bluuebunny/crossref_metadata_2025 huggingface dataset:
Metasomatism of cumulus magnesian olivine by iron-rich postcumulus liquids in the upper Critical Zone of the Bushveld Complex
Authors: Scoon Roger N.
Published: December 2006

Hybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.
Peraluminous sapphirine from the Aileron district, Arunta Block, central Australia
Authors: Warren R. G. , Hensen B. J.
Published: December 2006

Specimens collected from a small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, have unusual bulk compositions and mineral assemblages. One sample consists of phlogopite enclosing blue spinel (mg 96) with minute granules of corundum and sapphirine at the margins; a second of phlogopite enclosing porphyroblasts of corundum and peraluminous sapphirine. In the first the sapphirine is close to the 7 : 9 : 3 composition; in the other it is markedly peraluminous, e.g. (Mg1.628Fe0.028)Al4.714Si0.636O10, intermediate between the 7 : 9 : 3 and 3 : 5 : 1 members. The texture suggests that this sapphirine is a stable phase in equilibrium with eastonitic phlogopite and corundum. The very potassic, very magnesian bulk composition of the rocks is attributed to potassium metasomatism of a protolith consisting of magnesian chlorite and quartz.
A basic magnesium carbonate, a possible dimorph of artinite, from Unst, Shetland
Authors: Livingstone A.
Published: December 2006

Efflorescent outgrowths and compact vein-like material from brucite-rich specimens is a basic magnesium carbonate and possibly a dimorph of artinite. X-ray, infrared, chemical and thermal data reveal the material to be different from known natural basic magnesium carbonates.
Non-hydrocarbons of significance in petroleum exploration: volatile fatty acids and non-hydrocarbon gases
Authors: Cooles G. P. , Mackenzie A. S. , Parkes R. J.
Published: December 2006

Non-hydrocarbon gas species (CO2, N2, H2) are locally important in exploration for gas, and there is a growing body of evidence that acid water originating in shales materially affects the diagenesis of nearby sandstones. These gases have been studied by analysing the products of closed-vessel hydrous pyrolysis of known petroleum source rocks, and comparing the results with field observations. Alteration of petroleum source rocks at temperatures >250°C yields a significant amount of non-hydrocarbon components. Ethanoate and higher acid anions are liberated in substantial quantities; the yield appears to be related to the oxygen content of the sedimentary organic matter present.

The non-hydrocarbon gases CO2, H2 and N2 are frequently the dominant gaseous products from hydrous pyrolysis: in the natural environment the same rock sequences at a higher maturity preferentially generate hydrocarbon gases—mainly methane. This discrepancy may be attributed to reaction and phase thermodynamic effects between laboratory and natural systems, behaviour that has important implications in the prediction of gas generation and composition in nature by source rock pyrolysis in the laboratory.
The replacement of sandstones by uraniferous hydrocarbons: significance for petroleum migration
Authors: Parnell John , Eakin Paul
Published: December 2006

Hydrocarbons (bitumenssensu lato) in sandstones have been recorded in several instances to have partially replaced their host rock, including quartz grains. Many replacive hydrocarbons are uranium-rich: associated non-uraniferous hydrocarbons are not replacive. Uranium is transported as carbonate complexes, which may be decomposed by organic acids to yield UO22+ions and CO2. The UO22+will be absorbed onto hydrocarbons before reduction to a mineral phase, generally uraninite; and the CO2may be aggressive towards the silicate grains of the host sandstone. Accretionary nodules of replacive uraniferous hydrocarbon in red beds (e.g. at the cores of reduction spots) can provide valuable information about petroleum migration. They occur particularly in the vicinity of faults, and may record the interactions between metal-rich groundwaters and hydrocarbons leaking along a fault from an underlying reservoir. A trial study in Devonian sandstones of Easter Ross successfully traced a hydrocarbon-bearing sandstone reservoir from an occurrence of uraniferous hydrocarbon nodules.
The release of trace elements and volatiles from crinoidal limestone during thermal decrepitation
Authors: Rankin A. H. , Miller M. F. , Carter J. S.
Published: December 2006

A suite of coarsely crystalline samples of crinoidal limestone adjacent to a PbS-CaF2-CaCO3 vein in the North Pennine orefield (Greenhow Rake) was analysed by three independent methods [Inductively Coupled Plasma emission spectroscopy (ICP), Mass Spectrometry (MS) and Gas Chromatography (GC)], in order to determine the nature and origin of trace elements and volatiles released on heating and thermal decrepitation, to establish whether fluid inclusions from the host limestone differed in composition from those within the vein, and to assess the value of the results obtained for mineral exploration. The ICP method gave reproducible results for Ca, Na and K, and significant values for Pb and Zn. Na and K correlate with H2O levels determined by MS/volumetric analysis, suggesting their coexistence within fluid inclusions. No such correlation was found for Pb and Zn, suggesting that these elements were derived by volatilisation, perhaps from traces of galena and sphalerite. Methane showed no correlation with H2O and a source other than aqueous fluid inclusions is thought probable for this volatile.

