A silver-palladium alloy with structural formula close to AgPd has been found in laterite from the Bahia lateritic gold deposit. The alloy occurs in a void of an iron oxide nodule, associated with goethite and hematite. The angular shape and protuberances of the alloy grains suggest crystal growth in a lateritic environment, indicating that the alloy is a secondary mineral precipitated during lateritisation. The oxidation of sulphides of the parent rocks probably favoured the migration of palladium and silver as transient thiosulphate and sulphite complexes. Destruction of the thiosulphate and sulphite ligands could result in precipitation of both palladium and silver as an alloy. Eh-pH phase diagrams for Pd-H2O-C1 and Ag-H2O-C1 systems show that both palladium and silver are stable in lateritic environments under lower redox potentials. Such an environment may exist at the top of the ferruginous zone due to the abundant organic matter near the surface.

Zr-rich pyrochlore crystals in carbonatite from San Vicente (Cape Verdes Islands) show cryptic, concentric and rhythmic chemical zonation with Ca increasing and Ti, U and Zr decreasing towards the rims. In one carbonatite, taken from the Camile dyke, previously undocumented cryptic sector zonation of Ti, U and Zr is also observed in these crystals. The chemical variation is investigated by wavelength-dispersive electron microprobe, with both single spot and crystal map analyses.

The concentric and rhythmic zonation, marked by element substitution, was generated by magma heterogeneity and/or element diffusion kinetics, but it is suggested that the sector zonation, marked by differential site substitution, was governed by protosite variation between octahedral and cubic faces.

Olivine-plagioclase corona textures occur in ophitic to sub-ophitic olivine gabbros at Black Hill, South Australia. Contrasting with many corona and symplectite textures previously described, these do not involve spinel or garnet as reaction products and did not form under high-pressure conditions. Rather, the coronas formed at no more than 1 kbar pressure and are composed of a shell of orthopyroxene around the olivine often succeeded by a shell of amphibole or occasionally biotite. Beyond this, a vermicular symplectite of anorthite containing orthopyroxene and rarer amphibole vermicules extends out to host plagioclase of labradorite composition. Textural relations are used to infer a subsolidus igneous origin for all but the orthopyroxene shell which may have formed in the presence of some magma. Compositional zonation is absent from all the constituent phases except the amphibole shell which is strongly zoned in Mg# and may have a late origin. An average maximum corona width of 150- 200 μm indicates a limiting distance for subsolidus chemical diffusion. The corona products involve the reactants olivine and plagioclase in the proportions 1:3 and symplectite formation may have been promoted by a Na potential gradient. The system must also have been open to minor components including H2O and TiO2, with H2O possibly being derived from a hydrothermal system. Such systems may have been set up in the country rocks on intrusion of the magma and subsequently collapsed inwards into the pluton during sub-solidus cooling.

Högbomite is reported from two upper-amphibolite and granulite-facies, sapphirine-bearing, Al–Mg–Fe-rich and silica-poor lens-shaped layers within the Bamble Sector, south Norway. Primary assemblages, indicating peak metamorphic conditions of 773-844°C at 7 kbar (Mg–Fe exchange thermometry), are spinel–sapphirine–biotite–gedrite, spinel–corundum–sapphirine–cordierite and orthopyroxene–biotite–cordierite–plagioclase. Högbomite formed by hydrous alteration and oxidation of primary spinel and rutile and/or ilmenite according to the generalised reaction:

Suggested conditions of högbomite formation are 550–620°C and 6–7 kbar. The högbomites contain 10.2–14.7 wt.% MgO, 04).3 wt.% ZnO, 58.9–62.1 wt.% Al2O3 and 15.6-17.6 wt.% Fe as FeO. The two högbomite may belong to different polytypes, as suggested by their differing TiO2 (9.9-10.1 versus 5.7–5.8 wt. %) and calculated Fe3+ – and H2O–contents. The partitioning of Zn between spinel and högbomite is not uniform and is considered to depend upon prevailing fo2 and aH2O.

