Three groups of minerals are discussed, which are typified by pyroaurite and sjögrenitc, hydrocalumite, and ettringite. All show interesting structural features. In the pyroaurite-sjögrenite group, brucite-likc layers carrying a net positive charge alternate with layers in which the oxygen atoms of carbonate groups and water molecules are statistically distributed on a single set of sites; other anions may replace the carbonate, especially in synthetic phases. Hydrocalumite and related synthetic phases also have layer structures in which positively charged main layers alternate with intermediate layers containing anions and water molecules; the anions occur in cavities and their nature can again vary widely. In the ettringite group, the structures are based on positively charged columns, between which occur channels containing anions and sometimes also water molecules. This group includes thaumasite, the only natural mineral known to contain silicon six-coordinated by oxygen that is not a high-pressure phase. The thermal dehydration behaviour in each group is briefly considered.

Three-dimensional counter-diffractometer data corrected for absorption and a full matrix least-squares method have been used to refine the crystal structure of an oxy-kaersutite in the space group C2/m. The chemical composition of the amphibole is with cell parameters a 9·8920(11), b 18·064(2), c 5·3116(7)Å and β 105·388(5)°. Cation site occupancies were refined using bulk chemical constraints and the A-site positionally disordered. The final conventional R-factor for 1041 observed reflections is 4·0%.

The rotation of the tetrahedral chains in calcic clino-amphiboles is shown to be a function of the Aliv content of the tetrahedral sites. Comparison of this oxidized amphibole with an hydroxyamphibole of similar chemical composition indicates that on oxidation the maximum amount of structural relaxation takes place in the vicinity of the M(I) octahedra, which distort to a high-energy configuration.

‘Blue John’ andother purple fluorites have been studied by a wide range of techniques. Hydrocarbons are not the cause of the colour of these fluorites. No significant difference in the concentrations of 73 elements between the purple and colourless zones of Blue John has been detected. The spectroscopic and thermal bleaching behaviour of the purple zones is consistent with the presence of ‘colloidal’ calcium as colouring agent, and coloration of fluorites with calcium vapour gives material showing similar properties. The distribution of colour suggests trapping of the calcium in lattice defects along {111} (cleavage) planes through {001} (growth) zones. The ‘colloids’ could be produced by aggregation of calcium atoms liberated by irradiation from sources found in the geological environment of Blue John. The banding could arise from variations in the concentration of lattice defects caused by variations in growth rates of the fluorite.

The mole fractions of the silicons present in melilites as monomeric (SiO4) and dimeric (Si2O7) groups depends both upon the composition of the melilite and also upon the degree of Al-Si ordering. It is shown that the absolute numbers of monomeric and dimeric silicons in a 10 mg polycrystalline sample can be determined by a relatively rapid chromatographic method.

A statistical theory is developed to treat the data; this contains a variable k-parameter that describes the degree of ordering.

Experimental results are presented and interpreted for one natural and five synthetic melilites. The synthetic gehlenite-rich melilites are appreciably, but not completely disordered. The natural melilite is well ordered, but heating at 1050 °C introduces considerable disorder.

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 behaviour of 25 major and trace elements, together with mineral changes, has been examined in a deep weathering profile on the Carnmenellis granite, Cornwall. The bulk chemical and mineralogical composition of the regolith shows only minor changes from the parent rock indicating a low degree of chemical weathering. However, within the regolith marked differences occur between the fine fractions from the various horizons. Chlorite and degraded muscovite characterize the fines of horizons A+B and C whereas gibbsite and kaolinite characterize horizon D. Secondary minerals becoming base deficient with depth is the reverse of normal behaviour and two possible theories are advanced to explain this anomaly. Most of the trace elements are concentrated in the < 10 µm fraction of the various horizons, in part by an association with degraded muscovite. Tin and Zr occur in resistates and Be, Sr, and Co are mainly lost during weathering.

The behaviour of iron in the weathering of biotite in a sedentary soil profile developed on appinite has been investigated using the Msbauer effect. Both Fe2+ and Fe3+ in sites with cis and trans hydroxyl groups were observed at various stages in the weathering sequence. The results indicate that there is little or no ordering of the octahedral cations in the fresh biotite and hydrobiotite, but in the more weathered interstratified vermiculite-chlorite samples Fe3+ concentrates in the more distorted sites. No evidence was obtained for the presence of an anisotropic recoil-free fraction.

A series of albites was synthesized in the presence of aqueous NaOH fluids (concentration 0 to 4N NaOH, Pfluid = 15000 1b/in2) over the temperature range 300 to 500 °C. At fixed temperature-pressure conditions increasing NaOH concentration of the initial fluid can be correlated with decreasing obliquity of the albites.

Chemical analyses, cell parameters, and optical data are presented for conjugate cummingtonite and hornblende, which occur in a paragneiss. A metasomatic origin for the development of hornblende from cummingtonite, during retrogressive metamorphism, is proposed.

Biphosphammite, ammonian KH2PO4, syngenite, aphthitalite, monetite, hannayite, whitloekite, apatite and minor gypsum, taylorite, brushite, mirabilite, guanine, and unidentified compounds form crystalline crusts on the walls and floors of a chamber of Murra-el-elevyn Cave, Cocklebiddy, Nullarbor Plain, Western Australia. The minerals are derived from the interaction of bat urine, droppings, and limestone. Analyses for CaO, MgO, Na2O, K2O, SO3, P2O5, CO2, NH3, H2O-, total N, and ignition loss are given. High K2O and low total N figures are characteristic of the analyses.

Two occurrences of rutile are reported from Big Bell in Western Australia containing up to 5·8 wt % WO3, 1·7 wt % Sb2O5, and 1·7 wt % FeO. One occurs in a mica schist, and the other in a pegmatite, and both contain a fine intergrowth of ilmenite lamellae. Small grains of gudmundite are associated with the rutile in the pegmatite. Microprobe analyses do not conclusively establish the mechanism of isomorphous substitution.

Three generations of homogeneous titanomagnetite in a hawaiite from Saint-Clément may be defined by differences in size, habit, or reflectance, and are believed to correspond to different crystallization stages of the lava. Chemical compositions were determined by electron microprobe, and by X-ray and thermomagnetic methods.

Larger crystals are of intratelluric origin and occur either as inclusions in phenocrysts of early clinopyroxene (generation 1) or separately in the groundmass (generation 2); in the latter case, they always show evidence of resorption. Post-eruptive titanomagnetite (generation 3) is smaller and so highly oxidized that it is better termed titanomaghemite. Since the maghemitization, which is a low temperature process, alters the metallic ratios (especially the Fe/Ti ratio) the metallic contents at the time of the high-temperature crystallization are exactly known only for the intratelluric titanomagnetites.

As intratelluric crystallization proceeds Ti content increases greatly (5 to 14%); Mn also increases (but slightly), A1 and Mg decrease, while results for Cr are inconclusive. Apparently these changes go on until the post-eruptive crystallization stage.

The titanium trend is contrary to common belief. However, it is in accordance with predictions from the Fe-Ti-O system and may be explained by a decrease in oxygen fugacity during magma ascent.

Rhombohedral cleavage faces of quartz have been etched hydrothermally and with fused KOH. Stratigraphical etch patterns were observed on isolated cleavages, matched cleavage faces, and opposite sides of a thin flake. The angle between the () cleavage plane and the planes responsible for the stratigraphical patterns was almost equal to the interfacial angle, 46° 16′ between adjacent rhombohedral faces. It is concluded that the stratigraphical patterns formed by preferential etching reveal the edges of weak planes deposited during growth. The implications are discussed.