bluuebunny/crossref_metadata_2025 huggingface dataset:
Uranoan thorite in lithophysal rhyolite–Topaz Mountain, Utah, USA
Authors: Foord Eugene E. , Cobban Robert R. , Brownfield Isabelle K.
Published: December 2006

Uranoan thorite crystals have been found occurring as a sparse constituent in lithophysae in 6.1 to 6.8 Ma alkali rhyolite flows at Thomas Mountain, Utah, USA. The crystals are associated with sandidine, quartz, topaz, hematite, magnetite, and calcite; they are leek to dark grass green, transparent, well-formed, euhedral prisms, showing development of forms {100}, {101}, and {111}. The mineral is both optically isotropic, with a refractive index of 1.86(1), and X-ray amorphous, indicating its metamict state. Electron microprobe analysis yields: SiO2 17.3, ThO2 56.8, UO2 25.4, total 99.5%, and a structural formula of (Th0.72U0.31)Σ1.03Si0.97O4.

This appears to be the first reported occurrence of thorite in volcanic rocks of rhyolitic composition, and may account for part or all of the Th reported in alkali rhyolites and so-called ‘topaz-rhyolites’ of the western USA.
Genesis of chrysoberyl in the pegmatites of southern Kerala, India
Authors: Soman K. , Nair N. G. K.
Published: December 2006

In southern Kerala, India, chrysoberyl occurs in granitic pegmatites in association with quartz in alkali feldspar and is thought to crystallize earlier than beryl and sillimanite. The pegmatites are thought to have derived from the residual melts of granitic liquids formed by partial melting of the khondalites. In the absence of pyrometasomatic, desilication or aluminium-contamination processes, the genesis of chrysoberyl is explained by the CO2activity in residual melts. This model is at variance with the known concepts and may also explain the genesis of chrysoberyl in Sri Lanka where it is still controversial.
Ion microprobe determination of rare earth elements in accessory minerals
Authors: Reed S. J. B.
Published: December 2006

The application of ion microprobe analysis to REE determination is discussed, with special reference to REE-bearing accessory minerals (e.g. allanite, monazite, apatite, xenotime, sphene). The main analytical problems are shown to be (a) interferences in the mass spectrum caused by molecular ions, and (b) matrix effects, i.e. variations in ion yield with sample composition. Interference suppression using either high mass resolution or secondary-ion energy discrimination is fairly effective, but entails a significant sacrifice in peak intensity. In the absence of a viable model for predicting matrix effects, it is proposed that empirical standardization should be used. Often better absolute accuracy can be obtained by combining ion microprobe and electron microprobe data for REE present in sufficiently high concentrations. Detection limits for REE are in the part per million region at present, with spatial resolution typically around 10 μm, but there is considerable scope for improvement. Better standards and improved understanding of matrix effects should also lead to greater accuracy in quantitative analysis.
Stability constants and formation of Cu(II) and Zn(II) phosphate minerals in the oxidized zone of base metal orebodies
Authors: Clara M. , Magalhães F. , de Jesus Julio Pedrosa , Williams Peter A.
Published: December 2006

The free energies of formation of the minerals libethenite, Cu2PO4OH, pseudomalachite, Cu5(PO4)2(OH)4, cornetite, Cu3PO4(OH)3, tarbuttite, Zn2PO4OH, spencerite, Zn2PO4OH · 1.5H2O, and scholzite, CaZn2(PO4)2 · 2H2O, have been determined at 298.2 K using solution techniques. The values of ΔfG° (298.2 K) for the above minerals are, respectively, −1228.8±3.0, −2840.3±4.2, −1600.9±5.9, −1632.1±4.0, −1982.4±3.1, and −3556.7±7.6 kJ mol−1. These results, together with others from the literature concerning hopeite, Zn3(PO4)2·4H2O, hydroxyapatite, Ca5(PO4)3(OH), and the synthetic salt Cu3(PO4)2 · 2H2O have been used to construct a model for the formation of suites of Zn(II) and Cu(II) phosphate minerals found in the oxidation zone of base metal orebodies. The distributions and modes of occurrence of many natural assemblages of these minerals are readily explained by the model. Particular attention has been focused on deposits at Broken Hill, Zambia, and Saginaw Hill, Arizona, USA.
Ferrierite from Tapu, Coromandel Peninsula, New Zealand, and a crystal chemical study of known occurrences
Authors: Sameshima T.
Published: December 2006

Ferrierite has been found at Tapu, Coromandel Peninsula, New Zealand, as a mineral vein with calcite in altered hornblende andesite lava of the Miocene Beeson's Island Volcanics. The ferrierite is low in SiO2 (63.67%) and high in Al2O3 (13.75%), MgO (3.48%), and BaO(2.35%). The large a axis and cell volume (a 19.236(4), b 14.162(6), c 7.527(3) Å, V 2050 Å3) are consistent with the low SiO2, high Al2O3, high MgO chemistry. Optical orientation and optical sign (a = Z, b = X, c = Y, 2V(-)55°) of the mineral are different from those of the Lake Kamloops ferrierite reported by Graham (1918). Refractive indices α 1.487, γ 1.489, and density 2.136 were measured. Cleavage observed on (100) is perfect and on (001) is imperfect. Using data from eighteen occurrences so far reported including the Tapu mineral, the crystal chemistry of ferrierite has been studied.
Detrital and authigenic minerals in sediments from the western part of the Indian Ocean
Authors: Tsirambides A. E.
Published: December 2006

Mineralogical and oxygen isotope analyses have been performed on nine western Indian Ocean core samples in order to distinguish the detrital from authigenic minerals in the sediments. Following the removal of carbonates, organic constituents and Fe and Mn oxides, the residue was separated into five size fractions, the principal minerals present being feldspar, quartz, clinoptilolite, and clay minerals.

