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.

Quetzalcoatlite is a new mineral from the Bambollita mine in the Sierra La Huerta near Moctezuma, Sonora, Mexico. The colour is Capri blue (R.H.S. 120A) with a pale streak. H = 3; G meas. = 6·05±0·3 G calc. = 6·12. Crystals up to 0·4 mm long were not measurable but are acicular to prismatic, hexagonal, with fair cleavage on {100}; a 10·097 Å, c 4·944, space group C632. Z = I. Strongest lines are 8·752 (100), 2·748 (70), 2·520 (42B), 3·531 (40), 3·273 (31), 1·766 (28), 5·054 (21), 2·250 (20). Electron probe analysis gives: CuO 20·3%, 18·9, 20·9; ZnO 41·2, 38·2, 38·7; TeO2 29·4, 31·3, 30·6. Water by Penfield method on 2.265 mg gave 10·1%. Empirical cell contents are Cu4·01Zn7·73Te3·04O8·88(OH)17·89 or Cu4Zn8(TeO3)3OH18. Pleochroic with ε = colourless, ω = pale blue-green; uniaxial (-) with εD = 1·740, ωD = 1·802. Named for the Toltec god Quetzalcoatl.

Tertiary alkali basaltic rocks from the Southern Highlands, New South Wales, contain two types of clinopyroxene crystals, which exhibit excellent morphological development of hour-glass zoning. These are small, quench clinopyroxene crystals and large, abundant, euhedral phenocrysts. Electron microprobe analyses show that the different zone sectors derive from compositional differences, with variation in TiO2 and Al2O3 contents inversely with SiO2 content being most significant. Increased depth of colour of titanaugites depends on increased coupled substitution of Al for Si in tetrahedral sites and of Ti3− in an M1 site, while pleochroism is enhanced by increasing Fe content. It is suggested that the formation of hour-glass zoning is determined by an environment of crystallization where initial crystal growth is rapid relative to ionic diffusion in the melt, resulting in different chemistry for different growth directions within the crystal. This implies that, under certain conditions, kinetic factors, as well as the chemical composition of the host magma, may be significant in determining the chemical composition of phases precipitated, particularly where more than one crystallographic direction of growth is possible.

Chemical analyses of the Lorne volcanic rocks show that the basalts and andesites are a closely related suite of lavas, notably rich in alkalis, especially potassium. The rhyolites of the Lorne area do not appear to form a continuous series with the more basic rocks. The various lava types are compared with the plutonic rocks of the area, and a relationship is suggested between the basic lavas and appinitic intrusions. The acid lavas are not comparable to granites in the area.

The late orogenic igneous activity in this part of the Caledonides therefore appears to involve three generations of magma, produced separately but within a short time of one another. The assumption that all the late Caledonian igneous rocks of the Scottish Highlands are differentiates of a common parent magma is not justified.

Glaukosphaerite, a new secondary basic copper-nickel carbonate was first determined and described from Widgiemooltha (31° 30′ S., 121° 34′ E.), Western Australia, by R. C. Morris at the W.A. Government Chemical Laboratories in 1967. The mineral has since been found at the nickel mines at Kambalda, Windarra, Scotia, Carr Boyd Rocks, and St. Ives, Western Australia, and is apparently an indicator of copper-nickel sulphide mineralization. The name is derived from the colour and spherulitic formation. Associated minerals are goethite, secondary quartz, paratacamite, gypsum, nickeloan varieties of magnesite and malachite, and clays. Glaukosphaerite also fills joints in fresh basic rocks.

The type material is from Hampton East Location 48, 3 km N. of the Durkin Shaft, Kambalda. Glaukosphaerite is monoclinic, a 9·34 Å, b 11·93 Å, c 3·07 Å, β 90–1°, space group indeterminate, c axis disordered, six strongest X-ray powder lines are 2·587 (10b) 201, 3·68(7b) 220, 2·516(4) 240, 211, 5·04(3) 120, 2·124(3b), 1·473(3). The mineral occurs in green spherules of fibres cleaved and elongated along c, D = 3·78 to 3·96 increasing with Cu, is brittle, H. 3 to 4, and has dull to subvitreous to silky lustre.

α 1·69–1·71 green, β ≈ γ 1·83–1·85 yellow-green, α:[001] 7°.

Chemical analysis on the type material, D 3·78, containing 0·5 % goethite impurity and minimal inseparable malachite, gave CuO 41·57, NiO 25·22, CoO 0·07, ZnO 0·02, Fe2O3 0·47, MgO 1·23, SiO2 < 0·01, CO2 21·70, H2O+ 9·85, H2O− < 0·01, sum 100·13%. After deduction of goethite the unit cell contains 4[(Cu,Ni,Mg,Co)1.74C0·96H2·12O4·73]. This unit-cell content is deficient in metal ions and oxygen and contains excess H, when compared to 4[(Cu,Ni)2(OH)2CO3]. Two analyses of malachite-bearing glaukosphaerite from Widgiemooltha and Carr Boyd Rocks show the same deficiencies and excess not previously recorded in malachite, rosasite, or aurichalcite analyses. Glaukosphaerite, 4[(Cu,Ni)2(OH)2CO3] is the nickel analogue of rosasite 4[(Cu,Zn)2(OH)2CO3].

