In May 1950 the writer received from Mr. Henry Burgess, of Spiggie, Shetland Isles, a specimen of a material which had been found by a local quarryman. The property of the material which had particularly attracted attention was that when it absorbed water it became soft and pliable and looked like wet leather. The specimen, measuring approximately 4x2X1 inches, had a brownish iron-stained outer skin, and showed slight signs of foliation parallel to its length. Internally it consisted of a white to greyish-white compact material, in which were embedded portions of the syenitie country-rock. This white material has been shown to consist dominantly of the mineral palygorskite (attapulgite).

The high-pressure behaviour of manganese carbonate was investigated by in situ synchrotron X-ray powder diffraction up to 54 GPa with a laser-heated diamond anvil cell. A phase transition from rhodochrosite to a new structure form was observed at 50 GPa after laser heating. The diffraction pattern of the new high-pressure form was reasonably indexed with an orthorhombic unit-cell with a = 5.361 A, b = 8.591 A and c = 9.743 Å. The pressure-induced phase transition implies a unit-cell volume reduction of ∼5%. This result does not support the direct formation of diamond by dissociation of solid state MnCO3 reported in a previous study. Fitting the compression data of rhodochrosite to a second-order Birch-Murnaghan equation of state (Ko’ = 4) gives K0 = 126(±10) GPa. The c axis of the unit-cell parameter was more compressive than the a axis.

Mayo Belwa, a highly shocked enstatite achondrite, fell during the evening of 3 August 1974 in the Adamawa District, NE. Nigeria (8° 58' N., 12° 05' E.). The stone weighed 4·85 kg and was deposited in the Geological Survey of Nigeria and loaned for study to the British Museum (Natural History). It is composed principally of enstatite (0·02 FeO%), with some olivine (Fo100), diopside, feldspar (An9 Ab88 Or3), and minor Fe-Ni metal and the sulphides oldhamite, daubréelite, ferroan alabandine, and troilite. The kamacite contains between 0·15% and 1·2% Si and the troilite contains 1·1% Ti. The meteorite possesses vuggy cavities, which contain fluor-amphibole needles projecting from their walls, suggesting the presence at some time of a volatile-rich phase. The titanium-rich troilite, the nearly ironfree enstatite and the Mg/Si ratio are typical of the E-achondrites.

The crystal structure of caratiite, K4Cu4O2 (SO4)4MeCl (a 13.60(2), c 4.98(1) Å, Z = 2) was solved in space group I4 from single crystal data, and refined to R = 0.035 for 654Fo > 6σ(Fo). The atomic arrangement is characterized by Cu2O(SO4)2 chains parallel to [001], with Cu in (4 + 1) co-ordination and O surrounded tetrahedrally by four Cu atoms. These chains are interconnected by K atoms to a framework, the channels of which house the Me and Cl atoms. The nature of the Me atoms is not completely clear; most probably the position is half occupied by Cu, fully occupied by Na, or occupied by a mixture of the two.

The electron probe was used to analyse for Fe, Ti, Mg, Al, and Mn. Vanadium could not be determined because of interference from Ti. Calcium was shown to be present only in inclusions of apatite and silicate; it does not oceur in the titanomagnetite structure.

Homogeneous grains (magnetite-ulvöspinel solid solutions) do not vary greatly in bulk composition, averaging close to 61% Fe and 8 % TiO2. Inhomogeneous grains contain variable amounts of hematite-ilmenite lamellae produced by progressive oxidation of the homogeneous grains. In the early stages there is rapid build-up of titanium in the rhombohedral lamellae, which are initially relatively poor in iron. As oxidation proceeds the almost titanium-free cubic phase becomes strongly enriched in magnesium, aluminium, and manganese, while the iron: titanium ratio in the rhombohedral phase increases. The end-product may be a homogeneous titanhematite, if initial concentrations of Mg, Al, and Mn are low. Otherwise residuals of a relatively iron-poor almost titanium-free (Mg, Al, Mn)-enriched cubic phase remain in the titanhematite. The behaviour of iron and titanium agrees with recent synthetic work in the system. Some of the oxidized grains contain transparent spinel lamellae exsolved from the cubic phase. Exsolution may commence at different stages in the oxidation process, but the reason is not clear—there are no significant differences in the composition of phases with and without spinel lamellae.