The vein calcite showed much lower K/Na fluid ratios (0.05 cf 0.1–0.2), than the limestones. The limestone-hosted fluids occur predominantly in crinoid ossicles and are believed to represent early pre-mineralisation fluids, unrelated to those associated with veining. Combined ICP and MS techniques may be of value in investigating samples where fluid inclusions are too small for microthermometric and optical analysis.
A possible role for light hydrocarbons in Pb/Zn mineral exploration
Authors: Ferguson J.
Published: December 2006

A method of base metal exploration in sedimentary rocks which depends on detecting anomalous amounts of the light hydrocarbon gas methane in the host rocks has been applied in a number of orientation surveys in the British Isles and in Europe. The results of these studies are described, and the limitations of the method discussed. It is suggested that the principal difficulty lies in the extraction technique which is based on heating, and alternative methods are proposed. The preliminary results from these new methods are discussed and compared with those from the original study. A number of the samples analysed by the new methods are calcites of differing diagenetic origins. It is postulated that the differences in methane found in these samples may give a clue to the organic geochemistry of the rocks at the time when the calcites were deposited.
Calderite-rich garnets from metamorphosed manganese silicate rocks of the Sausar Group, India, and their derivation
Authors: Dasgupta Somnath , Bhattacharya P. K. , Banerjee H. , Fukuoka M. , Majumdar N. , Roy Supriya
Published: December 2006

Manganiferous garnets occur in metamorphosed Mn silicate-oxide and silicate-carbonate-oxide rocks of the Sausar Group, India. The garnets of the carbonatic rock show maximum calderite content (∼50 mole%). Increased Ca content in the bulk has been observed to be sympathetically related to the concentration of calderite, rather than the expected andradite component of the garnets. This is the consequence of the preferential partitioning of Ca in coexisting pyroxmangite and/or carbonate. Petrochemical characteristics of the diverse assemblages in response to the physical conditions of metamorphism show that the calderite solubility in manganiferous garnet is not only a function of pressure. Such characteristics only indicate that the mobile components in the bulk influenced the mineralogy of the assemblages at the ambient physical conditions of metamorphism, and this in turn controlled the calderite solubility in garnet.
Holmquistite-bearing amphibolite from Greenbushes, Western Australia
Authors: Frost M. T. , Tsambourakis G. , Davis J.
Published: December 2006

Holmquistite idioblasts occur in a hornblende-titanite-quartz-plagioclase amphibolite, in contact with a lithium (spodumene) pegmatite at Greenbushes, Western Australia. Unit cell parameters, chemical data and the results of a mineralogical investigation of holmquistite and associated minerals in the amphibolite are given.
Primary allanite in andesitic rocks from the Poros Volcano, Greece
Authors: Mitropoulos P.
Published: December 2006

Primary allanite crystals occurring in andesitic rocks from Poros, NE Peloponnese, Greece were studied. Although the allanite shows high REE content, its presence does not affect the whole-rock REE content of the andesite. It is suggested that the formation of allanite crystals reflects the crystallization history of the andesitic rocks where allanite occurs. These andesitic rocks are the most basic rocks in which allanite has been reported.
Lower crustal granulite xenoliths in carbonatite volcanoes of the Western Rift of East Africa
Authors: Thomas C. W. , Nixon P. H.
Published: December 2006

Sporadic small garnet granulite and two-pyroxene granulite xenoliths found in the carbonatite tuffs and lavas near Fort Portal, South West Uganda, are chiefly silica-saturated and rich in Al2O3(> 20 wt. %) and Na2O (c.2 to 4 wt. %). ThreeREEpatterns are distinguished:LREEenriched—HREEdepleted with a positive Eu anomaly;LREEdepleted—HREErelatively enriched and flat; andLREEslightly enriched with a very weak Eu anomaly and high overallREE. The xenoliths are considered to represent original basaltic melts and fractional crystallisation products, varying with the dominance of the clinopyroxene, plagioclase or olivine crystallising phase. It is thought thatREEabundances were established before metamorphism.

