First-principles simulations and high-pressure experiments were used to study the stability of BaCO3 carbonates at high pressures. Witherite, which is orthorhombic and isotypic with CaCO3 aragonite, is stable at ambient conditions. As pressure increases, BaCO3 transforms from witherite to an orthorhombic post-aragonite structure at 8 GPa. The calculated bulk modulus of the post-aragonite structure is 60.7 GPa, which is slightly less than that from experiments. This structure shows an axial anisotropicc ompressibility and the a axis intersects with the c axis at 70 GPa, which implies that the pressure-induced phase transition reported in previous experimental study is misidentified. Although a pyroxene-like structure is stable in Mg- and Ca-carbonates at pressures >100 GPa, our simulations showed that this structure does not appear in BaCO3.

The formation of ilmenite from titanomagnetite frequently shows separate generations of production. The ilmenite products can be combined with the magnetite host to represent the intermediate stages in this development. Use of the Buddington and Lindsley (1964) geothermometer in conjunction with this method provides a number of points in the temperature and oxygen-fugacity history of a single, complex magnetiteilmenite grain.

Application of the method is illustrated by an example from the Freetown layered gabbro. A titanomagnetite exsolved granular ilmenite whilst cooling and this process ceased at 930° and an oxygen fugacity of log ƒ O2 = −11·5. Further cooling and exsolution continued until the grain reached 662 °C and log ƒ O2 = −19·0, producing distinct ilmenite lamellae.

It is suggested that the name shoshonite be revived for a group of potash-rich basalts and that the magma from which monzonites, potash-rich syenites, many lamprophyres, nepheline monzonites, shonkinites, eovites, ijolites, leucite basalts, leucitophyres, etc., have crystallized be called the shoshonite magma-type. This series shows parallels with the alkali basalt magma series, and the question of the relation of the two magma-types is discussed.

The name latite is discussed and rejected as a name for this potash-rich magma type. It is further shown that rocks of this composition invade geosynelinal regions that have been recently stabilized, one group of shoshonites being associated with an environment that is still subjected to minor folding and showing some relation to the cale-alkaline rocks, the other a true alkaline suite associated with non-orogenic conditions.

It has recently been shown by W. Nieuwenkamp that matlockite is identical in chemical composition and crystal-structure with artificial lead fluochloride PbFC1. His conclusion is based upon powder photographs of the two substances and a fluorine determination of a specimen of matlockite from Matlock, Derbyshire. The present work was undertaken primarily to check Nieuwenkamp's interesting results. Access to an exceptionally fine suite of matlockite specimens in the British Museum collection made possible single crystal X-ray measurements, accurate optical determinations, and a new chemical analysis.

Studies are described of various specimens of diamonds made by the General Electric Company of New York, and by Allmänna Svenska Elektriska Aktiebolaget, Västeras, Sweden. The G.E. synthetic diamonds always contain single crystal inclusions of nickel or of a Ni-rich face-centred cubic compound. These are strictly parallel to the diamond surrounding them. If the nickel inclusions are twinned on (111), the diamond is similarly twinned; and this is usually the case. It is suggested that epitaxial growth is part of the mechanism of the graphite-to-diamond transformation in the G.E. technique. Other powdered or single-crystal inclusions are also found; some of those correspond to compounds that occur in meteoritic carbon. There is no parallel growth in such cases. The Swedish synthetic diamonds do not contain nickel, but they are generally less well-crystallized than the G.E. specimens. Both, however, give good diffraction spots indicating a well-ordered structure with (except very rarely) no trace of graphite present.

Brunogeierite, Fe2GeO4, a = 8.4127(7) Å, V = 595.4(1) Å3, is a rare germanate spinel from Tsumeb, Namibia. Its structure has been refined to an R index of 2.2%. The oxygen parameter, u, is 0.2466(1), indicating nearly ideal cubic close-packing of oxygen atoms. There is exact agreement between the observed a unit-cell dimension and that calculated from the observed Ge–O and Fe–O bond lengths. The cations Ge and Fe are fully ordered at tetrahedral (A) and octahedral (B) sites, respectively, in keeping with synthetic germanate spinels, all of which are fully-ordered normal spinels.

