Mountkeithite [(Mg,Ni)9(Fe3+,Cr,Al)3(OH)24]3+[(CO{in3,SO4)1.5(Mg,Ni)2(SO4)2(H2O)11]3− occurs with pyrite, magnetite, magnesite, hexahydrite, and morenosite in a low-temperature hydrothermal vein system in the Mount Keith disseminated nickel sulphide deposit, Western Australia. Electron-microprobe analyses gave the following average composition: MgO 31.7, NiO 6.1, CuO 0.1, Cr2O3 6.1, Fe2O3 8.3, Al2O3 2.6, SO3 14.7; microanalysis for C and H gave CO2 3.9, H2O 30.6; total 104.1. The X-ray powder diffraction pattern has the following strongest lines: 11.30(10), 5.63(8), 4.63(2), 3.765(6), 2.645(4), 2.545(4), 1.554(5), and 1.505(3); these can be indexed on a hexagonal cell with a = 10.698 Å, c = 22.545 Å.

Mountkeithite occurs as friable aggregates and rosettes composed of soft, pearly to translucent, pale pink to white flakes with perfect basal cleavage. Optical properties are uniaxial negative with ɛ = 1.51 and ω = 1.52; weakly dichroic from colourless to palest pink. Specific gravity is 2.12 (calc. 1.95). Mountkeithite is soluble, with efferves-cence, in dilute HCl. Mountkeithite changes to a pyroaurite-like phase, with a 7.8.Å basal spacing when immersed in water, but can be changed back to an 11.3 Å phase by immersion in 1M MgSO4; mountkeithite also collapses to a 7.8 Å basal spacing under vacuum. Phases similar to mountkeithite have been synthesized by immersing grains of stichtite in 1M MgSO4, at 20, 50, 105, and 200°C.

The structure of mountkeithite is interpreted as consisting of positively charged brucite-like hydroxyl layers, containing both divalent and trivalent cations, separated by about 7 Å-thick, negatively charged interlayers containing carbonate and sulphate anions, magnesium sulphate, and water. Carrboydite, motukoreaite, and hydrohonessite are minerals with similarly large basal spacings of 10–11 Å, and are also believed to have pyroaurite-related structures with expanded interlayers.

Minute crystals of columbite occur in tungstate ore in the quartz veins at Carrock Fell Mine. Accompanying wolframite crystals show pronounced niobium enrichment adjacent to growth faces. The columbite grains, however, occur as inclusions in minerals formed during metasomatic replacement of the wolframite. They are chemically variable, containing 7–18 % WO3 by weight, up to 1.9% Sc2O3, and up to 4.7% TiO2. Tin, a common minor element in columbite, was not detected. The columbite grains are spatially associated with carbonate minerals involved in wolframite replacement, and it is suggested that their formation occurred principally when carbonate precipitation caused a reduction in the stability of dissolved niobium complexes.

FREE energies of formation of boleite, Pb26Cu24 Ag9Cl62(OH)47·H2O, and pseudoboleite, Pb5Cu4 Cl10(OH)8·2H2O, have been determined from solution studies at 298.2 K. ΔG°f values for the minerals are −19097.9±4.1 and −3705.4±5.5 kJ mol−1 respectively. These values, together with results of earlier studies (Humphreys et al., 1980) have been used to construct the stability field diagram shown in fig. 1. The boundaries for boleite and pseudoboleite are shown separately. If fields for the two minerals are plotted together, boleite has no thermodynamic stability at the silver ion activity chosen. At higher activities of Ag+(aq) the boleite field is negligible in extent.

The results can, however, be rationalized in terms of kinetics of mineral formation, rather than thermodynamic considerations alone. Since pseudoboleite is never found without boleite upon which it is observed to grow epitaxially (Winchell, 1963), it is clear that boleite must form metastably prior to any pseudoboleite deposition. Accordingly, boleite has a large range of solution compositions, from which it may precipitate. The hatched area of fig. 1 shows this at the Cu2+(aq) and Ag+(aq) activities chosen. The field is terminated at high aCl− by the AgCl line, above which silver is precipitated as AgCl(s), chlorargyrite.