A blue-green scaly coating on a slaty specimen from the Bastenberg mine, near Ramsbeck, Germany, yielded an X-ray powder pattern akin to schulenbergite. Chemically it is the zinc analogue of schulenbergite. Electron-probe data, XRF and ICP/OES analyses are presented which demonstrate two distinct zinc-rich compositions. Thermogravimetric analysis, together with CHN results, suggest the phase may contain 3-4 water molecules. Powder data are indexed on a hexagonal cell with a 8.293(2) Å, c 7.247(4) Å and cell volume 431.6(2) Å3, with Z = 1.

Clinotobermorite, Ca5Si6(O,OH)18·5H2O, has been found as a vein-forming mineral in gehlenitespurrite skarns at Fuka, Okayama Prefecture. It is associated with tobermorite, plombierite, apophyllite, and calcite. The clinotobermorite is colourless or white and occurs as tabular or acicular crystals. It is monoclinic with the space group Cc or C2/c. The unit cell dimensions are a 11.331, b 7.353, c 22.67 Å, β 96.59°. Microtwinning and stacking disorder on (001) are observed. On heating the clinotobermorite at 300°C the 002 spacing is reduced from 11.3 to 9.3 Å. Its refractive indices are α 1.575, β 1.580, γ 1.585, and the density 2.58 g/cm3 (meas.), 2.69 g/cm3 (calc). The Moh's hardness is 4.5. Calculation of the analytical data on the basis of six tetrahedral cations shows that this mineral has a simplified chemical formula Ca5.3Si6(O,OH,F)18·5H2O. The chemical composition and the unit cell are closely related to those of tobermorite. It is most likely that clinotobermorite is a low-temperature polymorph of tobermorite.

Coronas containing Ca-amphibole with aluminous minerals have been characterised optically and by scanning electron microscopy, analytical transmission electron microscopy and electron-probe microanalysis. The layers nearest to plagioclase are amphibole + epidote + kyanite, followed by amphibole + epidote + staurolite + spinel. These assemblages are consistent with waterundersaturated conditions, possibly at lower metamorphic grade than the commoner assemblage amphibole + spinel. Observed mineral proportions and compositions were used in a seven-layer model of steady-state, diffusion-controlled growth with local equilibrium. This model is not fully realistic, because the observed amphibole is strongly zoned from tschermakitic to actinolitic away from plagioclase, suggesting disequilibrium. However, the four-mineral layer has been successfully modelled assuming local equilibrium, with diffusion coefficients Lii larger for i = FeO and MgO than for SiO2, AlO3/2, CaO and FeO3/2. Retarded grain-boundary diffusion of the latter components is explicable by crystal-chemical effects. The number of minerals per layer is constrained by a modified form of the metasomatic phase rule of Korzhinskii, with the role of 'inert' components played by relatively immobile ones (having relatively small fluxes and relatively small diffusion coefficients).

New stability constant data are presented for the minerals blixite, mendipite and the compound Pb7O6Cl2.2H2O at 298.2 K and P = 105 Pa. Mendipite is in fact a metastable phase at this temperature, being thermodynamically stable under the appropriate conditions at temperatures above about 29°C Kinetic influences are of some significance with respect to the sequence of formation of solid phases in the PbO-HCl-HH2O system, and these have been elucidated for some important reactions. Penfieldite and fiedlerite appear to be metastable phases at all temperatures at 105 Pa. The results have been used to reassess the conditions of formation of the lead(II) oxy- and hydroxychloride phases that are known to form as minerals and as corrosion products of lead-containing artefacts. The effect of CO2 on the system is also described.

An iron-rich vesigniéite, [Ba(Cu,Fe)3(VO4)2(OH)2] with Cu : Fe near 4 : 1, was identified in samples from the submarine-exhalative Gorob-Hope copper deposit, Namibia. The olive-colour mineral occurs as mammillary encrustations and vein fillings in the gossan zone of this deposit; it often has a brownish oxidation coating. The main X-ray lines are 3.21 Å (100) [204, 113], 2.72 Å (63) [313,002] and 2.94 Å (61) [020, 311]. The cell parameters are a = 10.236, b = 5.905, c = 14.253 Å and B = 103.71°. The formation of vesigniéite took place during the alteration of the ore under strongly alkaline and slightly reducing to oxidizing conditions. While copper and barium were derived from the submarine exhalative mineralisation, vanadium was leached from the countryrock, a micea schist.