Cámaraite — ideally Ba3NaTi4Fe8 2+(Si2O7)4O4(OH)4F3 — is triclinic, space group C , a = 10.6965(7) Å, b = 13.7861(9) Å, c = 21.478(2) Å, α = 99.345(1)°, β = 92.315(2)°, γ = 89.993(2)°, V = 3122.6(4) Å3, Z = 4, D calc. = 4.018 g cm–3, from the Verkhnee Espe alkaline deposit, Akjailyautas Mountains, Kazakhstan, has been solved and refined to R 1 5.87% on the basis of 6682 unique reflections (Fo >4σF). The crystal structure of cámaraite can be described as a combination of a TS block and an intermediate (I) block. The TS (titanium silicate) block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a minimal cell based on translation vectors t 1 and t 2, with t 1 ~5.5 and t 2 ~7 Å and t 1 ^ t 2 close to 90°. We describe the crystal structure of cámaraite using a double minimal cell, with 2t 1 and 2t 2 translations. In the O sheet, there are eight [6]-coordinated M O sites occupied mainly by Fe2+ and Mn, with minor Fe3+, Mg, Zr, Ca and Zn with <MO–ϕ> = 2.185 Å. Eight M O sites give, ideally Fe8 2+ p.f.u. In the H sheet, there are four [6]-coordinated M H sites occupied almost solely by Ti (Ti = 4 a.p.f.u.), with <M H–ϕ= = 1.963 Å, and eight [4]-coordinated Si sites occupied solely by Si, with <Si—0> = 1.621 Å. The topology of the TS block is as in Group II of the Ti-disilicates (Ti = 2 a.p.f.u. per minimal cell) in the structure hierarchy of Sokolova (2006). There are six peripheral (P) sites, four [8–12]-coordinated Ba-dominant A P sites, giving ideally 3 Ba p.f.u., and two [10]-coordinated Na-dominant B P sites, giving ideally 1 Na p.f.u. There are two I blocks: the I 1 block is a layer of Ba atoms (two A P sites); the I 2 block is a layer of Ba (two A P sites) and Na atoms (two B P sites). Along c, there are two types of linkage of TS blocks: (1) TS blocks link via AP cations which constitute the I 1 block, and (2) TS blocks link via common vertices of MH octahedra (as in astrophyllite-group minerals) and A P and B P cations which constitute the I2 block. Cámaraite is the only mineral of Group II with two types of linkage of TS blocks and two types of I blocks in its structure. The relation of cámaraite to the Group-II minerals is discussed.

Oxygen isotope fractionation between rutile and water has been studied from 300 °C to 700 °C, P H2O = 1 kb, using aqueous oxidation of titanium metal as the equilibration reaction. The mechanism of rutile formation (which is critical to the assessment of isotopic equilibrium) is an ‘armouring’ reaction in which rutile grows around grains of titanium metal by solution-precipitation processes. Mean fractionation factors expressed as 103 In αTiO2-H2O obtained in the present study are:

−6.20±0.23‰ at 304±5 °C

−6.64±0.27‰ at 405±6 °C

−6.11±0.16%. at 508±6 °C

−4.45±0.28%. at 608±6 °C

−3.38±0.15%. at 698±6 °C.

These data agree with those obtained at temperatures above 500 °C by Addy and Garlick (1974) but do not accord with theoretical predictions by Bottinga and Javoy (1973). A minimum in the calibration curve 103 ln α versus 106 T −2 occurs between 300 °C and 500 °C but from 500 °C to 700 °C 18O fractionation between rutile and water may be expressed by the equation:

103 ln α = −(4.72±0.40)106 T −2+(1.62±0.53).

Oxygen isotope analyses of rutile and quartz from metamorphic eclogites and schists from the Tauern Window, Austria, yield isotopic temperatures at about 550 °C in agreement with results obtained on similar rocks from the Sesia Zone (Western Alps, Italy) and elsewhere by other workers. Petrologic studies indicate that the latest metamorphism of the Tauern eclogites reached about 450 °C Thus the measured partitions of 18O between rutile and quartz indicating temperatures around 550 °C have been inherited from an earlier metamorphic event.

Ortho- and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures and variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers and serpentine. Chemical and geothermometric data suggest that the pyroxenes crystallized at 1240–1051°C, followed by subsolidus exsolution at slightly lower T (1145–1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs and a 1033–982°C range for orthopyroxene lamellae + clinopyroxene matrix pairs).