Analyses and normative compositions are presented for the different phases in each grain, where possible. The compositional changes accompanying oxidation satisfactorily explain the slope of the thermomagnetic curves : there is no need to invoke bulk compositional variations within a given ironsand sample.

Samples from an ilmenite and a non-titaniferous magnetite concentrate were also briefly examined.

The humites are structurally analogous to olivine wherein the replacement of four oxygen by four (F,OH) anions in the slightly distorted, hexagonal close-packed array is balanced by the replacement of one tetrahedrally coordinated Si by a tetrahedral void, according to the general formula Mg2xSix-1O4x-4(F,OH)4 where x = 3, 5, 7, 9. In humites the key structural units are not ‘olivine and sellaite (or brucite) layers’, as previously assumed, but are zigzag chains of edge-sharing octahedra, just as in olivines. It is shown that for humites and olivines alike the unit cell parameters a, b, and d001/n and the cell volume (normalized to one-half the mean anion-anion distance along the normal to (001)) vary linearly with the average radius of the octahedrally coordinated cation in the chain.

Substitutions of (F,OH) for O and vacancies for Si have second-order effects on the unit cell parameters, causing a linear decrease of the normalized cell volume with increase in F/O ratio in the synthetic series forsterite-humite-norbergite. Comparison of the crystal structures of forsterite and norbergite shows that the polyhedral distortions in norbergite are smaller than in forsterite in accord with the decreased number of shared edges: the fluorines in norbergite are bonded to three Mg atoms whereas all anions in forsterite are bonded to three Mg and one Si atom.

Tetrahedrite fahlores from the Coeur d'Alene mining district (Idaho) have been found to be enriched in Ag by the Ag–Cu exchange reaction

which occurred during cooling following galena mineralization. This solid–state reaction resulted in quantitative removal of Ag (in a AgSbS2 component) from galena and development of bournonite coronas on fahlore grains. The reaction produced a distinct population of high-Ag fahlores found in galena-rich samples and accounts for all of the bournonite mineralization. The most argentian of these high-Ag fahlores examined in this study (molar Ag/(Ag+Cu) = 0.303±0.011 and 0.336±0.011) are found in samples which achieved saturation with respect to other Ag-sulfosalts, namely pyrargyrite and polybasite and diaphorite, respectively. Multiple lines of evidence for the Ag-Cu exchange reaction are presented in this paper which applies mass-balance constraints and a thermodynamic database for sulfides/sulfosalts to microprobe analyses and textural observations, and to bulk production data. This solid-state reaction may explain why previous district studies have been unable to demonstrate convincingly primary fahlore zoning. Based on the Ag/(Ag+Cu) of fahlores coexisting with other Agsulfosalts and Fe-Zn partitioning between fahlore and sphalerite, we estimate that fahlore compositions were frozen in by 175°C. Examining the composition data for other Ag tetrahedrite fahlores found in the Coeur d'Alene district and elsewhere, we conclude that the thermodynamic database provides an accurate description of phase equilibria, except possibly for very Fe-rich systems where further studies of the Fe-Zn partitioning between fahlore and Fe-rich (Zn,Fe)S phases and of the thermodynamics of Fe-rich (Zn,Fe)S phases are warranted.

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.

Paragonite has been found in two chloritoid-chlorite-mica schists from the Dalradian, north of Stonehaven. The principal bulk-chemical constraints on the occurrence of this mineral within the chloritoid zone appear to be the ratios of soda to potash and the Al2O3 contents of the rock. The composition of paragonite coexisting with muscovite indicates a crystallization temperature of 550 °C for rocks some way above the chloritoid isograd. Although this result is supported by other mineral equilibria, the muscovite compositions suggest a temperature of 690 °C which is too high because the mineral assemblage chloritoid-paragonite is unstable above about 600 °C By combining data on the breakdown of the chloritoid-paragonite assemblage with a mineral equilibrium for the assemblage garnet-plagioclase-muscovite-biotite, it has been deduced that rocks close to the staurolite isograd crystallized at about 7·5 kb and 600 °C.

