The structural formula of stilpnomelane averaged from 37 representative literature analyses and based on the determined structure is (Ca,Na,K)4(Ti0·1Al2·3Fe35·5Mn0·8Mg9·3)[Si63Al9](O,OH)216·nH2O. Ferrostilpnomelanes have up to 12 R3+ cations in the octahedral sheet, ferristilpnomelanes commonly have between 24 and 30 R3+ cations, rarely up to 37. The two groups can be distinguished by accurate X-ray powder diffraction, ferrostilpnomelanes having d001 less and a greater than have ferristilpnomelanes. The unit cell is triclinic, a = b∼21·8 Å, γ = 120°, d001∼12·2 Å, P. Hexagonal groups or ‘islands’ of 24 silicon-oxygen tetrahedra co-ordinate to the octahedral sheet, the centres of the ‘islands’ are displaced by 5a/12, 8b/12 on either side of the octahedral sheet as a result of warping of this sheet to permit articulation with the smaller tetrahedral sheet. The 24-tetrahedra ‘islands’ are linked laterally by 6-member rings of inverted tetrahedra, which in turn link at their apices to like rings connecting the next sheet of ‘islands’. Thus a single tetrahedral layer is 4 tetrahedra thick and has symmetry 6/m. The 6-member rings of tetrahedra act as hinges permitting up to 0·5 Å extension or contraction of the tetrahedral sheet to accommodate variations in the octahedral sheet dimensions with varying R2+/R3+ ratio.

The Zaisho meteorite, one of only four known pallasite falls, contains pyroxene, which is rare in pallasites, and it is only the fourth meteorite known to contain phosphoran olivine. Except for a recent report by Goodrich (1984) such olivine appears to be restricted to pallasites, and to those containing the rare Mg phosphate, farringtonite. It is unique among olivines in having P in tetrahedral sites and having vacancies, for charge balance, in octahedral sites. Zaisho also contains stanfieldite and a phosphate having a composition between that of stanfieldite and farringtonite. Its metallographic cooling rate is similar to that of many other pallasites, but its average olivine composition, Fa18.3, places it in a small group with more Fe-rich olivines than most pallasites.

Magnetite—apatite deposits associated with the Atacama Fault Zone of northern Chile are interpreted here, on field criteria, as being the products either of hydrothermal fluids with a strong magmatic signature, or of late-stage Fe-rich magmas mixed with an aqueous fluid. Even in the Chilean iron belt, apatite-rich magnetite deposits are a rarity. Variations in F- and Cl- contents in apatites, strongly zoned with respect to halogens, are indicative of primary variations in fHF and fHCI in the hydrothermal fluids. Small variations in halogen fugacities in the aqueous fluid are capable of buffering large variations in halogen content within apatite crystals in equilibrium with that fluid. The recorded halogen zonation profiles are inconsistent with crystallization of the apatites simply from a volatile-rich, late-stage fractionation Fe-rich magma, or its derived magmatic vapour. It is more likely that they are the result of mineral—fluid buffering with a fluid that represents the mixing of a magmatically-derived aqueous fluid with a meteoric fluid that has variably scavenged Ca and Cl from within the country rocks. The source magma of the former is probably an Fe-P enriched acidic magma, derived by fractionation of primary calc-alkaline basic magmas.

The volume proportions of the mineral phases in two strongly deformed olivine-orthopyroxene rocks have been quantified by whole-pattern stripping of fixed geometry X-ray powder diffraction data. The results were compared with the phase proportions as determined by Rietveld refinement of time-of-flight neutron powder diffraction data, and were shown to be in excellent agreement. The X-ray technique not only provides a very rapid and cost-effective method of determining phase proportions, but it also circumvents several of the problems associated with obtaining this information by image analysis. Moreover, the technique is particularly advantageous in strongly textured rocks or in rocks that contain significant residual strains. As such it offers a powerful technique for analysing the mineralogical composition of fine-grained and/or deformed experimental run products, which makes it of considerable potential for monitoring in situ the progress of mineral reactions during laboratory experiments.

Classical clay mineralogy determinations and electron microprobe analyses of weathering minerals developed in altered two-mica granites indicate that the chemical forces that produce new minerals are often constrained to small volumes, frequently on the scale of a mineral grain or contact between two grains in the granite.

Chemical potentials such as pH, alkali and alkaline earth and silica activity in the altering aqueous solutions provoke a destabilization of pre-existing minerals, which recrystallize locally to give a new multimineral product. The chemical composition of the new phases is largely governed by the relative concentrations of the elements present in the former minerals.