Minute CaMg-bearing rhodochrosite inclusions were discovered in spessartine crystals of small coticule globules enclosed within redschists that form the country rock of the famous coticule bands of Belgium. This points to a former metamorphic reaction relationship between the two minerals, such as rhodochrosite + pyrophyllite = spessartine + quartz + CO2 + H2O. The coexistence of spessartine with Mg-chlorite in the coticule globule could also indicate more complex reactions such as rhodochrosite + sudoite + quartz = spessartine + Mg-chlorite + CO2 + H2O. Our finding casts some doubt on the suggestion by Kramm (1976) that spessartine formed from Mn-montmorillonite in previously glassy volcanic tufts.

The stabilities of antierite, Cu3SO4(OH)4, and a synthetic compound whose stoichiometry is here established as Cu3SO4(OH)4.2H2O, have been determined at 5°C intervals between 10°C and 45°C using solution methods. The results of the experiments show that antlerite is stable with respect to the compound Cu3SO4(OH)4.2H2O only at temperatures above 30°C Below 30°C a change in the relative stabilities of these two basic copper(II) sulfates occurs, and the compound Cu3SO4(OH)4.2H2O, although unknown at present as a mineral, instead, is stable. Under these conditions, it does not react to give antlerite if kept in contact with water. Once isolated from its mother liquor, however, the dihydrate undergoes rapid dehydration to yield antlerite. Thermodynamic quantities for the two phases have been derived from equilibrium measurements. At 298.2K, values of AfG° for the compounds Cu3SO4(OH)4.2H2O(s ) and antlerite are −1919.6(14) and −1445.0(10) kJ mol−1, respectively. The results have been used to construct stability field diagrams for the system CuO—H2O–SO3 at 25°C and at 35°C These diagrams have been used to illustrate the chemical conditions under which the two compounds might be expected to form in the oxidised zones of cupriferous base metal orebodies.

The true formula of abhurite is Sn21Cl16(OH)14O6. A stability constant for the phase has been determined at 298.2 K. For the reaction Sn21Cl16(OH)14O6 (s) + 26H+ (aq) ⇋ 21Sn2+ (aq) + 16Cl− (aq) + 20H2O (1), log KH+ (298.2 K) is equal to -39.9 (7). This value is used to assess the relative stability of abhurite in the natural environment and to evaluate its modes of occurrence in relation to other secondary Sn(II) and Sn(IV) species.

The new mineral tooeleite occurs with pyrite, arsenopyrite, scorodite, and jarosite at the U.S. Mine, Tooele Country, Utah. Analysis by electron microprobe gave Fe2O3 43.56%, As2O5 38.61%, SO3 7.24%, and H2O by the Penfield method 9.80%. For this analysis, the suggested formula is Fe8-2x3+[(AS1-xSx)O4]6(OH)6.5H20; x is about 0.2.

Crystals are orthorhombic, perhaps Pbcm or Pbc21, with a = 6.416, b 19.45, c = 8.941 Å. The strongest diffraction lines are [d in Å, (hkl), (I)] 9.75 (020) (10); 3.208 (200,132) (9); 3.047 (061,220) (5); 4.476 ({002) (4) and 2.680 (240,113) (4). The mineral is optically negative with a small 2V; α = 1.94, β = 2.05, γ = 2.06. Hardness = 3, Dmeas = 4.23, Dcalc = 4.15 g/cm3 for Z = 2.