The crystal structures of synthetic Pb3O2(CO3) and NaPb2(OH)(CO3)2, have been solved by direct methods and refined to R = 0.062 and 0.024, respectively. Pb3O2(CO3) is orthorhombic, Pnma, a = 22.194(3), b = 9.108(1), c = 5.7405(8) Å, V = 1160.4(3) Å3, Z = 8. There are four symmetrically distinct Pb2+ cations in irregular coordination polyhedra due to the effect of stereoactive s2 lone electron pairs. The structure is based upon double [O2Pb3] chains of [O(1)Pb4] and [O(2)Pb4] oxocentred tetrahedra and CO3 groups. The [O2Pb3] chains are parallel to the c axis, whereas the CO3 groups are parallel to the (010) plane. NaPb2(OH)(CO3)2 is hexagonal, P63mc, a = 5.276(1), c = 13.474(4)Å, V = 324.8(1) Å3, Z = 2 and has been solved by direct methods. There are two symmetrically distinct Pb2+ cations in asymmetric coordination polyhedra due to the effect of stereoactive s2 lone-electron pairs. The single symmetrically unique Na+ cation is in trigonal prismatic coordination. The structure is based on hexagonal sheets of Pb atoms. Within these sheets, Pb atoms are located at vertices of a 36 net, such that each Pb atom has six adjacent Pb atoms that are ~5.3 Å away. Two sheets are stacked in a close-packing arrangement, forming layers that contain the (CO3) groups. The layers are linked by OH groups that are linearly coordinated by two Pb2+ cations. Na+ cations are located between the layers. The structure is closely related to the structures of other lead hydroxide carbonates (leadhillite, macphersonite, susannite, hydrocerussite, ‘plumbonacrite’).

In the Fen complex (Telemark, S.E. Norway), carbonatites of different compositions have penetrated feldspathic fenites (alkali feldspar(s) + aegirine augite ± alkali amphibole) or older carbonatites, inducing different types of contact metasomatic alterations in their wall-rocks. (1) Pyroxene søvite has induced alkali metasomatism (i.e. fenitization s.s.), with alkali feldspars remaining stable and aegirine-augite transformed to nearly pure aegirine. (2) Søvite and dolomite carbonatite with phlogopite and/or alkali or alkali-calcic amphibole have caused replacement of feldspathic fenite by phlogopite, i.e. magnesium metasomatism. (3) Granular (dyke facies) ferrocarbonatite has increased the ferromagnesian components in calcite in wall-rock søvite. (4) Heterogeneous (pyroclastic) ferrocarbonatite induced pseudomorphic replacement of phlogopite by chlorite (leaching of alkalis). The different contact metasomatic processes reflect contrasts in compositional character among carbonatite magmas in the Fen complex, which may be evaluated in terms of differences in alkali and magnesium carbonate activities. The different types of carbonatite magma represent the products of local evolutionary trends, and are genetically related to spatially associated silicate rocks, rather than to a single ‘primitive’ carbonatite parent magma.

Large albitite bodies associated with explosive brecciation, fenite aureoles, intrusive carbonate rocks, olivine dolerites, and copper mineralization are described from the Dead Sea rift region in the south-eastern Sinai Peninsula. The main carbonate phases comprise breunnerite and dolomite with actinolite the main phase in the fenites. Field and isotopic evidence indicates that the intrusive carbonate rocks are of carbonatitic origin. The cogenetic, almost monomineralic main phases can be attributed to fractional crystallization and liquid immiscibility acting on a highly gascharged, slow-cooling gabbroic magma. The complex, which is probably of Cretaceous (or older) age, appears to be related to early continental separation along the Dead Sea branch of the African rift system. The porphyry-type copper deposits resemble others that are located along crustal accretion (Iceland) or subduction (Chile) zones.