Three mineral facies were observed in the weathered granites: initially a sericite-beidellitic type, then a beidellite-kaolinitic type, and finally a last stage kaolinite-oxide facies assemblage. The position of each facies is not restricted to a given depth in the profile but the relative proportions of each facies found in a thin-section size sample change towards the kaolinite-oxide facies.

The global rock chemistry reflects the type facies predominant in each sample. The first two facies are roughly silica conservative while the kaolinite-oxide facies loses silica as well as alkali and alkaline earths.

Geochemical and clay mineral studies of rock alteration should consider problems of mineral genesis at very localized sites.

A high-pressure modification of NaAlSiO4, with the calcium ferrite structure was synthesized at pressures higher than 24 GPa using a double-stage split-sphere type high-pressure apparatus. The unit cell dimensions are: a=10.1546(8), b=8.6642(8), c= 2.7385(4) Å, and V = 240.93(3) Å3 with the space group Pbnm. Calculated density is 3.916(1) g/cm3. The crystal structure was determined by the ordinary powder X-ray method. Both M1O6 and M2O6 octahedra run parallel to the c-axis, forming edge-shared double chains. The shared edges exhibit remarkable shortening (2.25–2.39 Å). Sodium atoms are located in the ‘tunnel’ formed by the linked double chains, being in eightfold co-ordination. The structure is very similar to that predicted by Dempsey and Strens (1976) with the aid of the DLS method. The possible instability of the calcium ferrite type of Mg2SiO4 is discussed.

The results of reconnaissance melting experiments on Tertiary granophyres and Torridonian arkose at 1000 kg/cm2 water-vapour pressure show that the melting behaviour of these rocks can be viewed in relation to the synthetic system NaAlSi3O8-KAlSi3O8-SiO2-H2O. By analogy with this system, P-T curves are constructed for Skye granophyres and, viewed in relation to the stability of tridymite, the depth and temperature of emplacement are estimated. Further estimates of the likely depth of magma generation, and the behaviour of the country rocks at that level, lead to the conclusion that most of the Tertiary granitic rocks were produced through partial melting of Lewisian basement rocks, the heat being provided by the basic magmas available during this volcanic episode. Subsidiary amounts of granitic magma were similarly derived from higher levels, within the Torridonian arkose group, and evidence is provided for the depth and temperature at which these, together with tridymite-bearing metamorphosed arkose in Rhum, were produced.

The Late Cretaceous Itatiaia complex is made up of nepheline syenite grading to peralkaline varieties, quartz syenite and granite, emplaced in the metamorphic rocks of the Serra do Mar, SE Brazil. The nepheline syenites are characterized by assemblages with alkali feldspar, nepheline, Fe-Ti oxides, clinopyroxene, amphibole, apatite and titanite, while the peralkaline nepheline syenites have F-disilicates (rinkite, wöhlerite, hiortdahlite, låvenite), britholite and pyrophanite as the accessory phases. The silica-oversaturated rocks have alkali feldspar, plagioclase, quartz, amphibole, clinopyroxene and Fe-Ti oxides; the chevkinite-group minerals are the featured accessory phases and are found with allanite, fluorapatite, fluorite, zircon, thorite, yttrialite, zirconolite, pyrochlore and yttrocolumbite. The major- and trace-element composition of the Itatiaia rocks have variations linked to the amount of accessory phases, have smooth, enriched chondritenormalized rare-earth element (REE) distribution patterns in the least-evolved nepheline syenites and convex patterns in the most-evolved nepheline syenites. The REE distribution patterns of the quartz syenites and granites show a typical pattern caused by fractional crystallization of feldspar and amphibole, in an environment characterized by relatively high oxygen fugacity (>NiNiO buffer) and high concentrations of H2O and F, supporting the crystallization of hydrous phases, fluorite and F-disilicates. The removal of small amounts of titanite in the transition from the least-evolved to the most-evolved nepheline syenites stems from petrogenetic models involving REE, and is shown to be a common feature of the magmatic evolution of many other syenitic/ trachytic/ phonolitic complexes of the Serra do Mar and elsewhere.

Calcium silicate hydrate (C-S-H) alteration was studied with flow-through experiments at 25°C and pH 9.2. Three materials with apparent Ca/Si ratios (C/S ratios) of 1.47, 1.38 and 0.86 were investigated. Physical (thermogravimetric analyses/differential thermal analysis), mineralogical (X-ray diffraction) and chemical (electron probe microanalysis, transmission electron microscopy/energy dispersive X-ray spectrometry) analyses were performed to characterize the reacting minerals. Initial stoichiometric C/S ratios were 1.22, 1.22 and 0.85, respectively. The excess of Ca is attributed mainly to the presence of calcium hydroxide intimately mixed in with C-S-H particles.