Investigation by transmission electron microscopy of exsolved enstatite and augite reveals a multistage hydration process. The first stage (highest T, probably in the amphibole stability field) leads to the formation of biopyribole lamellae within exsolved augite, leaving the enstatite matrix unaffected. The second stage (~500–300°C) corresponds to the topotactic replacement of enstatite by layer silicates (talc + chlorite + serpentine, with (001)layer silicates parallel to (100)enstatite). Enstatite is also replaced by randomly oriented, poorly crystalline serpentine. The last hydration stage (<300°C) corresponds to the disappearence of augite and recrystallization of serpentine, leading to completely hydrated bastites with random lizardite lamellae, polygonal serpentine and minor chrysotile.

The Llandovery Folly Sandstone near Presteigne, Powys, contains disseminated thorium mineralization. The thorium occurs as inclusions of thorite and thorianite within sub-millimetre-scale bitumen nodules throughout the sandstone. The nodules grew replacively in situ. The two square kilometres outcrop/subcrop of the sandstone may contain over 500 tonnes thorium.

Retrograde metamorphism of gneisses and pegmatites leads in part to the destruction of feldspar and its replacement by late-stage lobate myrmekite and muscovite. Reactions promoted by retrogression suggest a range in volume of quartz production that may supplement that developed by exsolution and lead to deviations from the strict proportionality relationship suggested by previous workers. There is no need, however, to propose that quartz in myrmekite originates by constriction of pre-existing quartz within exsolved albite.

Pyrrhotites in polished sections from more than twenty stratabound massive sulphide and magmatic nickel-copper deposits in Norway were studied under the microscope using the magnetic colloid method. In both types of deposits, two distinct styles of intergrowths between monoclinic and hexagonal pyrrhotites were found: crystallographically-controlled lamellar intergrowths and fissure-controlled irregular intergrowths.

Lamellar intergrowths consist of crystallographically oriented monoclinic lamellae occurring in a hexagonal matrix and were produced originally by exsolution from hexagonal pyrrhotite on cooling. Irregular intergrowths comprise blades and patches of monoclinic pyrrhotite occurring along fissures and grain boundaries of hexagonal pyrrhotite, and were formed by interactions between hexagonal grains and sulphur-rich hydrothermal solutions.

Increase in lamella thickness and spacing, development of lamella zonations, wedge-shaped composite ends, boxworks and composite lamellae were caused by progressive lamellae coarsening and maturation during natural annealing, which could have been promoted by anisotropic stress. Metamorphic recrystallization and annealing tend to homogenize pyrrhotite and erase preexisting exsolution lamellae.

Ancient ivory, from the Chengdu Jinsha and Guanghan Sanxingdui sites in China, has been buried for several thousand years. In order to determine the degradation mechanisms and to provide a scientific basis for protecting them, these ancient ivory samples have been compared with modern ivory using infrared spectroscopy in the frequency range 400–4000 cm–1. By combining chemical analysis data we compare the crystallinity and crystal chemistry of the apatite component, as well as the structural characteristics of the ivory. These investigations showed that the ancient ivory consists almost entirely of hydroxyl-carbonate apatite as the predominant phase. Compared with the modern ivory, the PO43– and CO32– bands are stronger, the PO4RF values are obviously greater, and an extra OH– band at 3569 cm–1 is observed in the ancient ivory. These results indicate that there is a greater degree of apatite crystallinity in the ancient ivory and also imply that there has been incorporation and recrystallization of CO3 2– in the apatite during burial. Positive correlations have been found between the apatite crystallinity, CO3 2– and OH– ion contents, and burial time. The organic matter in ancient ivory has been lost or decomposed as the organic bands (e.g. at 1238 cm–1 and 1337 cm–1) have disappeared. This may be the main reason that ancient ivory is easily dewatered and readily friable after being unearthed.

An X-ray and chemical examination of deweylites reveals that they are intimate mixtures of very poorly ordered trioctahedral 2:1 and 1:1 layer silicates. The 2:1 mineral exhibits no swelling in the presence of ethylene glycol or water and is best described as an extremely fine-grained and highly disordered form of talc, for which the term kerolite is often used. Stevensite is not a component of deweylites. The 1:1 component most closely resembles a disordered chrysotile.