Sphalerite is a common constituent of the mineralized rocks and host metasediments of the Foss baryte-base metal deposit, located near Aberfeldy in the central Scottish Highlands. Microprobe analyses of sphalerite show a wide range in minor element content (0–17 mol. % FeS, 0–3 mol. % MnS), and sphalerites of contrasting composition are often found in the same rock. This suggests that equilibrium domains in some rocks were minute (< 1 mm), during regional metamorphism. Pressures derived from the selective application of the sphalerite geobarometer are consistent with other mineralogical evidence of peak metamorphic pressures in the range 7–10 kbar, at 540–580 °C. However, there is considerable evidence of partial retrograde re-equilibration of sphalerite, by continued buffering with pyrite+ pyrrhotine and by outward diffusion of iron. Marginal depletion of Fe and Mn in sphalerite within coarse carbonate rocks is attributed to partitioning reactions following recrystallization. Sphalerite which has retained its pre-metamorphic composition shows systematic variations in composition through profiles of mineralized beds, which may be related to depositional environments. Bimodal primary compositions can be explained by precipitation of zinc sulphides under the contrasting chemical environments of hydrothermal vents and cooler, exhaled brine layers on the sea floor.

Reversed brackets on As and Sb partitioning between tetrahedrite-tennantite fahlores {∼CU10(Fe,Zn)2(Sb,As)4S13} and bournonite-seligmannite solid solutions {CuPb(Sb,As)S3} (400°C evacuated silica tubes, NH4Cl flux) indicate that the maximum nonidealities associated with the As-Sb substitution are about 165 (bournonite) and 250 (fahlore) cal/gfw on a per atom exchange basis. The experimental constraints are consistent with the following calibration of the As-Sb exchange reaction between these phases:

where kcal/gfw, X2 = Zn/(Zn + Fe) (or one-half the number of Zn atoms/formula unit) and X3 = As/(As + Sb) refer to atomic ratios in fahlore, and ΔG°23 = ΔH°23 = 2.59 ± 0.14 kcal/gfw (Raabe and Sack, 1984; Sack and Loucks, 1975; O'Leary and Sack, 1987; Sack, 1992).

The crystal structure of synthetic kogarkoite has been determined from X-ray data collected on an automatic diffractometer. The refinement was performed by a least-squares method employing anisotropic thermal parameters. The 3157 reflections with I > 3σ(I) converged to a conventional R value of 0.033. The cell content is 12 Na3SO4F, the space-group P21/m, a = 18.074, b = 6.958, c = 11.443 Å, β = 107.71°.

Kogarkoite presents a marked trigonal subcell with c′ corresponding to [102] of the monoclinic cell. The tridimensional framework can be considered built up by nine differently stacked layers of Na atoms approximately perpendicular to the c′ axis (five sheets are present in galeite, six in sulphohalite, and seven in schairerite). The very close structural relationships between these minerals are discussed.

Twenty-five new analyses of 10 spilitic pillows from the British Isles offer evidence of considerable variability in distribution of components, notably Si, Na, Mg.Fe2+, Ca, Fe3+, and water. Selvedges of pillows have compositions strongly contrasted with but essentially complementary to those of cores. Redistribution of components is believed to have occurred during devitrification. Geosynclinal spilites are regarded as basalts which have suffered such redistribution under conditions of diagenesis or low-grade metamorphism.

The investigation of the Launton, Warbreccan, and Cronstad meteorites was undertaken with the object of placing on record descriptions of these stones. Launton and Warbreccan were found to conform very well with the type of Baroti and Wittekrantz, containing from 7-9 per cent. of nickel-iron, described in a previous paper. Cronstad, however, which contained as much as 18 per cent. of nickel-iron, showed so much variation from that type as to suggest a passage towards the rare Hvittis class of chrondritic stone containing even larger amounts of nickel-iron.