Oxygen isotope compositions for two samples reflect an authigenic origin for clinoptilolite by the submarine alteration of volcanic material. Oxygen isotope compositions of two separates (free from feldspar and clays) suggest a detrital origin for the quartz in this area. The same mode of origin is apparent for the other components too, except possibly for some smectite which may have formed authigenically.
A new barium uranyl oxide hydrate mineral, protasite
Authors: Pagoaga M. K. , Appleman D. E. , Stewart J. M.
Published: December 2006

Protasite, a new barium-containing member of the uranyl oxide hydrate group, occurs as bright orange pseudo-hexagonal platelets associated with uraninite and uranophane on unidentified rock matrix from Shinkolobwe mine, Zaïre. Protasite is monoclinic, Pn with a 12.295(2), b 7.221(1), c 6.9558(8) Å, β 90.40(2)°, and V 617.50(11) Å3. The tabular pseudo-hexagonal crystals are flattened on {010}, 0.1 mm to 0.5 mm wide and up to 0.1 mm thick. They are biaxial negative, 2 V = 60–65°, β and γ 1.79–1.83, and X = b. Sector twinning is common. Microprobe analysis shows BaO 15.0, UO3 78.0, H2O(diff.) 7.0%. The structural formula is Ba[(UO2)3O3(OH)2]. 3H2O, Z = 2, and density(calc.) = 5.827(3) g cm−3. Complete crystal-structure analysis shows protasite to be the simplest model structure of the uranyl oxide hydrate group.
The crystal chemistry of rhodizite: a re-examination
Authors: Pring A. , Din V. K. , Jefferson D. A. , Thomas J. M.
Published: December 2006

The crystal chemistry of rhodizite was re-examined using data from high-resolution electron microscopy (HREM), magic angle spinning nuclear magnetic resonance (MASNMR), a single crystal X-ray structure refinement, and a new chemical analysis. The analysis calculates to the formula: (K0.46Cs0.36Rb0.06 Na0.02)Σ0.90Al3.99Be4(B11·35Be0.55Li0.02)O28· The distribution of alkali cations was shown to be truly random by HREM images and computer image simulations. The distribution of boron and beryllium was monitored by MASNMR, the spectra for both elements gave only single resonances indicating that all beryllium and boron atoms are located in chemically equivalent sites. The structure of rhodizite was refined by single crystal X-ray diffraction techniques. The mineral is cubic a = 7.318(1) Å, space group P3. A full matrix least-squares refinement using 152 unique observed reflections [F > 3σ(F)] converged to R = 0.0344. The refinement confirmed the basic structure as determined by Taxer and Buerger (1967), 4 beryllium atoms of the unit cell were found to occupy a 4e special position, the remaining 0.5 being randomly distributed with the 11.35 boron atoms over the 12h sites.
The Delaney Dome Formation, Connemara, W. Ireland, and the geochemical distinction of ortho- and para-quartzofeldspathic rocks
Authors: Leake Bernard Elgey , Singh Dharm
Published: December 2006

A number of geochemical plots are suggested by means of which metamorphosed quartzofeldspathic sedimentary rocks (meta-sandstones, meta-arkoses, quartzites, etc.) can be distinguished from their metamorphosed igneous mineralogical analogues (metarhyolite, meta-granite, etc.). These plots are applied to an enigmatic, partly mylonitized, quartzofeldspathic series, the Delaney Dome Formation, which is involved in a major thrust zone. The formation is shown to be igneous and is probably a meta-rhyolite. The plots proposed should have general application in identifying the precursors of both high-grade and low-grade quartzofeldspathic rocks of unknown origin.
Eclogitic amphibolites from the Grampian Moines
Authors: Baker A. J.
Published: December 2006

Amphibolites from the Grampian Moines of Scotland contain clinopyroxene-plagioclase symplectites and plagioclase-hornblende coronas around garnet. The clinopyroxene-plagioclase symplectites are interpreted as former omphacites. It is inferred that an eclogitic garne-tomphacite-quartz assemblage once existed, but has since undergone partial reaction to an amphibolite facies hornblende-plagioclase assemblage. An early high-pressure metamorphic event occurred in parts of the Moine.
Geochemistry of ultramafic xenoliths and their host alkali basalts from Tallante, southern Spain
Authors: Dupuy C. , Dostal J. , Boivin P. A.
Published: December 2006

Ultramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REE due to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.
Mineralogy of the Nandewar Volcano, northeastern New South Wales, Australia
Authors: Stolz A. J.
Published: December 2006