Evaluation of the chemical data, including values for H2O +, shows that both end-members are hydrated to various degrees. This hydration is most probably attributable to the presence of water molecules and hydroxyls associated with the large surface areas and unbalanced surface bonds. Formulae for the end-members are approximately R 3Si4O10(OH)2·0·3 0·7 H2O and R 3Si2O3(OH)4·0·3–0·7 H2O where R is principally Mg, but samples with more or less H2O are entirely possible due to variations in crystallinity. No specific formula can be given to deweylites as they are mixtures with variable proportions of the components. The name is useful, however, as a field or ‘box’ term, similar to the use of garnierite and limonite.

Uraniferous bitumens from Great Britain, Scandinavia and South Africa have been studied by oilimmersion reflected-light microscopy and categorised into those formed either by replacement of preexisting uraninite and pitchblende or by complexation/reduction mechanisms in pre-existing hydrocarbons. The former are characterised by displaying normal replacive textures, and containing high concentrations of non-mineral-bound uranium, or later, occasionally exotic, uraniferous fracturefilling phases. Uraniferous bitumens formed during complexation/reduction reactions display monotonous mineralogies and ordered mineral-inclusion distributions.

Radiolytic alteration of uraniferous bitumens induces both chemical and mechanical alteration. Early alteration is marked by the generation of mobile hydrocarbons during 'cracking reactions' with subsequent within-sample migration to form globular bitumens and dendritic interspersions of mineralrich and -poor uraniferous bitumen. Mobile hydrocarbons may act as lubricants during mechanical deformation. Advanced organic alteration is characterised by well-documented increased reflectance around uraniferous grains, and by fracturing of the bitumens.

Detrital chrome-spinels are contaminant grains within ilmenite concentrates produced from heavy-mineral deposits along the coast of southeast Africa. The presence of even minor levels of chromia in the predominantly ilmenite-rich concentrates, downgrades their market value as potential feedstocks for the production of titania pigment. An understanding of their composition can assist in their removal from the ilmenite concentrates.

Compositions from a database of close to 900 chrome-spinel analyses shows the major element components and their ranges (in wt.%) are: Cr: 0.4-45.3, Al: 0.0-31.0, Fe: 8.5-69.6 and Mg: 0.0-12.2. Minor components include Ti: 0.1-11.4 and Zn: 0.0-13.7.

The chrome-spinel data fall into two compositionally distinct groups. The first group of spinels is dominated by a strong trend reflecting the mutual substitution between Al3+ and Cr3+ in the spinel structure. The second group of spinels is characterized by compositions containing abundant Fe3O4magnetite component. The clear division between chrome-spinel compositional types indicates the grains are derived from at least two chemically dissimilar provenances.

The compositional differences between the chrome-spinel groups has a positive impact on subsequent ilmenite upgrading treatments as the spinels which contain the highest magnetite component are easily removed via low-intensity magnetic separation procedures.

Samples from a traverse across the Blå Måne Sø perthosite unit in the Tugtutôq Central Complex of the Gardar province, South Greenland have been examined for cathodoluminescence characteristics, microporosity and δ18O isotopic values. Reddening of cathodoluminescence colours in alkali feldspars (normally blue) from the unit may be correlated with increased microporosity of the feldspars as determined using scanning electron microscopy. δ18O values of all samples lie within the range of values expected of juvenile fluids, independent of the level of alteration indicated by cathodoluminescence studies.

Observations are consistent with previous suggestions that levels of alkali feldspar microporosity and levels of fluid alteration (as determined by cathodoluminescence of alkali feldspars) are related phenomena. Oxygen isotope ratios suggest that the fluid is largely juvenile in origin, with, perhaps, some meteoric (low δ18O) component.

Evaluation of X-ray powder diffraction data shows that argon atoms (radius 1·9 Å) lie at the centres of the large holes in the cordierite structure. The reflection intensities for hydrous cordierite differ from those for anhydrous cordierite but to a lesser degree than those for argon-bearing cordierite. Although analysis is uncertain, it appears that the smaller water molecules (radius 1·4 Å) do not occupy either the centres of the six-membered rings or the centres of the large cavities. It is possible that they may be attached to the walls of the large cavities or occur partly as hydroxyl groups in the framework. Single-crystal X-ray analysis is necessary to resolve this problem.

In the course of an investigation on the determination of soil carbonates a sample of dolomite, from the Ibis dolomite quarry, Mr. Flinders, Queensland, was examined by X-ray diffraction and found to contain palygorskite. As this appears to be the first recognition of the occurrence of the mineral in Australia and as it has a number of commercial uses (e.g. as a drilling mud, catalyst, decolorizer, and absorbent), further work was undertaken.