A skarn mineral assemblage occurs at the junction between vent pyroclastics and a xenolithic Cretaceous chalk block which subsided into the collapsed caldera of the Central Ring Complex, Isle of Arran, Scotland. Adjacent to the metachalk marble an andradite garnet exoskarn zone has developed at the expense of the carbonate. An andradite grossular/diopsidic clinopyroxene endoskarn zone has formed in the surrounding agglomerate, and a magnetite exoskarn is present in places between the andradite and garnet/pyroxene zones. The andraditic exoskarn garnets have fluor-hydrogarnet components, indicating that fluorine was present in the metasomatic fluid. From petrographic evidence, three distinct episodes of exoskarn garnet crystallization can be recognized, in which the fluor-hydrogarnet component steadily increased as a function of time, which probably reflects falling temperature. The REE compositions of the exoskarn minerals are regarded as having been largely inherited from the carbonate, and the exoskarn garnets increasingly fractionated HREE with time. The endoskarn and agglomerate have also been epidotized. The REE signatures of epidotes appear to be inherited partially from precursor clinopyroxenes or feldspars, which have been replaced by epidote. Late-stage vein minerals include prehnite, laumontite and K-rich laumontite, and their REE compositions appear to have been derived from the marble, probably via REE fluoro-complexes in the fluid.

In a thin specimen X-ray absorption and fluorescence can, to a first approximation, be ignored and the observed X-ray intensity ratios, IA/IB, can be converted into weight fraction ratios, , can be converted into weight fraction ratios, CA/CB, by multiplying by a constant , by multiplying by a constant kAB;

kAB values can be calculated or determined experimentally. The major correction which may have to be made to the calculated weight fraction ratio is for X-ray absorption within the specimen. The activated volume for analysis in a thin specimen is approximately 100 000 × less than in a bulk sample. Beam spreading within the specimen can be estimated using a simple formula based on a single elastic scattering event at the centre of the specimen. Examples are given of the application of the technique to obtain both qualitative and quantitative analyses from thin mineral specimens. The minimum detectable mass and the minimum mass fraction which can be measured using the technique are estimated.

Hybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.

Hydrocarbons (bitumens sensu lato) in sandstones have been recorded in several instances to have partially replaced their host rock, including quartz grains. Many replacive hydrocarbons are uranium-rich: associated non-uraniferous hydrocarbons are not replacive. Uranium is transported as carbonate complexes, which may be decomposed by organic acids to yield UO22+ ions and CO2. The UO22+ will be absorbed onto hydrocarbons before reduction to a mineral phase, generally uraninite; and the CO2 may be aggressive towards the silicate grains of the host sandstone. Accretionary nodules of replacive uraniferous hydrocarbon in red beds (e.g. at the cores of reduction spots) can provide valuable information about petroleum migration. They occur particularly in the vicinity of faults, and may record the interactions between metal-rich groundwaters and hydrocarbons leaking along a fault from an underlying reservoir. A trial study in Devonian sandstones of Easter Ross successfully traced a hydrocarbon-bearing sandstone reservoir from an occurrence of uraniferous hydrocarbon nodules.

A suite of coarsely crystalline samples of crinoidal limestone adjacent to a PbS-CaF2-CaCO3 vein in the North Pennine orefield (Greenhow Rake) was analysed by three independent methods [Inductively Coupled Plasma emission spectroscopy (ICP), Mass Spectrometry (MS) and Gas Chromatography (GC)], in order to determine the nature and origin of trace elements and volatiles released on heating and thermal decrepitation, to establish whether fluid inclusions from the host limestone differed in composition from those within the vein, and to assess the value of the results obtained for mineral exploration. The ICP method gave reproducible results for Ca, Na and K, and significant values for Pb and Zn. Na and K correlate with H2O levels determined by MS/volumetric analysis, suggesting their coexistence within fluid inclusions. No such correlation was found for Pb and Zn, suggesting that these elements were derived by volatilisation, perhaps from traces of galena and sphalerite. Methane showed no correlation with H2O and a source other than aqueous fluid inclusions is thought probable for this volatile.