The paper discusses the mineralogy of eruptives from the Nandewar Volcano, which range in composition from hawaiite and trachyandesite to comendite via tristanite and mafic and peralkaline trachyte. Olivine, Ca-rich pyroxene, and amphibole display marked decreases in 100 Mg/(Mg + Fe) ratios in the sequence trachyandesite to comendite, reflecting variation in host rock compositions. The presence of tscher-makitic subcalcic pyroxene and aluminous bronzite megacrysts in several trachyandesites indicates that these experienced intratelluric crystallization at elevated pressures (6–8 kbar). Some titanomagnetite and plagioclase phenocrysts in trachyandesites may also be moderate pressure cognate precipitates. Groundmass pyroxenes of some trachytes and comendites are strongly acmitic. The presence or absence of coexisting alkali amphiboles and aenigmatite appears to reflect stability over a relatively broad range of fO2 conditions. Aenigmatite rims on titanomagnetite and ilmenite microphenocrysts in several peralkaline eruptives provides support for a ‘no-oxide’ field in T-fO2 space. The Fe-Ti oxide compositional data indicate that magmas spanning the spectrum trachy-andesite-comendite crystallized under conditions of decreasing T and fO2 which broadly coincided with the FMQ synthetic buffer curve. However, a voluminous group of slightly older associated rhyolites appear to have crystallized under significantly more oxidizing conditions.
New data on painite
Authors: Shigley James E. , Kampf Anthony R. , Rossman George R.
Published: December 2006

A crystal of painite discovered in 1979 in a parcel of rough gem spinel from Burma represents the third known crystal of the species. This crystal is similar to the type crystal in most respects. Chemical analyses of both the new crystal and the type crystal confirmed the essential constituents reported by Moore and Araki (1976) and showed in addition the presence of trace amounts of Fe, Cr, V, Ti, Na, and Hf. Optical absorption spectra suggest that the red colour of painite is caused principally by Cr3+ and V3+.
Chemistry, optics, and crystal growth of milarite from Strzegom, Poland
Authors: Janeczek J.
Published: December 2006

Milarite was found in a cavity of a small pegmatitic segregation in association with albite, stilpnomelane and chabazite. Hexagonal, well-shaped crystals of milarite up to 4 mm in length are transparent, pale green in colour, and show bright green cathodoluminescence. The refractive indices are: ω = 1.535, ε = 1.534(Na). D = 2.55 g cm−3. The unit cell dimensions are: a 10.418(4), c 13.817(7) Å, V 1298.7 Å3. Anomalous biaxial sectors and birefringent optical patterns are visible in basal section. 2V varies from 34° in the {0001} sector to 64° in {101} sector. OAP azimuths are related to the external hexagonal symmetry of the crystals. Microprobe analyses of sections perpendicular and parallel to the c-axis revealed a uniform distribution of alkalis, mainly K2O, whereas CaO and Al2O3 contents are slightly higher in prismatic and pyramidal sectors than in the basal sector, but no systematic chemical zoning can be distinguished. The Al2O3 content decreases in the outer zone of the crystal studied due to Be-Al substitution. The average ‘anhydrous’ chemical formula of the Strzegom milarite is: (K0.98Na0.01)Ca1.93Mn0.02(Be2.16Al0.93)(Si12O30). Infra-red data indicate the presence of H2O molecules in the milarite. Internal growth structures of the crystals indicate rhythmic fluctuations of the growth rate resulting in an internal strain, inducing anomalous optical patterns. HRTEM study did not reveal any significant distortion of the milarite lattice.
Laphamite, an arsenic selenide analogue of orpiment, from burning anthracite deposits in Pennsylvania
Authors: Dunn Pete J. , Peacor Donald R. , Criddle Alan J. , Finkelman Robert B.
Published: December 2006

Laphamite, ideally As2(Se,S)3, is monoclinic, P21/n, with a = 11.86(1), b = 9.756(9), c = 4.265(9) Å, β = 90.17°, V = 493.5 Å3, Z = 4, D (calc.) = 4.60, D (meas.) = 4.5(1) g/cm3. Microprobe analysis yielded: As 47.0, Se 43.7, S 8.7, sum = 99.4 wt. %. In plane polarized light in polished section at about 3200 K laphamite is moderately bireflectant from white to grey. Reflection pleochroism is absent but fiery red internal reflections are characteristic as are golden yellow reflections along scratches. Anisotropy is moderate with grey rotation tints, masked in some sections by internal reflections. Reflectance spectra in air and oil between 400 and 700 nm are given. Colour values for the CIE illiminant C in air are, for R1 followed by R2: Y% 29.0, 33.5, λd 480, 478; Pe% 9.1, 8.7. Laphamite forms dark red crystals, up to 5.0 mm, associated with orpiment, arsenolite, and an unnamed phase, NH4AlF4, in a burning coal waste-dump at Burnside, Northumberland County, Pennsylvania.
A re-examination of the structure of ganophyllite
Authors: Eggleton Richard A. , Guggenheim Stephen
Published: December 2006

The superstructure of ganophyllite has been analysed using subcell single crystal X-ray data and electron-optical observations. The full cell (supercell), space group A2/a, a = 16.6, b = 26.6, c = 50 Å, β = 94° has the approximate formula (K,Na,Ca)6+7.5(Mg,Fe,Mn)24[Si32.5Al7.5]O96(OH)1621H2O, Z = 8. A structure is proposed in which triple chains of silica tetrahedra parallel to X form narrow 2 : 1 layers with sinusoidal Mn-octahedral sheets. The triple chains are connected laterally, and across the interlayer, by pairs of inverted tetrahedra, linking to each other into four-member rings parallel to (010). Polytypes are generated by various displacements of the inverted tetrahedra.
Thermodynamic models of reactions involving garnet in a sillimanite/staurolite schist
Authors: Foster C. T.
Published: December 2006