The process of transforming hydromagnesite to magnesite is analysed in the context of the theory of solvent-mediated transformations. A series of experiments at 120, 150 and 180ºC with different heating times was designed to determine, by powder X-ray diffraction, the amount of magnesite generated as a function of time. The aqueous-phase composition was monitored by inductively coupled plasma-mass spectrometry and carbonate alkalimetry. From the analytical data, the evolution of saturation indexes with respect to both phases was determined using the geochemical code PHREEQC. Finally, two different methods were applied to obtain the activation energy of the process and a TTT (Temperature- Transformation-Time) graph was constructed to define suitable conditions in which to obtain magnesite.

Toxic trace elements present an environmental hazard in the vicinity of mining and smelting activities. However, the processes of transfer of these elements to groundwater and to plants are not always clear. Tharsis mine, in the Iberian pyrite belt (SW Spain), has been exploited since 2500 BC, with extensive smelting taking place from the 1850s until the 1920s. Sixty four soil (mainly topsoils) and vegetation samples were collected in February 2001 and analysed by ICP-AES for 23 elements. Concentrations are 6—6300 mg kg-1 As and 14—24800 mg kg-1 Pb in soils, and 0.20—9 mg kg-1 As and 2—195 mg kg-1 Pb in vegetation. Trace element concentrations decrease rapidly away from the mine, with As and Pb concentrations in the range 6—1850 mg kg—1 (median 22 mg kg—1) and 14—31 mg kg—1 (median 43 mg kg—1), respectively, 1 km away from the mine. These concentrations are low when compared to other well-studied mining and smelting areas (e.g. 600 mg kg—1 As at 8 km from Yellowknife smelter, Canada; >100 mg kg—1 Pb over 270 km2 around the Pb-Zn Port Pirie smelter, South Australia; mean of 1419 mg kg—1 Pb around Aberystwyth smelter, Wales, UK). The high metal content of the vegetation and the low soil pH (mean pH 4.93) indicate the potential for trace element mobility which could explain the relatively low concentration of metals in Tharsis topsoils and cause threats to plans to redevelop the Tharsis area as an orange plantation.

The North Qôroq centre comprises a series of concentric nepheline syenite intrusions and forms part of the Igaliko Nepheline Syenite Complex, in the rift-related Gardar Province of South Greenland. The North Qôroq syenites range from mildly undersaturated augite syenite to strongly peralkaline agpaitic nepheline syenite. Extensive in situ fractional crystallization has been postulated for the chemical variation both within units and throughout the centre. Many of the rocks have been affected by metasomatic fluids associated with the emplacement of younger syenite units, and this complicates their interpretation. In this study, the trends and compositions exhibited by pyroxene and amphibole from North Qôroq are examined and related to either primary crystallization or metasomatic activity (e.g. controls of fO2, peralkalinity). Implications thus drawn are used to interpret the chemical processes inherent in the chemical and fluid evolution of alkaline magmas, and, in particular, the transition from miaskitic to agpaitic magmatism. In general, the major phases of the North Qôroq syenites records the increasing evolution of the units by crystal fractionation, towards peralkaline compositions. The composition of olivine, in the least evolved syenites, also points to a relatively high state of fractionation of the parent magma, whilst pyroxene and amphibole record an overall decrease in Mg/Mg+Fe), and a general increase in Fe3+ and alkali content, with increased fractionation. The increasing peralkalinity of the magma also governs the evolution of pyroxene and, to a lesser degree, amphibole towards higher Zr and Ti contents in the more Na-rich compositions. The trends for pyroxene from metasomatized syenite show similar patterns, but lower Fe2+ enrichment, suggesting the source of the metasomatic fluids is similar to the evolving syenites. The presence of amphiboles in metasomatic rocks, and high F contents attest to the F-rich nature of the metasomatic fluids, which is in agreement with results previously reported for metasomatic fluorapatite.