The vein calcite showed much lower K/Na fluid ratios (0.05 cf 0.1–0.2), than the limestones. The limestone-hosted fluids occur predominantly in crinoid ossicles and are believed to represent early pre-mineralisation fluids, unrelated to those associated with veining. Combined ICP and MS techniques may be of value in investigating samples where fluid inclusions are too small for microthermometric and optical analysis.

The Loe Shilman carbonatite sheet complex comprises an extensive amphibole sovite which is intruded by minor biotite sovite and amphibole ankeritic carbonatite. The carbonatites have fenitized the country rocks to form a metasomatic zone c. 100 m wide of alternating mafic and felsic mica-bearing banded fenites which grade into slates and phyllites. Phlogopite-rich micas occur nearest to the carbonatite contact. The biotites occur in K-feldspar + albite ± Na-amphibole ± aegirine and ± phengite fenites produced by the intrusion of the early amphibole sovite. Aegirine buffered the iron distribution and the biotites became more magnesian. Veins cross-cutting the fenites consist of biotite and/or Ba-bearing K-feldspar, and are interpreted to result from solutions emanating from the biotite sovite. The ankeritic carbonatite is responsible for the formation ofphlogopite in the fenites in a c. 3 m wide zone adjacent to the carbonatite, and evidently are the result of fenitizing fluids rich in Mg. Chemical equations calculated to balance the reactions interpreted to have taken place in the fenites suggest that about 10% of the Al and Si in the protolith was mobilized and moved towards the carbonatites during fenitization, and that the fenitizing solutions were strongly alkaline and oxidizing.

Iron ore sinters prepared for the blast furnaces at Scunthorpe, Humberside, comprise iron oxides bound in a matrix of aluminium- and silicon-rich calcium ferrites with some calcium orthosilicate and a residual silicate glass. A super-silicon-deficient clinopyroxene with up to half of the Si in the chain replaced by Fe3+ and Al3+ precipitates from the glass in some areas. The iron oxides are principally magnetite and hematite with some wüstite in highly reduced areas. The calcium orthosilicate has been previously reported as the mineral larnite, β-Ca2SiO4. This study found a range of Fe for Ca substitution and three polymorphs were recognised: the range Ca2SiO4 to Ca1.85Fe0.15SiO4 has the structure of larnite; the range Ca1.85Fe0.15SiO4 to Ca1.6Fe0.4SiO4 has the structure of bredigite, α′-Ca2SiO4. Iron substitution beyond this, and up to a maximum of about Ca1.5Fe0.5SiO4, was not recognisable as a polymorph of Ca2SiO4, but an orthorhombic cell is tentatively proposed. The complex ferrite SFCA (silico-ferrite of Ca and Al) is variable in composition, but has a minimum of about 3 wt. % Al2O3 and contains 20 wt. % CaO and SiO2 with a Ca: Si ion ratio of about 2 : 1. A C-face centred monoclinic cell has been deduced: a 15.70, b 9.70, c 8.48 Å; β 105°30′. The Al appears critical for the formation of SFCA in sinter; without it, a eutectic of magnetite and bredigite precipitates in preference.

Calderitic garnet and manganoan deerite have been found in Al-poor, Fe-Mn-rich quartzites associated with metabasic schists on the island of Andros, Cycladic blueschist belt, Greece. Calderitic garnet (30–45 mole % calderite, 55–70 mole % andradite, 0–2 mole % spessartine) occurs in paragenesis with quartz, rhodonite, rhodochrosite, Ni-Mn-bearing magnetite (2–20 mole % trevorite, 6–12 mole % jacobsite, 73–91 mole % Fe3O4), baryte, and orthite. Deerite is associated with quartz, garnet (rich in spessartine, almandine, and andradite), phengite, aegirine, magnetite (containing up to 12 mole % jacobsite), and crossite/riebeckite. Deerite contains 33.0–34.4 wt. % SiO2, 0.53–5.5 wt. % TiO2, 0.09–0.31 wt. % Al2O3, 11.9–33.9 wt. % MnO, 17.5–44.2 wt. % FeOtot, and 1.5–2.8 wt. % MgO. MnO contents correspond to 29–77 cation % Mn2+ per total R2+ cations. Deerites from Andros are thus closer to the hypothetical end-member mangano-deerite, Mn122+Fe3+6Si12O40(OH)10, than any other analysis of deerite so far published.