Textures produced by reactions involving garnet in a sillimanite-staurolite schist have been investigated using an irreversible thermodynamic model. The model predicts that the local production or consumption of garnet is strongly influenced by the proximity of garnet to growing sillimanite and dissolving staurolite. Garnets within sillimanite segregations should dissolve near the sillimanite-bearing centre of the segregation while garnets within or adjacent to staurolite poikilo-blasts should grow as the staurolite is replaced by a muscovite-rich pseudomorph. Garnets located in the matrix may grow, not react at all, or dissolve depending on the local configuration of nearby sillimanite, staurolite, and garnet. Textures predicted by the model are similar to those observed in thin section: many garnets are truncated within sillimanite segregations; garnets located in pseudo-morphs after staurolite are equant and have thick unpoikilitic rims that suggest growth; most matrix garnets also have thick inclusion-free rims that suggest growth but a few have thin rims, indicating little growth, or are irregularly shaped, suggesting dissolution.
The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risör, Norway
Authors: Joesten Raymond
Published: December 2006

Coronas of symplectic pargasite [XMg = 68, XCa = 69, XSi = 68] + spinel [SP45] and orthopyroxene [EN70] separate cumulus plagioclase [AN64] and olivine [FO64] in troctolitic gabbro from Risör, Norway. Coronas with a primary microstructure characterized by low-angle grain boundaries have the mineral assemblage layer sequence,

PLAGIOCLASE : PARGASITE + SPINEL : ORTHOPYROXENE + SPINEL : ORTHOPYROXENE : OLIVINE, whereas those with an annealed microstructure have the layer sequence,

PLAGIOCLASE : PARGASITE + SPINEL : PARGASITE : ORTHOPYROXENE : OLIVINE.

Primary symplectite consists of fan-shaped grains of pargasite that open toward plagioclase and are ribbed by rods of spinel. On annealing to a single grain, low-angle grain boundaries disappear and spinel rods rotate into parallel positions. Primary orthopyroxene, initially radial to layer contacts, anneals to a single grain, rimming olivine. Irregular discontinuous layers and plumes of symplectic orthopyroxene+spinel disappear on annealing and a monomineralic layer of pargasite forms between orthopyroxene and the amphibole symplectite.

The identity of composition of intercumulus pargasite and orthopyroxene with the same phases in primary coronas, the occurrence of primary and annealed coronas rimming cumulus ilmenite and the occurrence of radial orthopyroxene separating grains of cumulus olivine and separating cumulus olivine and plagioclase argue for the origin of primary coronas by fractional crystallization of spinel-saturated troctolitic magma at a pressure greater than 5 kbar.

Irreversible thermodynamic calculations show that the mineral assemblage layer sequence of the primary coronas is diffusionally unstable and that this instability provides the driving force for their spontaneous transformation to the stable mineral assemblage layer sequence of the annealed coronas.
The role of magmatic reaction, diffusion, and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risör, Norway: a discussion
Authors: Ashworth J. R.
Published: December 2006

The data of Joesten (1986) are re-interpreted. The petrography of the coronas is not consistent with magrnatic origin. Both microstructural types described by Joesten (1986), here re-named ‘columnar’ and ‘tabular’, formed by solid-state replacement of plagioclase and of adjacent olivine or ilmenite. Tabular microstructures are not annealed, but result from overgrowth or epitaxy of amphibole and pyroxene on pre-existing grains. Since the diffusion-controlled models of Joesten (1986) can account for major aspects of the coronas, it seems possible that a slightly modified, less simplified theory might explain them fully. Open-system behaviour must be admitted, with some constraints provided by symplectites. It may also be necessary to develop the theory in more than one dimension, and to allow for departures from local equilibrium at layer boundaries.
Rare earth element, Sr- and Nd-isotope evidence for petrogenesis of Permian basaltic and K-rich volcanic rocks from south-west England
Authors: Thorpe R. S. , Cosgrove M. E. , van Calsteren P. W. C.
Published: December 2006

Permian basic/ultrabasic lavas from south-west England may be divided into a ‘basaltic’ and a K-rich group. Both groups have enrichment of large-ion lithophile (LIL) elements relative to high field strength (HFS) elements, and the K-rich group show large degrees of LIL enrichment (c.50–500 times primordial mantle) in association with varied transition element concentrations. Samples from both groups 87Sr/86Sri = 0.704–0.705 and 143Nd/144Ndi = 0.5123–0.5127 and plot close to the mantle array on an ɛSr−ɛNd diagram. These data are interpreted in terms of derivation of the lavas from magmas resulting from partial melting of mantle which had experienced less (for the basaltic group) or more (for the K-rich group) enrichment in LIL elements as a result of migration of mantle melts. Such enrichment accompanied or followed subduction of oceanic lithosphere below south-west England. The resultant magmas experienced fractional crystallization of olivine and pyroxene prior to eruption.
Complex sequential pyroxene growth in tholeiitic hypabyssal rocks from southern West Greenland
Authors: Hall R. P. , Hughes D. J. , Friend C. R. L.
Published: December 2006

