological structures, the tectonic control on the eruptive style, the thickness and composition of the lithosphere. Some implications for a passive rift geodynamic environment are also considered.

Cenomanian glauconite grains from the Isle of Wight often possess a thin, pale green shell or ‘corona’ of glauconite. Optical investigation led to recognition of two types of microstructure in these shells; an outer layer with tangentially orientated c-axes, and an inner isotropic layer. A third type of layer with radially orientated c-axes occurs both subjacent to the corona and at lower levels in glauconite grains. This latter occurrence is frequently a relic of a former corona, and indicates two or more periods of glauconite formation. The disposition of the microstructures allows some conclusions to be drawn with respect to the depositional history of individual glauconite grains.

Crystal growth experiments in the system BaO–Al2O3 yielded single crystals of a new Ba oxo-aluminate with composition Ba4Al2O7. The title compound is orthorhombic, with cell parameters a = 11.3126(5), b = 11.7045(9), c = 27.1850(14) Å, space group Cmca. The structure was solved by direct methods followed by difference Fourier synthesis from X-ray single crystal diffraction data (R1 = 0.048 for 1213 independent observed reflections and 135 parameters). The main building units of tetrabarium aluminate are Al3O10 trimers and isolated AlO4 tetrahedra. Seven crystallographically different Ba sites crosslink between the tetrahedral groups and are co-ordinated by six to nine oxygen ligands. The structure is closely related to the mineral kilchoanite (Ca6[SiO4][Si3O10]) and the first representative of an unbranched oligo-aluminate with triple tetrahedra.

A dolostone layer is found in spatial association with the Emirli epithermal Sb-Au deposit in western Turkey. It occurs within an argillic alteration zone adjacent to the major Emirli fault zone, which is the controlling structure for the deposit, and is composed of large closely packed subhedral to anhedral planar and nonplanar dolomites. Pyrite is the only accessory mineral in the layer and occurs as disseminations and veinlets up to 100 μm wide. Dolomite crystals are petrographically homogeneous and have consistent deep-red cathodoluminescence (CL) colour with no zoning which implies single-stage dolomitization of a calcitic precursor which is partly preserved as remnant patches of orange-coloured CL zones. Some crystal boundaries have dark (or no) CL colour. Electron microprobe line-scan analyses across these regions indicate intense enrichment of Fe and Mg depletion, revealing late-stage Fe-metasomatism (ankeritization) especially prevalent near pyrite veinlets and disseminations.

Dolomite crystals are composed of 52.4–55.0 mol.% CaCO3, 29.9 –41.2 mol.% MgCO3, 1.8 – 14.4 mol.% FeCO3 and 0.75 –3.2 mol.% MnCO3 indicating ferroan dolomite. The relationship between Ca and Mg is not stoichiometric due to substitution of Mg2+ by Fe2+ after dolomitization, as demonstrated by a strong negative correlation between Fe and Mg. Whole-rock Fe contents of the dolostone layer increases toward the Emirli fault zone.

The δ13C(PDB) compositions of the Emirli dolomite, calcitic marbles, and graphite-schists are in the ranges of –1.6 to 0.8‰, 1.5 to 1.5‰ and –6.6 to –23.5%, respectively, indicating that dolomite was formed due to interaction of light-carbon-enriched fluids with calcitic marbles; light-carbon may have been derived from decarboxylation of the graphitic schist layers. δ18O(PDB) values of dolomite and marble range –15.2 to – 11.2‰ and –2.4 to – 3.5‰, respectively. This large isotopic difference between dolomite and marble was probably inherited from oxygen isotope exchange between the dolomitizing fluid and the precursor calcites, as well as other minerals enriched in light-oxygen.

Fluid inclusions in dolomite are two-phase, and homogenize into liquid within the temperature interval 242 – 362ºC with a mode of 290ºC, and have salinities of 1 – 3 wt.% NaCl equiv. Using this modal temperature, the average δ18O(PDB) composition of water in isotopic equilibrium with the Emirli dolostone was estimated to be –19.4±2.1‰, which is interpreted as an indication of modified surface-waters; this interpretation is also supported by low fluid salinity and Na and Sr contents. These fluids migrated along graben-related faults, penetrating deeper levels where they were transformed into hydrothermal fluids due to the high heat-flow of the Küçük Menderes graben system, and flowed-up mainly through the Emirli and Haliköy faults that control mineralization in local deposits of Sb-Au and Hg, respectively. Due to interaction with chlorite-bearing graphite-schists, the fluid may have dissolved Mg2+ from chlorite and been enriched in isotopically-light carbon due to decarboxylation of graphite. Dolomitization occurred as a result of the interaction of these fluids with a calcitic marble band adjacent to the Emirli fault zone. Subsequent introduction of Fe2+ caused ankeritization along dolomite crystal boundaries during first-stage Sb-Au mineralization.