The clinopyroxenes are low-jadeitic augites, the orthopyroxenes, aluminous hypersthenes and the garnets, pyrope-almandine with constant grossularite. Plagioclase varies with increasing metamorphic grade from labradorite to andesine-oligoclase. Scapolite (meionite), alkali-feldspar, quartz, mica, amphibole, rutile and apatite are minor phases and some appear to be metasomatic.

Calculated temperatures of metamorphic equilibration range from 580 to 800°C at pressures > 4 kbar for the two-pyroxene granulites and > 6 kbar for the garnet granulites. A known geophysical discontinuity marking a density change at 16 km in the Western Rift may be due to the presence of two-pyroxene granulite, calculated to become garnet-bearing at depths greater than 23 km. The absence of proven omphacite-bearing eclogite xenoliths (with no plagioclase) indicates that the greatest depth of crustal sampling by the carbonatite in the Fort Portal field is about 25 km which could be the depth of the Moho in this area.
Rare earth and uranium minerals present as daughter crystals in fluid inclusions, Mary Kathleen U-REE skarn, Queensland, Australia
Authors: Kwak T. A. P. , Abeysinghe P. B.
Published: December 2006

At least six separate rare earth and uranium-bearing daughter crystals occur in fluid inclusions hosted by andraditic garnet from the Mary Kathleen REE-U ore skarn, Queensland, Australia. The daughter minerals are particularly high in La, Nd and Ce which reflects the relatively high concentration of these in the bulk ore (La2O3 = 33.5%, Nd2O3 = 9.1% and Ce2O2 = 51.5% of the 2.6 wt. % REE common in the ore). The host garnets themselves contain up to 7600 ppm REE and 5 to 2700 ppm U. The energy-dispersive spectra (EDS) are consistent with the following minerals: a (Y, Ce, U, Ca, Fe, Nb, Ta) mineral; a (Ca, Fe, Ce) carbonate(?) mineral; a (Fe, Ca, Y, Ce, Nb, Ta) mineral; a possible carbonate of La, Mn and Nd; a chlorite of Mn and La as well as a possible chloride or oxychloride of K, Mg, Mn and La. Their occurrence infers that relatively high concentrations of REE and U prevailed in the original, oxidized, concentrated (30–70 wt. % total dissolved salts), high-temperature (550–670°) ore solutions. Their presence as daughter crystals may be due to the fact that CaCl2 is a dominant salt in the solutions and that the latter's solubility was sufficiently high to ‘salt out’ the less soluble REE-U components.
Rhodochrosite from Islay, Argyllshire and Dalroy, Inverness-shire, Scotland
Authors: Nicholson Keith
Published: December 2006

Rhodochrosite has been discovered as a minor component of two manganese deposits in Scotland. At Islay it occurs as coatings and veinlets on and within hollandite-pyrolusite-hematite-goethite mineralisation. Two foram microfossils were found within the carbonate which is considered to have formed from the interaction of organic-rich groundwater and seawater. The Dalroy rhodochrosite has the form of stromatactis-like cavity infill structures and veinlets in a mineralogically complex manganese-iron ore in which braunite, bustamite, manganoan ilmenite and pyrophanite have been identified. Intergrowths of rhodochrosite and native copper grains occur in a calcite matrix within the infill structures. These intergrowths are typically one micrometre across, but achieve diameters of up to 50 µm. The genesis of the Dalroy rhodochrosite is more problematical, although a hydrothermal source is most likely.
Chlorine-rich potassium hastingsite from West Ongul Island, Lützow–Holm Bay, East Antarctica
Authors: Suwa Kanenori , Enami Masaki , Horiuchi Tatsuro
Published: December 2006

Chlorine-rich potassium hastingsite occurs in a calcareous pegmatite, a replacement zone and an amphibolite lens within hornblende gneiss on West Ongul Island, Lützow-Holm Bay, East Antarctica. The amphibolite lens and hornblende gneiss were metamorphosed to the kyanite-sillimanite grade of the granulite facies during Proterozoic metamorphism. Chemical analysis (3.27 wt.% Cl), unit cell parameters and optical properties of the Cl-rich potassium hastingsite are given. Cl-rich (> 3 wt.%) calcic amphiboles reported from various rock types are mostly more than 0.9 in (Na+ K) content, more than 0.4 in K/(Na + K) ratio, more than 0.75 in Fe2+/(Fe2+ + Mg + Mn) ratio and more than 1.9 in AlIV content (total iron as FeO and O = 23). The unit cell volume of Cl-rich hastingsite is distinctly larger than that of Cl-poor hastingsite.
Fluid activity in the lower crust and upper mantle: mineralogical evidence bearing on the origin of amphibole and scapolite in ultramafic and mafic granulite xenoliths
Authors: Stolz A. J.
Published: December 2006

Xenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increasedPH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevatedfo2andfso2conditions.
A new interpretation of round embayments in quartz crystals
Authors: Donaldson C. H. , Henderson C. M. B.
Published: December 2006

The surfaces of quartz crystals that were partially dissolved in superheated, H2O-saturated rhyolite melt are covered with hemispherical embayments; each embayment is judged to have formed where a gas bubble in the melt approached the crystal. ‘Flux-line attack’ and ‘upward-drilling’ of the refractory lining of glass tanks are analogous processes. As a bubble nears a dissolving solid it enters a compositional boundary layer in the melt, resulting in melt of variable surface tension surrounding the bubble. This unstable situation results in small-scale convection of the melt about the bubble (Marangoni convection) which can cause locally enhanced dissolution rate of the solid. It is suggested that this mechanism could cause round embayments to form in quartz phenocrysts in acid volcanic and sub-volcanic rocks. Criteria by which embayed phenocrysts formed by dissolution can be distinguished from those formed by unstable growth are reviewed briefly.
Experimental crystallization of chevkinite/perrierite from REE-enriched silicate liquids at high pressure and temperature
Authors: Green T. H. , Pearson N. J.
Published: December 2006

Chevkinite/perrierite crystallized from a series of REE-enriched compositions, ranging from basalt through andesite to rhyolite at pressures from 7.5 to 20 kbar and temperatures from 900–1050°C. The Al2O3 content increased with increasing pressure, but no other consistent composition change was observed with pressure and temperature variation in this range. The REE partition pattern consistently favoured light over heavy REE, with a regular decrease in partition coefficients (D) from La through to Lu. D values increase with decreasing temperature and decreasing REE content (i.e. Henry's Law does not apply, as REE are essential structural constituents of chevkinite/perrierite), but appear unaffected by pressure and fo2. Chevkinite/perrierite may be a significant fractionating phase in evolving silicic magmas, or a residual phase from low degrees of partial melting of granulites. In both these circumstances it will exert a major control on the REE content of derivative liquids.
Compositions of trioctahedral micas in the Cornubian batholith
Authors: Stone M. , Exley C. S. , George M. C.
Published: December 2006

New major and trace element chemical analyses of trioctahedral micas of the Cornubian batholith emphasize their enrichment in a ‘trace-alkali element’ association (Li, Rb, Cs, F, Nb, Mn) and their deficiency in a ‘femic element’ association (Zr, Ce, Th as well as Mg and Ti) compared with micas from many other granite suites, although there are similarities with some Hercynian granites and with rare metal pegmatites. The new data demonstrate a continuous series from siderophyllite through zinnwaldite to lepidolite, principally as a result of Li-R2+ substitution as indicated by Foster (1960), although a more complete replacement is (Li, Al) = (Fe2+, Fe3+, Ti). It is suggested that the ranges of these micas are based upon the Li content in the unit cell formula and the ratio of Li to R2+, in effect, a compromise between the ranges proposed by Foster (1960) and Rieder (1970). Microprobe analyses lack Li2O, but can be plotted on FeO-SiO2 and FeO-Al2O3-SiO2 diagrams (wt. or atom %) in order to locate compositions within the trioctahedral LiFe micas and to distinguish between lepidolite and muscovite.

An examination of 55 new mica analyses shows that hornfels biotites are richer in Mg and that the Cornubian Type A granite (as classifed by Exley and Stone, 1982) micas are richer in Ti and Fe compared with those of Type B granites. Micas from microgranite dykes appear to be poorer in femic elements and richer in trace alkalis and F compared with their Type B granite hosts, consistent with their differentiation from the latter. Mica chemistry is consistent with the magmatic evolution of the A-B-microgranite sequence in the biotite granites, the transformation of Types B to D upon emplacement of E, the derivation of Types E from B by extreme differentiation or metasomatic transformation and mobilization, and the in situ differentiation of Types G from E.
Ecandrewsite, the zinc analogue of ilmenite, from Little Broken Hill, New South Wales, Australia, and the San Valentin Mine, Sierra de Cartegena, Spain
Authors: Birch W. D. , Burke E. A. J. , Wall V. J. , Etheridge M. A.
Published: December 2006