Chemically complex pyroxenes which occur in early Proterozoic tholeiitic dolerite dykes in southern West Greenland have been investigated using back-scattered electron (BSE) imagery, X-ray mapping and electron microprobe analysis. A wide variety of compositions occur within individual pyroxene grains in these rocks. They can be explained by simultaneous nucleation of different pyroxenes, the evolution of domains around these nucleii as a response to differential chemical gradients and the sequential precipitation of different pyroxenes at progressively lower temperatures. As an example, the individual grains in one dyke sample contain domains of bronzite, hypersthene, magnesium pigeonite, augite, and subcalcic augite. Olivine in this sample varies in composition from Fo70 to Fo33, although individual grains are only weakly zoned. The wide variation in pyroxene and olivine compositions suggests ranges of crystallization temperatures from c.1250° to as low as 825°C. Such compositionally variable pyroxenes are possibly characteristic of hypabyssal tholeiitic rocks.
The iron end-member of the pyrosmalite series from the Pegmont lead-zinc deposit, Queensland
Authors: Vaughan J. P.
Published: December 2006

Pyrosmalite [(Mn,Fe)8(OH,Cl)10Si6O15] has been identified by X-ray diffraction from the Pegmont lead-zinc deposit, north-west Queensland. Microprobe analyses indicate that it is the first reported occurrence of an extremely iron-rich member of the series. The characteristically high chlorine content of pyrosmalite in Pegmont and other similar base metal deposits may provide some evidence concerning the depositional environment of these stratiform Pb-Zn ores.
New varieties of mantle xenolith from the Massif Central, France
Authors: Hutchison R. , Williams C. T. , Henderson P. , Reed S. J. B.
Published: December 2006

Spinel lherzolite xenoliths from two localities in the Massif Central are undepleted in Al2O3, CaO, and Na2O. One suite from Tarreyres, is K2O depleted and amphibole-bearing whereas the other, from Monistrol d'Allier some 18 km away, is amphibole-free and has a higher mean K2O content of 0.035 wt.%. We present bulk major and minor element abundances in a harzburgite and a lherzolite from each locality and microprobe analyses of their constituent phases. Amphibole-bearing lherzolite and its pyroxenes are light-rare earth element (LREE) depleted, whereas amphibole-free lherzolite and its pyroxenes are LREE enriched. Both harzburgites and their pyroxenes are LREE enriched and one rock contains LREE enriched glass. The harzburgites are like harzburgite xenoliths from elsewhere but each lherzolite represents a previously unrecognized type of mantle in terms of the mineralogy and REE content. The implication for basalt genesis are briefly discussed.
The mineralogy of nepheline syenite complexes from the northern part of the Chilwa Province, Malawi
Authors: Woolley Alan R. , Garth Platt R.
Published: December 2006

The mineralogy, including the accessory phases låvenite, rosenbuschite, and catapleiite, and consequent petrogenetic implications have been investigated for a group of four overlapping nepheline syenite complexes (Chikala, Chaone, Mongolowe, and Chinduzi) and for spatially associated silica-saturated and over-saturated perthosites, from the northern part of the Chilwa Alkaline Province, Malawi.

The complexes are thought to have formed by injection into high-level chambers of magma pulses genetically related to a common source magma at depth. Evidence for the source magma is preserved in salitic cores observed in the pyroxenes and a trend to more hedenbergite-rich compositions is believed to have formed by evolution of this magma. Subsequent trends of acmite enrichment followed magma injection into the higher-level chambers; the actual pyroxene trend associated with each individual complex is a function of the evolution attained by the source magma, oxidation potential, and perhaps even alkali activity. On the basis of such a two-stage model, the pyroxene data suggest emplacement of the Chaone and Mongolowe magmas somewhat earlier than that of Chikala, with the Chinduzi magma migrating even later.

Amphiboles and biotites are believed to have formed after high-level injection of the magmas. Their compositions broadly reflect the nature of the crystallizing pyroxenes in that magnesian hastingsitic hornblendes and more Mg-rich biotites are associated with more Mg-rich sodic pyroxenes, whereas katophorites and annite-rich micas are generally associated with sodic pyroxenes somewhat richer in hedenbergite. Sub-solidus crystallization in some of the complexes is represented by aegirine and magnesio-arfvedsonite. Nepheline compositions indicate broadly similar crystallization temperatures within the complexes, namely 950 to 750°C. Oxygen fugacities for these magmas obtained from biotite/annite compositions vary from 10−19 to 10−14 bars for this temperature range. Mineralogical data, particularly from pyroxenes and amphiboles, strongly suggest that the perthosites, spatially associated with the nepheline syenite complexes, are genetically unrelated.
Garnet-quartz intergrowths in graphitic pelites: the role of the fluid phase
Authors: Burton Kevin W.
Published: December 2006

Garnets with an unusual inclusion pattern of cylindrical quartz intergrowths have been found to develop exclusively in the presence of graphite. The intergrowths consist of quartz rods, 1–5 µm in diameter, originating at the sector-zone interfaces in the garnet with the long axes normal to the crystal faces. The lattice orientation and continuity of the quartz suggests that the interphase boundaries between the quartz and garnet are epitaxially related and that new material was added to the tube as the crystal face of the garnet grew. In the presence of a C-O-H fluid, at the temperatures and pressures recorded, (P = 6.5 kbar, T = 500°C), the amount of CO2 present restricts the solubility of SiO2 in the intergranular fluid phase, where the oxygen fugacity (fo2) is below the Quartz-fayalite-magnetite (QFM) buffer, and within the stability field of graphite. The reduced solubility will lower the concentration of SiO2 in solution, and hence restrict its ease of transport via the fluid, resulting in an excess of SiO2at the site of garnet growth. Under such conditions the SiO2 is incorporated in the growing garnet in the form of the cylindrical quartz intergrowths.
Pyroxenes and coexisting minerals in the Cheviot granite
Authors: Haslam H. W.
Published: December 2006