The C-S-H chemical compositions were monitored during flow-through experiments in order to determine the mineral stoichiometry needed for reaction kinetics. Under our experimental conditions the stoichiometric C/S ratios decreased continuously with time. A close to stoichiometric dissolution was observed after 2 days of experiments. Using an integrated approach, the kinetics was found to be a function of the C/S.

A decrease in layer-to-layer distance in the early stage of the alteration process is interpreted as interlayer Ca/Na exchange (Na being part of the pH buffering solution). A second dissolution step, marked by a close to stoichiometric release of Ca and Si, undoubtedly results from layer dissolution. The structural similarity of C-S-H and tobermorite was confirmed.

Chernikovite and meta-ankoleite were synthesized with a relatively high crystallinity and the compounds were identified by means of chemical analysis and X-ray diffraction. The infrared spectra were recorded and the bands assigned. From the luminescence spectra, the band-gap energy for both compounds was calculated as 2.35 eV, indicating that they must be considered as insulators. The dependence of the solubilities of these compounds on the acidity of the solution was studied, and the dominant ionic species were determined. The pKsp values of chernikovite and meta-ankoleite were found to be 22.73±0.24 and 24.30±0.81 respectively.

The crystal structure of the high-pressure phase-II of cristobalite has been solved by neutron diffraction (space group P21/c, a = 8.3780(11) Å, b = 4.6018(6) Å, c = 9.0568(13) Å, β = 124.949(7)°, at P = 3.5 GPa). This phase corresponds to a distortion of the high-temperature cubic β-phase, rather than of the ambient temperature and pressure tetragonal α-phase.

The LGC (Leinster Granite Complex) in SE Ireland consists of five plutons or Units. Units 1 and 3 display complex magmatic disequilibrium textures described here for the first time. The textures indicative of disequilibrium include boxy cellular plagioclase, oscillatory zoning, calcic spike zones and combinations of these within individual plagioclase feldspar crystals. The textures are interpreted as having formed in a dynamic magmatic environment which facilitated acid-acid mixing on a variety of scales and at a variety of stages between source to emplacement. It is proposed that the restricted distribution of disequilibrium textures was a result of the complex strain regime operating within the crust during assembly of Units 1 and 3.

Goethites of well-crystallized appearance from a sedimentary ore deposit have been studied by Mössbauer spectroscopy and other methods following HCl corrosion, particle size fractionation, and annealing treatments.

The goethites contained an appreciable amount of Si, but were not significantly Al-substituted. The observed broad and asymmetric resonant lines of the Mössbauer spectra are considered to result from this Si which, interspersed in the goethite structure, would interrupt magnetic coupling and give rise to a reduced and inhomogeneous magnetic hyperfine field.

Measurements have been made of the magnetic susceptibility and of the polarized absorption spectra of epidotes of the Al-Fe, Al-Fe-Mn, and Al-Cr series. Magnetic susceptibility measurements at room temperature by the Guoy method indicate that Fe3+, Mn3+, and Cr3+ are in ‘high-spin’ electronic states in the epidote structure, and the electronic transitions observed in the absorption spectra have been assigned on this basis. The polarized spectra, measured over the range 4000 to 22 000 Å by a polarizing microscope using a universal stage technique, showed each epidote group to be distinctly pleochroic. This pleochroism correlates with the presence of ions in distorted coordination sites.

Extinction coefficients of Fe3+ in epidote are higher than those for other Fe (III) compounds, indicating that Fe3+ ions occupy the non-centrosymmetric (Al, Fe) site. The low value (13200 cm−1 at 0·155 Fe3+ and 12700 cm−1 at 0·915 Fe3+) of the crystal-field splitting parameter, Δ, implies that the site is easily expanded to accommodate ions larger than Al3+ There is also evidence that increasing Fe3+ content causes distortion of the (Al,Fe)O6 octahedron, suggesting a possible mechanism for the zoisite ⇌ clinozoisite transition.

The spectra of Al-Mu-Fe epidotes have been interpreted in terms of intense contributions from Mn3+ ions in the non-centrosymmetric (Al, Fe) site, and weak contributions from Mn3+ ions in the centrosymmetrie AlOH site. Values of Δ vary from 13730 cm−1 (0·082 Mn3+) to 13400 cm−1 (0·747 Mn3+). Both (Al, Fc) and AlOH sites have strong tetragonal distortions (c/a ~ 0·95), leading to the observed large Jahn-Teller splitting of 12000 cm−1.