The rare fluophosphate minerals wagnerite, ideally Mg2(PO4)F, and isokite, ideally CaMg(PO4)F, are intimately associated with magnetite-hematite deposits in sillimanite-, garnet-, and pyroxene-rich paragneisses and migmatites at the Benson Mines, near Star Lake in the west-central Adirondack Highlands of New York State. Coarsely crystalline wagnerite occurs in lenticular masses, typically 4 × 8 cm, delineated by sharply cross-cutting, sinuous, 2 cm-wide veins of fine-grained, fibrous to platy isokite and granular fluorapatite. These also penetrate transverse fractures across wagnerite lenses. Isokite formed from the introduction of Ca- and O-rich hydrothermal solutions into wagnerite. Both minerals are monoclinic: wagnerite crystallises in space group P21/a with a = 11.945, b = 12.717, c = 9.70 Å, β = 108.18°, V = 1400.2 Å3, D(calc) = 3.291 g/cm3 for Z = 16; isokite crystallises in space group A2/a with a = 6.909, b = 8.746, c = 6.518 Å, β = 112.20°, V = 364.7 Å3, D(calc) = 3.248 for Z = 4. Optical properties for wagnerite are: α = 1.5845, β = 1.5875, γ = 1.6010, 2V = 51°(calc.) disp = r < v weak, absorption α < β > γ with α = col., β = pale yel., γ = v. pale yel. For isokite only a mean index of refraction, n = 1.598, could be measured. Wet chemical analysis of wagnerite containing a calculated 11.4% of isokite as fine lamellae, gave the formula:

Crystallographic studies of micro-inclusions in diamond coat, carried out by analytical electron microscopic techniques, show that structures possessing rhombohedral symmetry form a significant fraction of the population of well-crystaUised particles. They are, however, less frequent than apatite or biotite, which were identified earlier by the same investigative methods. Thin-foil specimens of diamond coat were prepared by mechanical fine-polishing, and subsequently ion-beam milling, sawn and polished plates of coated diamonds oriented parallel to (100) or (110). The majority of crystalline inclusions analysed were ≤0.2 p,m in diameter. Data obtained on 15 individual inclusions, including composition analysis by energy-dispersive X-ray spectroscopy, are reported and discussed. The a-axis of the hexagonal unit cell, and the c/a ratio, were determined for all specimens, and are believed to be accurate to ≈1% in most cases. In 12 out of 15 specimens, the cations with Z ≥ 11 identified comprised substantially only Mg, Fe and Ca. Values of a in these specimens ranged from 4.81 to 4.92 Å; and for three of them the space group R, corresponding to the dolomite structure, was positively identified. These crystals are classed as ankerites. Representative Mg:Fe:Ca ratios in the ankerites are 28 : 18 : 54, with cell dimensions a = 4.87 Å, c/a = 3.37. The ankerites contained small amounts of Ba, typically 1-3%, and smaller amounts of Sr, typically 0.2-2%. Three out of the 15 specimens contained >10% Ba. In two of these, Ba and Ca concentrations were roughly equal, and in one of the two, which had cell parameters a = 5.11 Å, c/a = 3.50, the space group symmetry Rc, corresponding to the calcite structure, was verified.

Chernikovite, (H3O)2(UO2)2(PO4)2, was synthesized and subsequently transformed into metauranocircite II, Ba(UO2)2(PO4)2.6H2O, by ion-exchange experiments. The crystals are lemon-yellow and strongly fluorescent. From solubility data a mean pKs value at 25°C was determined as 51.79 ± 0.21. For a cell volume of 16.31.7 Å3 and Z = 4 the density was calculated as 3.970 g/cm3 whereas the measured density is 3.964 g/cm 3. The X-ray powder pattern shows the following strongest lines: 3.609(60)(114), 3.470(60)(123), 2.9272(90)(132)2.9006(90)(312), 2.6065(80)(116), 2.4581(75)(233), 2.2015(100)(143 ; 143), 2.1869(100) (241 ; 226 ;241).