Orthopyroxene and clinopyroxene, with biotite, are widespread in the high-level Cheviot granite, while amphibole is rare. The pyroxene compositions are very close to (Mg,Fe)2Si2O6 and Ca(Mg,Fe)Si2O6. Biotite compositions, with low Si and Al, are characteristic of biotites coexisting with pyroxenes. Geothermometric estimates based on oxide and pyroxene compositions show that present mineral compositions result from post-consolidation equilibration.
Carlosturanite (not ‘picrolite’) from Taberg, Sweden
Authors: Mellini Marcello , Zussman Jack
Published: December 2006

The second occurrence of the asbestiform silicate carlosturanite is reported. The mineral occurs in Taberg, Sweden, and it was previously reported as ‘picrolite’, that is fibrous antigorite. Carlosturanite from Taberg is non-pleochroic, green in colour and has positive optical elongation. The unit cell parameters area= 36.55,b= 9.31,c= 7.27Å,β= 101.05°. Based on the determined formula, (Mg20.47Fe0.59Mn0.06Cr0.01Ti0.01) (Si11.68Al0.28)O27.81(OH)34.19H2O carlosturanite can be expected to crystallize within the pure MgO-SiO2-H2O system as a low-grade metamorphic mineral.

Because there are close similarities in structure and properties of carlosturanite and the picrolite variety of antigorite, their distinction requires careful attention to X-ray and electron diffraction patterns and chemical compositions. The present identification of the Taberg specimen as carlosturanite puts a lower limit of 33.7 Å to the known range ofaparameters of antigorite.
The chemistry and structure of redledgeite
Authors: Gatehouse B. M. , Jones G. C. , Pring A. , Symes R. F.
Published: December 2006

Two distinct habits of redledgeite are described: small black bipyramidal crystals and yellow-green fibres. The mineral is a Ba-Cr-Ti oxide rather than a Mg-Cr-Ti oxide as previously supposed. Electron microprobe analysis gave Ba1.10(Cr1.82Ti5.95 Fe0.10V0.08)Σ7.95O16 and Ba1.27(Cr2.48Ti5.49Fe0.02)Σ7.99 O16 for the black and yellow-green forms respectively. The mineral is a monoclinic hollandite-type phase, space group I2/m, with a = 10.129(2); b = 2.959(1); c = 10.135(2) Å; β = 90.05(11)° and Z = 1; calculated density for the black form is 4.413 g cm−3. The crystal structure was refined to R 6.3%, Rw 7.4% using 1017 reflections with F > 3σ(F) from a set of 1062 unique reflections. Electron diffraction studies revealed weak superlattice reflections, with a period of 2.24b due to tunnel cation ordering.
Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques
Authors: Lloyd Geoffrey E.
Published: December 2006

Backscattered electrons (BSE) are incident electrons reflected back from a target specimen and imaged with the scanning electron microscope (SEM). Three distinct BSE signals exist: atomic number or Z-contrast, in which composition determines image contrast; orientation contrast, in which specimen crystal structure determines image contrast; and electron channelling patterns (ECP), which are unique for a particular crystal orientation. The origins of these three signals are described, with particular attention being given to the necessary SEM operational and specimen preparation requirements. Z-contrast images are relatively simple to obtain and also have a familiar appearance such that their usage should become commonplace. ECP in comparison require subsequent interpretation which depends on the crystal structure and the relationship between crystal and specimen coordinate systems. A general solution to ECP interpretation is therefore presented, involving the construction of reference ‘ECP-maps’ over the surface of a sphere. A brief summary of the applications and potential use of the three BSE signals in the geological sciences is also given.
Mineralogical applications of the analytical SEM in archaeology
Authors: Freestone I. C. , Middleton A. P.
Published: December 2006

The modern analytical SEM, which can provide high-quality imaging facilities together with quantitative elemental analysis using an energy-dispersive spectrometer, is finding wide application in the investigation of archaeological problems. Many of these investigations involve the study of silicate and carbonate-based artefacts which may be relatively unmodified from their original geological parent raw materials so that mineralogically based interpretations are often appropriate. In this paper we present a series of examples illustrating the role of the analytical SEM in the mineralogical investigation of archaeological problems, including the characterization and provenancing of geological raw materials, the elucidation of the processes used to transform those raw materials into useful objects and the recognition and characterization of changes which archaeological artefacts may have undergone during burial or during storage.
Convergent beam electron diffraction
Authors: Champness P. E.
Published: December 2006

In convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (⩽50 nm) area of the sample. Instead of the diffraction spots that are obtained in the back focal plane of the objective lens with parallel illumination in conventional selected-area electron diffraction, CBED produces discs of intensity. The point group can be determined uniquely from the symmetry within the individual discs and the overall pattern. In order to determine the point group, it is usually necessary to record a number of CBED patterns with the electron beam aligned along different zone axes, but sometimes only one, high-symmetry pattern is required. The positions of reflections in higher-order Laue zones can be used to identify the crystal system and lattice type and to detect the presence of certain glide planes. The repeat along the zone axis that is parallel to the beam can be calculated from the diameters of the Laue zones. Hence the presence ofpolymorphs can be detected. Doubly-diffracted discs in CBED often contain a ‘line of dynamic absence’, the orientation of this line with respect to the symmetry seen in the bright field disc allows the symmetry element responsible for it (glide plane or screw diad) to be identified. This allows 191 of the 230 space groups to be uniquely identified. The measurement of specimen thickness, extinction distance and cell parameters are also briefly discussed.
Iron ore sinter in the analytical transmission electron microscope
Authors: Mulvaney R.
Published: December 2006