The Al-Cr epidote spectra show weak absorption bands, attributed to Cr3+ ions in a centrosymmetric site of approximately orthorhombic symmetry (i.e. the AlO site). The value of Δ is about 16300 cm−1.

Review of all available data on the distribution of Fe3+, Mn3+, and Cr3+ between the sites gives the following site preferences: Fe3+, Al > AlO > AlOH; Mn3+, Al ≥ AlOH ≫ AlO ; Cr3+, AlO ≫ Al, AlOH.

Hydrous micas differ from normal micas in that they contain less potassium and more water. In the calculation of structural formulae from their chemical analyses, two methods have been used which rely on the following different assumptions:

1. As these minerals are often fine-grained and may contain adsorbed water, no reliance is placed on the value of H2O+ (water lost above 105° C.) as a measure of structural water. The water content is ignored and the structural formula is calculated on the basis of 20 oxygens plus 4 hydroxyls per structural unit. This method has been used by Hendricks and Ross and by Grim and Bradley.

2. The water which is recorded as H2O+ in the chemical analysis is assumed to represent hydroxyl groups in the aluminosilicate layers of the minerals. Thus, instead of having 20 oxygens and 4 hydroxyl ions per structural unit as in normal micas, formulae are derived which show less than 20 oxygens and more than 4 hydroxyls, the total oxygen plus hydroxyl remaining 24. This method has been used by Brammall, Leech, and Bannister.

The structural setting of a refoliated belt of sapphirine granulites in northern Uganda and petrography of two selected rocks are described. Electron-probe analyses of the following minerals are given: ilmenite, titanian hematite, rutile, magnetite, sapphirine, hyperstheue, brown and green biotite, garnet, and cordierite. Field and experimental data suggest the following paragenesis: deposition of ferruginous shales with siliceous bands, followed by burial and regional metamorphism under granulite facies conditions, and finally rapid unloading associated with refoliation and shearing and crystallization of sapphirine and cordierite.

The stabilityand structure of aqueous complexes formed by pentavalent antimony (SbV) with simple organic ligands (acetic, adipic, oxalic, citric acids, catechol and xylitol) having O-functional groups (carboxyl, alcoholic hydroxyl, aliphatic and aromatic hydroxyl) typical of natural organic matter (NOM), were determined at 25°C from potentiometric and X-ray absorption fine structure spectroscopy (XAFS) measurements. In organic-free aqueous solutions, spectroscopic data are consistent with the dominant formation of SbV hydroxide species, Sb(OH)5 and Sb(OH)6-, at acid and near-neutral to basic pH, respectively. Potentiometric measurements demonstrate negligible complexing with mono-functional organic ligands (acetic) or those having non-adjacent carboxylic groups (adipic). In contrast, in the presence of poly-functional carboxylic, hydroxyl carboxylic acids and aliphatic and phenolic hydroxyl, SbV forms stable 1:1 or 1:3 complexes in coordination 6 with the studied organic ligands, over a wide pH range pertinent to natural waters (3 ≤ pH ≤ 9). The XAFS measurements show that in these species the central SbV atom has an octahedral geometry with 6 oxygen atoms from hydroxyl moieties and adjacent functional groups (O = C—OH and/or C—OH) of the ligand, forming bidendate chelate cycles. Stability constants for SbV-oxalate complexes generated from potentiometric experiments were used to model SbV complexing with di-carboxylic functional groups of natural humic acids. Our predictions show that in an aqueous solution of pH between 1 and 4 containing 1 μg/l of Sb and 5 mg/l of dissolved organic carbon (DOC), up to 15% of total dissolved Sb maybe bound to aqueous organic matter via di-carboxylic groups.