From results based on X-ray rotation patterns of oriented rosasite fibres from Sardinia and Durango with supplementary electron-microprobe studies and Guinier powder-film data, the authors consider glaukosphaerite and rosasite to be separate mineral species from the related malachite, 4[Cu2(OH)2CO3].

Type material is preserved in the government collection at the Government Chemical Laboratories, Perth, Western Australia.

The crystal structure of buttgenbachite from Likasi (Congo) was solved by a three- dimensional Patterson function, computed using intensity data collected photographically by the Weissenberg method, and refined by successive Fourier maps and least-squares refinement to an R index 0·055 for 328 independent observed reflections. The cell content resulting from the structural analysis is Cu36·6Cl6·7(NO3)2·6(OH)63·9.2·1 H2O. The space group is P63/mmc with a = 15·750 Å and c = 9·161Å. The buttgenbachite structure is not completely ordered. It consists of a rigid three-dimensional skeleton formed by Cu co-ordination polyhedra sharing edges and corners. The large channels in the framework show a zeolite-like nature; inside the channels the disorder is caused by the different orientations of nitrate groups and by the occurrence of the substitution . The crystal structure of buttgenbachite is closely related to that of connellite. The mechanism of forming an isomorphous series is discussed: the present study shows that the replacement is more complicated than was supposed in the literature.

The mineral chernovite, described as a new mineral from the Urals by Russian mineralogists in 1967 (Goldin et al, 1967), has been found at three different localities in mineral fissures in the Binnatal, Switzerland, and a little to the south, in Italy. The mineral occurs in greenish-yellow bipyramidal crystals up to about I mm. A detailed single-crystal study of several specimens showed that the lattice constants vary considerably on either side of those published for the original chernovite. The refractive indices, too, display some variation. Microprobe analyses of this material, and of xenotime specimens from the same region, revealed that there exists at least partial solution between chernovite and xenotime. The three chernovite samples have molar percentages of 63·2, 73·2, and 82·0 % YtAsO4 (the Urals chernovite has about 95·2 %). These studies prove the existence of an isomorphous series between xenotime and chernovite, at least in the As-rich portion.

The liquidus temperature (1198 °C) and equilibrium phase relations of a sample of Columbia River basalt from the Picture Gorge section have been determined at I atmosphere by heating in a controlled atmosphere. When this basalt is cooled from above its liquidus temperature the liquidus phase (plagioclase) may fail to crystallize depending on the degree of undercooling and the duration of the experiment. A field in temperature-time space in which plagioclase fails to crystallize on cooling is separated from another in which plagioclase always crystallizes by a third in which the nucleation of plagioclase is unpredictable in terms of temperature and time. The extent to which this basaltic liquid can be supercooled without the crystallization of plagioclase is independent of the time it is held above the liquidus or the temperature in excess of the liquidus to which it is heated.

The exceptionally long times required to ensure the nucleation of plagioclase at or near the liquidus temperature suggest that many so-called ‘equilibrium’ phase relations determined from experiments of a few hours' duration could be in serious error if the ‘equilibration’ involves a nucleation process.

It is demonstrated that, over a range of cooling rates, the temperature at which plagioclase begins to crystallize on cooling varies markedly and the temperature and times required for both possible and certain nucleation of plagioclase are calculated for a range of constant cooling rates. The range of cooling rates over which the nucleation temperature of plagioclase varies is likely to occur in nature only in certain lava flows and small minor intrusions. In such cases this could lead to changes in the order in which the minerals appear on cooling and other petrologically significant effects.

The location and some characteristics of nine veins containing nigerite are reported. These veins are part of the province of tin-tantalum veins of central Amapá. They are intrusive in andalusite schist and commonly consist of pegmatite in the centre and of quartz-mica rock at the margins. The nigerite can occur in the pegmatite as well as in the quartz-mica rock; its presence in a sample of quartz-sillimanite rock is also mentioned. In both pegmatite and quartz-mica rock, the common associated translucent heavy minerals are cassiterite, tourmaline, chrysoberyl, and occasionally andalusite, sillimanite, and garnet. Associated minerals in the quartz-sillimanite rock are cassiterite and chrysoberyl. Nigerite and cassiterite commonly occur together but are not found in the presence of gahnite. A possible explanation for this is given.

The nigerite of Amapá occurs as thin hexagonal plates, transparent and golden brown, amber-yellow or colourless, with diameters from microscopic up to a few millimetres. The sizes and the optical sign of this nigerite are the same as those of Nigeria and of Siberia, whereas Portuguese nigerite occurs in very fine grains and is mostly optically negative. The existence of alluvial nigerite is reported.

