Li-mica (zinnwaldite and/or lepidolite)—topaz—albite granites in the Tregonning—Godolphin pluton and similar rocks in the St. Austell pluton appear to be petrogenetically unrelated to the spatially associated biotite granites. Evidence is provided by lack of development of Li-mica granites at roof zones of biotite granites and markedly different trends and composition fields in bivariate plots such as Li vs. Cs, Rb vs. Sr and Nb vs. Zr. Thus, differentiation of biotite granite magma is unlikely to have generated Li-mica granite magma, as also, on its own, is partial melting of biotite granite or biotiteabsent residual lower crust. However, partial melting of biotite-rich residual rocks involving biotite breakdown could yield a trace alkali- and F-enriched melt, although this would require marked femic mineral, K-feldspar and anorthite fractionation, and Na-enrichment. It is proposed that volatiles derwed from either a mantle source or the crust/mantle interface have aided metasomatism of either residual S-type crust that earlier provided S-type biotite granite magma, or basic (biotite-rich) granitoid, to produce a low-temperature, low-viscosity Li-mica granite melt that rose rapidly in the crust soon after the emplacement of associated biotite granites.

187Os/186Os ratios were determined for in-situ laurite grains in Alpine-type chromitites and platinumgroup minerals (PGM) in the associated alluvial placers in Borneo, Indonesia/Malaysia. The Osisotope ratios of laurite grains in chromite defne an 187Os/186Os ratio for the 100 Ma mantle source of c. 1.04. Thelow 187Os/186Os ratios in all grains confirm the essential derivation of these platinum-group elements (PGE) from the mantle. A minor variation in 187Os/186Os ratios was detected among PGM from placers, but no variation was found within individual grains, including a grain with chemical inhomogeneity. The values are similar to those for PGM in the associated chromitites. The data are consistent with a detrital origin of PGM in placers: the placer PGM originated in the ultramafic section of ophiolities and the release of these grains from igneous rocks and their deposition in placers was almost entirely by mechanical processes.

Banded sphalerite from Bleiberg, Austria, contains ubiquitous relics of Fe-thiosulphates and Fesulphites up to 50 μm in size. Zn, Pb, Ni and As form admixtures ranging from a few to 12 wt.% in these compounds. It is suggested that banded sphalerite is formed by a replacement of banded thiosulphate and thiosulphite precursors. This suggestion is supported by a significant admixture of Fe 0.41-8.20 wt.%) and the presence of goethite inclusions in banded sphalerite. In contrast, crystalline ZnS intergrown with the banded variety is Ni-free and contains only 0.06-0.14 wt.% Fe.

Banded pyrite contains inclusions of Fe-thiosulphate up to 200 μm in size. These thiosulphates contain (wt.%): 0.09-1.92 Zn, and 2.67−4.02 Pb. Pyrite formed by replacement (breakdown) of Fethiosulphate precursors contains 3.69-3.83 wt.% Pb.

It is suggested that banded sulphides precipitate from solutions carrying sulphur with mixed valances, and crystalline sulphides grow from solutions where the dominant dissolved sulphur is in the form of sulphidic anions.

Chromian spinel in volcanic rocks is a potential discriminant for magma chemistry. The TiO2 content of spinel, compared at similar Fe3+/(Cr + Al + Fe3+) ratios, can distinguish island arc basalts from intraplate basalts. MORB spinels are low in this ratio and are intermediate for the TiO2 level at comparable Fe3+ ratios. Spinels from back-arc basin basalts, although similar in TiO2/Fe3+ ratio, are more enriched in Fe3+ than the MORB spinels. Spinels in the oceanic plateau basalts are distinctly lower in TiO2 than other intraplate basalt spinels and even slightly lower in TiO2 than the MORB spinels. The data were successfully applied to estimate the kind of the magma from which spinelbearing cumulates, especially dunites, were formed. Original magma chemistry of altered or metamorphosed volcanics in which spinels survive can also be estimated by the chemistry of relict spinel alone. It is possible to estimate the magma type of source volcanics for detrital spinel particles of volcanic derivation.

Curie temperatures indicating non-titaniferous magnetite are common in Icelandic basalts of all ages, especially Tertiary ones. Yet, microprobe analyses of such samples have shown high titanium in the magnetite. To resolve this paradox, and the mechanism at work, the magnetic mineral fraction of eight basalt samples with Js-T curves characteristic for pure magnetite was subjected to a multi-disciplinary analysis including Mössbauer spectroscopy and X-ray diffraction. In most of the samples titanium in the magnetite, as analysed with the microprobe, ranged between 16 and 28 wt.%, indicating submicroscopic solvus exsolution in the titanomagnetite, beyond the power of resolution for the microprobe. More unexpectedly in view of the reversible Js-T curves, Mössbauer spectroscopy showed appreciable proportion of maghemite in the magnetic fraction. A three-stage mechanism is proposed for the formation of the mineral assemblages observed: (1) limited high-temperature oxyexsolution; (2) solvus exsolution during low-temperature hydrothermal alteration; and (3) maghemitization of the magnetite. Finally, the maghemite may transform to hematite with time. It is concluded that maghemite is much more common in Icelandic rocks than hitherto believed.

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.

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.

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 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:

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.

The crystal structure of sophiite, Zn2(SeO3)Cl2 (a = 10.251(4), b = 15.223(2), c = 7.666(5) Å, Z = 8) was solved in space group Pccn from single crystal X-ray data, and refined to R = 0.053 for 666 Fo > 4oFo. The mineral belongs to layer compounds. The threefold coordinated Se atom is at the apex of a pyramid, the base of which is formed by three O atoms; Se-O = 1.69 Å. Zn cations occupy two positions with distorted tetrahedral and octahedral coordinations. In the tetrahedra, Zn cations are surrounded by two O (2.02 Å) and two Ci (2.224 Å) atoms. Zn octahedra contain four O atoms (2.07 Å) and two CI atoms (2.701 Å). The atomic arrangement is characterized by rings containing two Se pyramids linked by their corners to two Zn tetrahedra. The rings are linked by their edges and corners to Zn octahedra to form layers parallel to (010). The layers are interconnected by residual Van der Waals bonds.

Experimental results on thermal phase transformations and deformations, crystal optics and other physical properties of sophiite are presented. A structural aspect of their anisotropy is discussed.