Specimens collected from a small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, have unusual bulk compositions and mineral assemblages. One sample consists of phlogopite enclosing blue spinel (mg 96) with minute granules of corundum and sapphirine at the margins; a second of phlogopite enclosing porphyroblasts of corundum and peraluminous sapphirine. In the first the sapphirine is close to the 7 : 9 : 3 composition; in the other it is markedly peraluminous, e.g. (Mg1.628Fe0.028)Al4.714Si0.636O10, intermediate between the 7 : 9 : 3 and 3 : 5 : 1 members. The texture suggests that this sapphirine is a stable phase in equilibrium with eastonitic phlogopite and corundum. The very potassic, very magnesian bulk composition of the rocks is attributed to potassium metasomatism of a protolith consisting of magnesian chlorite and quartz.

A method of base metal exploration in sedimentary rocks which depends on detecting anomalous amounts of the light hydrocarbon gas methane in the host rocks has been applied in a number of orientation surveys in the British Isles and in Europe. The results of these studies are described, and the limitations of the method discussed. It is suggested that the principal difficulty lies in the extraction technique which is based on heating, and alternative methods are proposed. The preliminary results from these new methods are discussed and compared with those from the original study. A number of the samples analysed by the new methods are calcites of differing diagenetic origins. It is postulated that the differences in methane found in these samples may give a clue to the organic geochemistry of the rocks at the time when the calcites were deposited.

Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims.

Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastnäsite. These are briefly described and microprobe analyses presented.

Recent TEM observations of two-phase microstructures and associated crystal defects in selected, rare dolomites have been extended to calcite-structured (R3̄c) carbonates and to other natural and synthetic carbonates that crystallize with the dolomite (R3̄) structure. The samples included siderites (FeCO3), smithsonites (ZnCO3), ankerites (Ca[Mg,Fe](CO3)2), and kutnahorites (Ca[Mn,Fe](CO3)2).

TEM methods show that the forms of second phases which result from the presence of common, divalent, metallic impurities are morphologically similar in R3̄c and R3̄ carbonates and occur more widely than hitherto realized. The most common form consists of thin ribbons of second phase which are coherent with and have the same crystallographic orientation as the host carbonate. Another form of microstructure, manifest as modulations in diffraction contrast, appears to be associated with incipient breakdown of single-phase carbonate. The results of extensive TEM/EDS microanalyses show that in siderite and ankerite the formation of ribbons is promoted by Ca impurity or Ca excess (with respect to R3̄c stoichiometry). In smithsonite, Cu and Ca impurities can play similar roles in relation to modulated microstructures. In kutnahorites, the perfection of grains and the absence of second-phase effects is strongly dependent on the ratio of Fe to Mn but is also affected by Ca in excess of the stoichiometric requirement. Electron diffraction results from several of the minerals show c-type spots, which can be interpreted as indicating ordering within basal layers of cations.

The results show that, by correlating analytical TEM data with the study of second phases and incipient two-phase microstructures, it should be possible to determine the limits of solid solubility in carbonate systems.

The form and thickness of the ferrihydrite coating on kaolinite basal surfaces was studied in mixes containing 1–10% ferrihydrite through a combination of TEM, shadowed replicas, selected area electron diffraction, and determinations of surface area and distribution of charge on the surfaces. At pH3 the ferrihydrite was found to attach initially as single particles to certain sites on the kaolinite. These particles grew laterally and vertically until with 4% ferrihydrite they were aggregates of from 8 to 25 layers of particles. With more ferrihydrite the aggregates coalesced. Despite the irregular cover on the kaolinite satisfactory electron diffraction patterns were obtained and as little as 2% ferrihydrite was detected in a mixture at pH3.