Woodallite is a new Cr-rich member of the hydrotalcite group from the large, low-grade Mount Keith nickel deposit, in the northeastern Goldfields district of Western Australia. Woodallite occurs as whorls and clusters of minute platelets up to 6 mm across in lizardite+brucite-altered dunite. Individual platelets are typically 10–100 µm in maximum dimension and are often curved. Associated minerals include chromite, lizardite, iowaite, pentlandite, magnetite, tochilinite and brucite. Electron microprobe analysis gave: Mg 25.90 wt.%; Cr 10.81; Fe 4.86; Al 0.68; Cl 9.89; S 0.03; Si 0.01; Ni 0.01; Na 0.01, yielding (after correction for loss of volatiles) an empirical formula of Mg6.19(Cr1.21Fe0.51Al0.15)∑1.87(OH)16[Cl1.62(CO3)0.17(SO4)0.01]·4H2O, by analogy with the hydrotalcite group. The simplified formula is Mg6Cr2(OH)16Cl2·4H2O. Combined thermogravimetric analysis and mass spectroscopy showed a two-stage weight loss of 12.7% and 27.3% occurring over the ranges 25–300°C and 300–660°C, respectively. The first weight loss is attributed to loss of interlayer water, chlorine-bearing species (e.g. HCl) and some CO2, the second to loss of hydroxide water, remaining CO2 and Cl species. The mineral is deep magenta to purple in colour, transparent, with a resinous to waxy lustre, and a perfect basal {0001} cleavage. Woodallite has a Mohs hardness of 1.5–2, and a pale-pink to white streak. The strongest lines in the X-ray powder pattern are [dobs (Iobs) (hkl)] 8.037 (100) (003); 4.021 (48) (006); 2.679 (1) (009); 2.624 (3) (012); 2.349 (5) (015); 2.007 (6) (0,0,12); 1.698 (2) (0,1,11); 1.524 (2) (23). These lines were indexed on a hexagonal cell with a = 3.103(2), c = 24.111(24)Å, V = 201.14 Å3 and Z = 3/8. The new mineral is isostructural with the hydrotalcite group and has space group Rm. The measured density is 2.062 gm/cm3. Woodallite is uniaxial negative with ω = 1.555 and ε = 1.535 (white light); pleochroism is distinct from violet to pinkish lilac. Woodallite forms as a result of hydrothermal alteration of primary magmatic chromite by Clrich solutions at temperatures <320°C. Relict chromite fragments are frequently present in the whorls, and associated magnetite is altered extensively to iowaite. The mineral is named after Roy Woodall, eminent Australian industry geologist.

High concentrations of indium (In) and selenium (Se) have been reported in the Neves-Corvo volcanic-hosted massive sulfide deposit, Portugal. The distribution of these ore metals in the deposit is complex as a result of the combined effects of early ore-forming processes and late tectonometamorphic remobilization. The In and Se contents are higher in Cu-rich ore types, and lower in Zn-rich ore types. At the deposit scale, both In and Se correlate positively with Cu, whereas their correlations with Zn are close to zero. This argues for a genetic connection between Cu, In and Se in terms of metal sourcing and precipitation. However, re-distribution and re-concentration of In and Se associated with tectonometamorphic deformation are also processes of major importance for the actual distribution of these metals throughout the whole deposit. Although minor roquesite and other In-bearing phases were recognized, it is clear that most In within the deposit is found incorporated within sphalerite and chalcopyrite. When chalcopyrite and sphalerite coexist, the In content in sphalerite (avg. 1400 ppm) is, on average, 2–3 times higher than in chalcopyrite (avg. 660 ppm). The In content in stannite (avg. 1.3 wt.%) is even higher than in sphalerite, but the overall abundance of stannite is subordinate to either sphalerite or chalcopyrite. Selenium is dispersed widely between many different ore minerals, but galena is the main Se-carrier. On average, the Se content in galena is ~50 times greater than in either chalcopyrite (avg. 610 ppm) or sphalerite (avg. 590 ppm). The copper concentrate produced at Neves-Corvo contains very significant In (+Se) content, well above economic values if the copper smelters recovered it. Moreover, the high In content of sphalerite from some Cu-Zn ores, or associated with shear structures, could possibly justify, in the future, a selective exploitation strategy for the production of an In-rich zinc concentrate.

In Niger, uranium occurs in upper Palaeozoic and lower Mesozoic continental sedimentary basins west of the Aïr mountains, but the source of the uranium had not been identified. Geochemical studies and fissiontrack observations on alkaline ignimbrites preserved in two Palaeozoic anorogenic centres in Aïr show that uranium is concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it is concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile. Tectonic uplift, anorogenic magmatism, followed by weathering and erosion of the volcanic cover, with sedimentation in nearby continental basins, have all contributed to the development of uranium mineralization in Niger. The petrological and geochemical similarities between the Palaeozoic ring complexes in Niger and the Nigerian Mesozoic ring structures suggest that sedimentary uranium deposits may also be found in Nigeria if the tectonic and sedimentological controls were favourable.

Enriched concentrations of uranium have been discovered in the exposed granitic roof zones of the Nigerian subvolcanic centres, with Th/U ratios approaching unity. Thus local vein deposits of uraninite, as well as dispersed uranium in recent sedimentary horizons, could be discovered particularly in the drainage systems entering the Chad Basin.