Cesbronite occurs at the Bambollita mine, near Moctezuma, Sonora, with a variety of other tellurites. The colour is ‘green beetle’ (R.H.S. 135-B) with a pale streak. H 3; Dmeas 4·45±0·2. Crystals are orthorhombic 2 mmm with a 8·624 Å, b 11·878, c 5·872 (all ± 0·016 Å); space group Pbcn. For Z = 2 the calculated density is 4·455 g/cm3. The strongest powder lines are 5·934 (100), 3·490 (92), 4·889 (71), 2·358 (70), 2·379 (38), 1·592 (34), 2·156 (28), and 1·698 (27). Electron-probe analysis gave: CuO 50·3 %, 50·3, 49·8, 49·4; TeO2 39·3 %, 39·2, 38·6, 38·6. Water determined gravimetrically 11·0 %. This gives Cu5(TeO3)2(OH)6.2H2O.

Cesbronite is pleochroic with absorption γ ≈ β ≫ α. Optically positive with 2Vγ calc. 72°; α 1·880 ‖[100], β 1·928 ‖[001], γ 2·029 ‖[010].

The history of the chemical analysis of minerals is shortly reviewed. Following rapid developments between 1770 and 1810, few important changes took place for close on 130 years; new procedures led to simpler and more convenient separations but not, on the whole, to greater accuracy. The older analyses are, in general, neither worse nor better than modern ones, and analyses can only be assessed on internal evidence, if available, or on the reputation of the analyst.

The development of microchemical and colorimetric techniques in the last 40 years has resulted in a great reduction in the amount of material required for a mineral analysis; 5 to 20 mg is usually sufficient, and in favourable cases a complete analysis may be possible on less than I mg. Some modern instrumental methods are considered, and it is concluded that they cannot wholly replace the classical wet-chemical methods in their modern developments.

W. F. Hillebrand (1894) deplored the tendency of some petrologists to demand large numbers of very incomplete rock analyses, and his remarks are considered apposite today.

The Galway Granite often has a highly siliceous aphyric alkali granite at the margin, followed inwards by a K-feldspar phenocrystic adamellite and then a steeply layered granodiorite. This previously puzzling arrangement, which seemed to conflict with the necessity to cool from the periphery and crystallize the most basic granite at the margin, is examined by a traverse of chemically analysed rocks and biotites. The Mg/(Mg+Fe+Mn) values indicate that the granodiorite probably crystallized from the outside inwards. The marginal aphyric granite crystallized from the acid residuum in the nearly solid adamellite. This was drawn out of the adamellite by blocks of country rock falling into the adamellite and creating a zone of rarefaction behind them. This also explains the absence of a chilled margin and the aplitic texture and mineralogy of the aphyric granite. The adamellite largely accumulated by crystal settling with gravity grading but the granodiorite crystallized in vertical layers during upward vertical movements of the magma that tipped up the gravity layering in the adamellite. Scattered microdioritic xenoliths are postulated to be disrupted dykes. Eighteen granite and twenty-seven biotite analyses are tabulated.

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.

The structure of julgoldite, , has been determined and refined to an R value of 4·6 % based on 2417 symmetry-independent reflections, of which 765 are unobserved (weighted R = 4·3 %). The cell dimensions are a 8·922(4), b 6·081(3), c 19·432(9) Å, β 97·60(6)°, the space group is A2/m with Z = 4 and Dcalc 3·56 g cm-3. Julgoldite is isostructural with pumpellyite; the Si-O and Ca-O distances of julgoldite are identical, within experimental accuracy, with corresponding distances in pumpellyite; the Fe-O distances of julgoldite are about 0·09 Å longer than the corresponding (Al, Mg)-O distances in pumpellyite. The four hydroxyl groups and the extent of O-for-OH substitution are identified by valence-sum calculations.

‘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.

A new variety of heterogenite was found in samples from Mindigi, Shaba, Zaïre. Photometric reflectance varies from 0·235 to 0·165 at λ 5890 Å in air. The bireflectance is 0·070, the degree of anisotropy is 0·7 and the main index in air is 2·7. A thermal weight-loss curve and a microprobe analysis are given and discussed, indicating the formula (Co,Ni) O.OH.

The new variety, referred to as heterogenite-H, is hexagonal with cell dimensions a 2·85, c 8·80, space group P63/mmc and cell-content 2 [(Co, Ni)O.OH)]. Indexed X-ray powder data are given. A crystal structure is suggested based on CoO.OH octahedra layers and indicating a polytypic relation between heterogenite-H, the 2H form, and normal heterogenite, the 3R form.

Grain boundary migration and the exsolution of albite from alkali feldspar, followed by the precipitation of myrmekite from the same source on to adjacent plagioclase grains, will explain the reversed zoning of these phases in contrast to the normal zoning seen in igneous rocks.