Conversion of randomly ordered illite-smectite to ordered illite-smectite in the Upper Jurassic Kimmeridge Clay Formation from the North Sea has been recorded in the literature as occurring within the ‘oil window’ and has been suggested as an indicator of oil source rock maturity. Studies of authigenic clay minerals in the fine fraction (>0.5 µm) of the Kimmeridge Clay Formation mudstones from fourteen locations along the UK onshore outcrop between Dorset and North Yorkshire show that they comprise mainly ordered illite-smectites. The onshore Kimmeridge Clay section is organically immature, suggesting that the illite-smectite ordering reaction cannot be extrapolated between basins as an inorganic indicator of ‘oil window’ levels of maturity. These results also have important implications in source rock hydrocarbon expulsion and migration models which involve shale dewatering as a flushing agent. However, dewatering of shales may aid migration as it could cause fracturing of the shale bands separating the organic-rich layers within the source rock, prior to hydrocarbon generation.

The occurrence and mineral chemistry of apatite, magnesian ilmenite, and an Na-Fe-Ti oxide in lavas from the Leucite Hills are reported. Magnesian ilmenite and apatite occur as xenocrysts and as crystals in amphibole-mica-pyroxenite xenoliths. Na-Fe-Ti oxide and also rutile occur as inclusions in ilmenite. The latter mineral contains up to 7.2% MgO and shows evidence of oxidation by, and reaction with, the host magma. The apatite differs from that which occurs as phenocrysts and microphenocrysts inasmuch as REE were not detected. The occurrence of these minerals, which are important repositories for REE and High Field Strength elements, together with phlogopite in the upper mantle source regions of ultrapotassic lavas, is important and may explain some unusual aspects of the geochemistry of such lavas (low K/Rb, P2O5/Ce, Ti/Zr, high Ti/V, Zr/Nb). The source regions must be grossly heterogenous and a two-component model is suggested for the source. This model is similar to that suggested for the source regions of other alkaline magmas and is capable of explaining the unusual Nd-Sr isotopic characteristics of the Leucite Hills lavas.

Delindeite and lourenswalsite are two new barium titanosilicate minerals found as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite is found as aggregates of flake-like crystallites in compact spherules, light pinkish grey in colour, with a resinous, pearly lustre. The flakes are biaxial positive with average n ∼ 1.813; the measured density is 3.3 g/cm3. Electron diffraction revealed a monoclinic unit cell in space group C2/m or subgroup, with a = 21.617(13), b = 6.816(5), c = 5.383(3) Å, β = 94.03(5)° (refined from X-ray powder data). The strongest X-ray lines are (hkl, dobs, Irel): (200, 10.80, 100); (311, 3.54, 24); (6̄01, 3.083, 28); (601, 2.888, 31); (2̄21, 2.806, 20); (910, 2.262,18). The crystals are submicroscopically twinned on (100) and also produce additional continuous diffraction streaks parallel to a*, which double the b and c axes. The formula derived from electron and ion probe analyses (H2O by difference), as constrained by density and molar volume data, is approximately (Na,K)2.7(Ba,Ca)4(Ti,Fe,Al)6Si8O26(OH)14, with Na > K, Ba ≫ Ca, Ti ≫ Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in rosettes, silver grey to light brownish grey in colour. The crystals are biaxial negative with very low 2V angle. Indices of refraction are nα = 1.815, nβ ≈ nγ = 1.840; the measured density is 3.17 g/cm = 1.840; the measured density is 3.17 g/cm3. X-ray and electron diffraction show a sharp pseudohexagonal lattice with a = 5.244 Å, but extremely diffuse diffraction streaks normal to the hk0 plane. In these streaks a period of 20.5 Å can be discerned. A hexagonal unit cell with a = 5.244(2) Å, c = 20.49(3) Å can be refined from the powder diffraction data but does not account for some lines, probably because of extreme layer disorder as shown by precession single-crystal patterns. The strong X-ray powder lines are (002, 10.22, 20); (-, 3.93, 20); (111, 2.608, 100); (300, 1.5145, 80); (220, 1.3111, 25). The formula given by microprobe analyses, constrained by density and molar volume data, is approximately (K,Ba)2(Ti,Mg,Ca,Fe)4(Si,Al,Fe)6O14(OH)12 with K > Ba, Ti ≫ (Mg,Ca,Fe), Si > Al > Fe; Z = 1. These minerals are formed under oxidizing weathering conditions, and iron is assumed to be in the Fe3+ state.