Iron ore sinters prepared for the blast furnaces at Scunthorpe, Humberside, comprise iron oxides bound in a matrix of aluminium- and silicon-rich calcium ferrites with some calcium orthosilicate and a residual silicate glass. A super-silicon-deficient clinopyroxene with up to half of the Si in the chain replaced by Fe3+ and Al3+ precipitates from the glass in some areas. The iron oxides are principally magnetite and hematite with some wüstite in highly reduced areas. The calcium orthosilicate has been previously reported as the mineral larnite, β-Ca2SiO4. This study found a range of Fe for Ca substitution and three polymorphs were recognised: the range Ca2SiO4 to Ca1.85Fe0.15SiO4 has the structure of larnite; the range Ca1.85Fe0.15SiO4 to Ca1.6Fe0.4SiO4 has the structure of bredigite, α′-Ca2SiO4. Iron substitution beyond this, and up to a maximum of about Ca1.5Fe0.5SiO4, was not recognisable as a polymorph of Ca2SiO4, but an orthorhombic cell is tentatively proposed. The complex ferrite SFCA (silico-ferrite of Ca and Al) is variable in composition, but has a minimum of about 3 wt. % Al2O3 and contains 20 wt. % CaO and SiO2 with a Ca: Si ion ratio of about 2 : 1. A C-face centred monoclinic cell has been deduced: a 15.70, b 9.70, c 8.48 Å; β 105°30′. The Al appears critical for the formation of SFCA in sinter; without it, a eutectic of magnetite and bredigite precipitates in preference.
Composition-induced microstructures in rhombohedral carbonates
Authors: Barber D. J. , Khan M. Riaz
Published: December 2006

Recent TEM observations of two-phase microstructures and associated crystal defects in selected, rare dolomites have been extended to calcite-structured (R3̄c) carbonates and to other natural and synthetic carbonates that crystallize with the dolomite (R3̄) structure. The samples included siderites (FeCO3), smithsonites (ZnCO3), ankerites (Ca[Mg,Fe](CO3)2), and kutnahorites (Ca[Mn,Fe](CO3)2).

TEM methods show that the forms of second phases which result from the presence of common, divalent, metallic impurities are morphologically similar in R3̄c and R3̄ carbonates and occur more widely than hitherto realized. The most common form consists of thin ribbons of second phase which are coherent with and have the same crystallographic orientation as the host carbonate. Another form of microstructure, manifest as modulations in diffraction contrast, appears to be associated with incipient breakdown of single-phase carbonate. The results of extensive TEM/EDS microanalyses show that in siderite and ankerite the formation of ribbons is promoted by Ca impurity or Ca excess (with respect to R3̄c stoichiometry). In smithsonite, Cu and Ca impurities can play similar roles in relation to modulated microstructures. In kutnahorites, the perfection of grains and the absence of second-phase effects is strongly dependent on the ratio of Fe to Mn but is also affected by Ca in excess of the stoichiometric requirement. Electron diffraction results from several of the minerals show c-type spots, which can be interpreted as indicating ordering within basal layers of cations.

The results show that, by correlating analytical TEM data with the study of second phases and incipient two-phase microstructures, it should be possible to determine the limits of solid solubility in carbonate systems.
A natural example of the disequilibrium breakdown of biotite at high temperature: TEM observations and comparison with experimental kinetic data
Authors: Brearley A. J.
Published: December 2006

Transmission electron microscopy has been used to investigate the mechanism of natural biotite breakdown under pyrometamorphic disequilibrium conditions. Biotite in a xenolith of pelitic gneiss collected from a Tertiary dolerite sill, Isle of Mull, Scotland, shows evidence of an incipient reaction, characterised by a darkening in colour and the appearance of areas of fine-grained reaction products. TEM and analytical electron microscope data show that the reaction can be described as:

The orientations of the product phase are controlled by the crystallography of the reacting biotite, demonstrating that the transformation proceeds by a topotactic mechanism. An empirical method, based on the Mg/(Fe2+ + Fe3+) ratios of coexisting spinel and biotite from experimental data, is used to deduce that the reaction occurred above ∼ 770 °C. A comparison of the natural reaction microstructures with those produced experimentally suggest that the xenolith was probably above 800 °C for less than 48 hours and cooled to temperatures of 770 °C after ∼ 150–200 hours.
Transmission electron microscopy of pyrometamorphic breakdown of phengite and chlorite
Authors: Worden R. H. , Champness P. E. , Droop G. T. R.
Published: December 2006

Phengite and chlorite have undergone decomposition during pyrometamorphism caused by the intrusion of a dolerite feeder pipe into Dalradian greenschists in Argyllshire, Scotland. All reaction products are extremely fine grained. Transmission electron microscopy has revealed that phengite pseudomorphs consist of biotite, spinel, mullite, sanidine and phengite, and that chlorite pseudomorphs consist of combinations of chlorite, spinel, orthopyroxene, magnetite, cordierite and biotite. Although the reactions were short-lived and did not go to completion, microprobe analysis and phase diagram analysis have revealed that there has been significant chemical interaction between the phyllosilicates and the surrounding rock. Numerous orientation relationships exist between the original minerals and their reaction products; the close-packed planes in the precursor phyllosilicates were inherited by their reaction products.
Palagonites of the Red Sea: a new occurrence of hydroxysulphate
Authors: Ramanaidou E. , Noack Y.
Published: December 2006