It is also evident from the chemical studies that the deposition of several assemblages in the leadcopper-chloride group of minerals is simply related to variations of chloride activity. With decreasing aCl− the associations cumengeite + boleite + pseudoboleite, diaboleite + boleite + pseudoboleite, and diaboleite + chloroxiphite are expected in turn. This relationship is apparently borne out by field observations of occurrences of the minerals.

The authors report the first occurrence of awaruite in Pakistan, in the Sakhakot-Qila ultramafites. Electron-microprobe analysis reveals a considerable variation in the compositions of awaruites from different parts of the complex. The presence of copper in awaruite, in amounts up to 5 atomic percent, is also reported for the first time. In one chromitite body, an iridian awaruite with up to 27 wt% of iridium occurs in association with ‘normal’ awaruite and a new Ru-Os-Ir-Ni-Fe alloy. The awaruites are discussed in relation to those from other localities.

In the extensive literature dealing with the growth twinning in crystals, two approaches have been followed. Friedel (1926) related twinning to a lattice common to both parts of the twin, while others have considered how the atoms at the junction must be packed to give substantially the same interatomic distance and bond angles as in the Z single crystal. The second approach has been adopted in this paper.

This investigation has been carried out by constructing ball-and-spoke models of the various twins. The balls constituting the boundary between the two parts of the twin have been drilled to accord with the full symmetry of the twinned crystal, while the rest of the model has the normal symmetry of the untwinned crystal. The orientation of the two sets of crystallographic axes is determined by the twin law. Only when it was possible to construct a model in which the environment of the boundary balls closely approximated to the normal environment, was the proposed atomic arrangement accepted.

Recent studies on the development of Sn-W mineralization in south-west England (Sheppard, 1977; Simpson et al., 1979; Jackson et al., 1979) indicate that the hydrothermal fluids dominantly consisted of local meteoric waters. These waters were heated up and drawn into the granite in a convective hydrothermal system. It is proposed that the fluids leached Sn and W from minerals such as the dark micas, during their passage through the granite. These metals were then deposited in veins at higher levels in the system.

New X-ray and optical studies show that thomsonite has a < b < c and its optical orientation is α = b, β = c, and γ = a. The refined cell dimensions of Old Kilpatrick thomsonite are a = 13.051, b = 13.092, and c = 13.263 Å. The systematic absences h+l odd for h0l, h + k odd for odd for hk0, and l odd for 0kl uniquely determine the space group as Pcnn for which the arrangement of the alumino-silicate framework is suggested. The exceptions to the above absences are the 704 reflection (due to Renninger effect) and many weak hk0 reflections which suggest that the space group might be Pcnm or Pcn2. The observed habit, cleavage, and parallel growth are discussed in the light of the proposed framework structure.

The advantages of polished ultra-thin sections (PUTS) in the study of very fine-grained materials, such as occur in some meteorites, have been illustrated by Fredriksson et al. (1978) whose technique is based on the earlier work of Beauchamp and WiUiford (1974). An essential feature of such methods for friable and heterogeneous materials is the use of a medium, usually an epoxy resin, to consolidate and partially impregnate them. Normally one polished side of the specimen is bonded to a glass slide during preparation, and the finished PUTS are integral with the slide on completion. PUTS are typically 2-5 microns in thickness.

This report extends the work of Corrigan and Gibb (1979) who determined losses of Fe and Na from a typical basaltic liquid during 1-atmosphere experiments using the wire-loop method (utilizing both Pt and Ag40Pd60 wires) by electronmicroprobe analyses of the quenched glasses. They found that Na losses were much more serious than Fe losses, the former being in direct linear proportion to the length of the experiment and in inverse proportion to the mass of the sample, such that the Na loss was 25 % after 120 hours for samples of 60 mg. Even using samples with weights in excess of 200 mg would mean that Na losses would be unlikely to be less than 5 % except in experiments shorter than a few days or on samples with very low initial Na contents.

Considerable recent interest has been shown in a group of double-chain silicate phases which includes the new mineral zektzerite, LiNaZrSi6O15 (Dunn et al., 1977; Ghose and Wan, 1978), and a family of isotypic phases (Marr and Glasser, 1979a and b). These phases are closely related structurally to Na2Mg2Si6O15 (Cradwick and Taylor, 1972), tuhualite, (Na,K)2Fe2+2Fe3+2Si12O30.H2O (Merlino, 1969), and emeleusite, Na2LiFeSi6O15 (Upton et al., 1978).