The uncommon sodium rare-earth phosphate mineral vitusite-(Ce) (Na3RE(PO4)2) can be considered as the extreme product of sodium and rare-earth substitution in the apatite structure. Lesser amounts of substitution provide sodium and rare-earth-bearing apatites up to about 80 mol.% exchange; beyond this point vitusite is the stable phase. The structure of vitusite, determined previously from a synthetic analogue, can also be considered as a derivative from apatite, but with cations exchanged on sites normally occupied by anions. Vitusite can therefore be considered as a sodium- and rare-earthrich apatite end-member, with a distinct, but apatite-derived, structure, formed in highly persodic and high rare-earth environments. From an examination of the literature on diffusion in apatite, vitusite in principle could be formed from apatite sensu stricto by subsolidus diffusion in response to late-stage Naand RE-rich hydrothermal fluids.

Spodumene pegmatites, aplites and schists from the Aclare Li prospect of SE Ireland host sporadically developed, small, red, euhedral garnets. Chemical data indicate that the pegmatite- and aplite-hosted garnets are rich in spessartine and are similar to garnets in many African Li-rich pegmatites. Partial melting as a source of the garnets is unlikely due to the large spessartine component, the abundance of inclusions within the garnets and the general P-T-X evolution of the pegmatite body. It is suggested that an aqueous fluid was exsolved during pegmatite crystallisation and complexed available Mn. Movement of this fluid to the intermediate zone of the pegmatite body produced areas of high Mn concentrations which permitted the crystallisation of spessartine-rich garnets at low pressures (<3 kbar).

Pressures of 10 GPa and above can bring about phase transformations in many oxides, an effect of great interest to geochemists and geophysicists. We can interpret such behaviour as due to the differential compressibility of 'anion' and 'cation' leading to a progressive rise in radius ratio with pressure, and hence, on the classic crystallochemical picture, eventually to an increase in co-ordination number (though with complications which make prediction difficult). More generally, pressure affects Gibbs free energy G directly; for oxides a pressure of 5 GPa gives, very roughly, the same contribution to G as 100°C in temperature (though with opposite sign). Thus high pressure significantly affects the shape and structure of phase diagrams, showing increasingly important effects above, say, 10 GPa—but again prediction can be difficult. However these two complementary approaches to the effects of pressure, helpful though they can be conceptually, are 'crystal-based' and totally neglect another rather littleknown but potentially important effect--the formation of amorphous solids; 'polymers' and glasses. Since amorphous materials are 'non-equilibrium' they are not readily dealt with theoretically; also, since they are difficult to detect by standard crystallographic techniques, they can be overlooked experimentally. The pressure-induced formation of amorphous solids could have significant implications for both geochemistry and geophysics.

At the Ashanti concession, Ghana, gold-bearing quartz veins and disseminated sulphide lodes occur in narrow (1-3 m) shear zones with altered argillites and metatholeiite host rocks. The mineralisation is concealed by up to 10 m of kaolinite-mica forest ochrosol soils, beneath which is a saprolitic zone of leached rock extending down 60-70 m to the hypogene ore zone. In the unweathered hypogene orebody, gold occurs as free grains in quartz, as sub-microscopic inclusions in the disseminated arsenopyrite, as gold tellurides and as aurostibite. The gold is released from the hypogene orebody by physical dissaggregation and chemical dissolution, the latter involving hydroxyl, thiosulphate, cyanide, and fulvate complexing. Dissolution and reprecipitation of the gold appears to have taken place largely in situ with little evidence of supergene enrichment. Consequently, the gold mineralogy of the soils is complex with residual and secondary gold grains exhibiting widely different textural and compositional characteristics. Residually enriched grains display pitted, rounded surfaces and have silver-depleted rims, while supergene gold grains are compositionally homogenous and have unpitted surfaces. The supergene grains display platelet, dendritic, irregular and octahedral habit. A fine grained spongy form of gold has also been observed from weathered telluride-bearing quartz veins. Much of the secondary gold is intergrown with iron oxides and hydroxides. The gold mineralogy of the Ashanti soils appears to be controlled by physico-chemical processes active during the lateritic pedogenesis producing residual and supergene enrichment of gold.