Palagonites from the Red Sea consist of two zones: an orange palagonite which is a mixture of Mg-Al double hydroxide, Al-hydroxide and an undetermined Si-K phase, and a white palagonite, similar to motukoreaite, a Mg-Al hydroxy-sulphate-carbonate. This mineral, frequent in experimental alteration of glass by seawater, is discovered for the first time in natural palagonite. Hydroxysulphates and hydroxides are the precursors of phyllosilicates, generally found in palagonites. The very young palagonites of the Red Sea are the first link between the natural and experimental observations.
The lattice dynamics of forsterite
Authors: Price Geoffrey D. , Parker Stephen C. , Leslie Maurice
Published: December 2006

We use an approach based upon the atomistic or Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the infrared and Raman vibrational frequencies of forsterite. We investigate a variety of interatomic potentials, and find that although all the potentials used reproduce the structural and elastic behaviour of forsterite, only one potential (THB1) accurately predicts its lattice dynamics. This potential includes ‘bond-bending’ terms, that model the directionality of the Si-O bond, which we suggest plays a major role in determining the structural and physical properties of silicates. The potential was derived empirically from the structural and physical data of simple oxides, and its ability to model the lattice dynamics of forsterite is a significant advance over previous, force-constant models, which have been simply derived by fitting to the spectroscopic data that they aim to model. The success that we have had in predicting the lattice dynamics of forsterite indicates that the potential provides the previously elusive yet fundamental, quantitative link between the microscopic or atomistic behaviour of a mineral and its macroscopic or bulk thermodynamic properties.
Manganoan deerite and calderitic garnet from high-pressure metamorphic Fe-Mn-rich quartzites on Andros Island, Greece
Authors: Reinecke Thomas
Published: December 2006

Calderitic garnet and manganoan deerite have been found in Al-poor, Fe-Mn-rich quartzites associated with metabasic schists on the island of Andros, Cycladic blueschist belt, Greece. Calderitic garnet (30–45 mole % calderite, 55–70 mole % andradite, 0–2 mole % spessartine) occurs in paragenesis with quartz, rhodonite, rhodochrosite, Ni-Mn-bearing magnetite (2–20 mole % trevorite, 6–12 mole % jacobsite, 73–91 mole % Fe3O4), baryte, and orthite. Deerite is associated with quartz, garnet (rich in spessartine, almandine, and andradite), phengite, aegirine, magnetite (containing up to 12 mole % jacobsite), and crossite/riebeckite. Deerite contains 33.0–34.4 wt. % SiO2, 0.53–5.5 wt. % TiO2, 0.09–0.31 wt. % Al2O3, 11.9–33.9 wt. % MnO, 17.5–44.2 wt. % FeOtot, and 1.5–2.8 wt. % MgO. MnO contents correspond to 29–77 cation % Mn2+ per total R2+ cations. Deerites from Andros are thus closer to the hypothetical end-member mangano-deerite, Mn122+Fe3+6Si12O40(OH)10, than any other analysis of deerite so far published.
Zirconolite, chevkinite and other rare earth minerals from nepheline syenites and peralkaline granites and syenites of the Chilwa Alkaline Province, Malawi
Authors: Platt R. G. , Wall F. , Williams C. T. , Woolley A. R.
Published: December 2006

Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims.

Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastnäsite. These are briefly described and microprobe analyses presented.
New data on the relation between caryinite and arseniopleite
Authors: Dunn Pete J. , Peacor Donald R.
Published: December 2006

Caryinite from Låmgban, Sweden, and arseniopleite from both the Långban and Sjö Mines in Sweden, are re-examined chemically and crystallographically. Both species are monoclinic, with space group C2/c, and similar lattice parameters. The general formula for caryinite is Na4Ca4Ca4(Mn,Mg)8(AsO4)12, and that for arseniopleite is Na4Ca4Mn4(Mn,Mg)8(AsO4)12. The species status of arseniopleite remains uncertain; it may have Mn dominant in the M(1) site.
Early basic magmatism in the evolution of Archaean high-grade gneiss terrains: an example from the Lewisian of NW Scotland
Authors: Rollinson H. R.
Published: December 2006

Amphibolite blocks from an Archaean (2.9 Ga) trondhjemite-agmatite complex in the Lewisian at Gruinard Bay have a varied trace element and REE content. Whilst some of the variability is attributable to element mobility during high-grade metamorphism and subsequent trondhjemite magmatism, it is for the main part considered to be a primary feature of the amphibolites. The observed trace element and REE chemistry is best explained in terms of source region heterogeneity and suggests a melting regime comparable with that beneath certain types of mid-ocean ridge. There are geochemical similarities between the amphibolites and the Lewisian layered gabbro-ultramafic complexes, and the two may represent the derivative liquid and associated cumulates respectively from a common parent magma. Thus there is a parallel between the processes which generated some Archaean amphibolites and layered gabbro complexes and those operating beneath modern ocean ridges. Hornblendite and amphibolite pods enclosed within tonalitic gneiss, also found as blocks in the agmatite complex, are geochemically distinct from the main group of amphibolites and are probably of calc-alkaline parentage.