Ecandrewsite, the zinc analogue of ilmenite, is a new mineral which was first described from the Broken Hill lode in 1970 and discovered subsequently in ores from Little Broken Hill (New South Wales) and the San Valentin Mine, Spain. The name ‘ecandrewsite’ was used in a partial description of the mineral in ‘Minerals of Broken Hill’ (1982), thereby establishing the Little Broken Hill locality, specifically the Melbourne Rockwell Mine, as the type locality. Microprobe analysis of ecandrewsite from the type locality gave ZnO 30.42 (wt.%), FeO (total Fe) 11.37, MnO 7.64, TiO2 50.12, total 99.6%, yielding an empirical formula of (Zn0.59Fe0.24Mn0.17)1.00Ti0.99O3 based on 3 oxygen atoms. All compositions from Little Broken Hill and the San Valentin Mine are ferroan manganoan ecandrewsite. The strongest lines in the X-ray powder diffraction data are (d in Å, (hkil), I/Io):2.746, (104), 100; 2.545, (110), 80; 1.867, (024), 40; 3.734, (012), 30; 1.470, (3030), 30; 1.723, (116), 25. Ecandrewsite is hexagonal, space group RR3¯ assigned from a structural study, with a = 5.090(1), c = 14.036(2)Å, V = 314.6(3)Å3, Z = 6, D(calc.) = 4.99. The mineral is opaque, dark brown to black with a similar streak, and a submetallic lustre. In plane polarized light the reflection colour is greyish white with a pinkish tinge. Reflection pleochroism is weak, but anisotropism is strong with colours from greenish grey to dark brownish grey. Reflectance data in air between 470 and 650 nm are given. At the type locality, ecandrewsite forms disseminated tabular euhedral grains up to 250 × 50 µm, in quartz-rich metasediments. Associated minerals include almandine-spessartine, ferroan gahnite and rutile. The name is for E. C. Andrews, pioneering geologist in the Broken Hill region of New South Wales. Type material consisting of one grain is preserved in the Museum of Victoria (M35700). The mineral and name were approved by the IMA Commission on New Minerals and Mineral Names in 1979.
The structure of reyerite, (Na,K)2Ca14Si22Al2O58(OH)8.6H2O
Authors: Merlino Stefano
Published: December 2006

The crystal structure of reyerite, (Na,K)2Ca14Si22Al2O58(OH)8.6H2O, Z = 1, was refined in the space group P, a = 9.765, c = 19.067Å, to R = 0.064 for 1540 reflections. The structure is composed of the following structural units: (a) tetrahedral sheets S1, with composition (Si8O20)8−, characterized by six-membered rings of tetrahedra; (b) tetrahedral sheets S2, characterized by six-membered rings of tetrahedra, with six tetrahedra pointing in one direction and two pointing in the other direction—the apical oxygens of these two tetrahedra connect two inversion-related S2 sheets to build double sheets, with composition (Si14Al2O38)14− and ordered distribution of aluminum cations; (c) sheets O of edge-sharing calcium octahedra. The various structural units are connected through corner sharing according to the schematic sequence ……; the corresponding composition is [Ca14Si22Al2O58(OH)8]2−. The charge balance is restored by alkali cations which are placed, together with water molecules, in the cavities of the structure at the level of the double tetrahedral sheet.
Ion Microprobe Determination of Rare Earth Elements in Perovskite from Kimberlites and Alnöites
Authors: Mitchell Roger H. , Reed Stephen J. B.
Published: December 2006

Ion microprobe analysis ofperovskite from kimberlites and alnöites permits the accurate determination of La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb at the part per million level. Other rare earth elements (REE) are subject to interferences in the mass spectrum caused by matrix-derived molecular ions and cannot easily be determined with comparable precision. Chondrite-normalized plots of the ion probe REE data are smooth curves, confirming the superiority of this technique over electron microprobe methods at the levels of REE abundance found in these perovskites. The perovskites analysed contain between 2.8 and 7.1 wt. % REE oxides and are highly enriched in the light REE, having La/Yb ratios of 577–3229. These La/Yb ratios are not representative of the parental magmas but result from REE fractionation during crystallization of the perovskite. Parental magma La/Yb ratios are estimated to be of the order of 120–650. The lower La/Yb ratios (80–200) found for whole-rock kimberlites are considered to result from contamination by relatively heavy REE-enriched crustal material.
Comparison of Cation Exchange in Ganophyllite and [Na + Al]-Substituted Tobermorite: Crystal-Chemical Implications
Authors: Komarneni Sridhar , Guggenheim Stephen
Published: December 2006

Ganophyllite and [Na+ + Al3+]-substituted tobermorite exhibited similar cation exchange properties and, in particular, selective cesium exchange. [Na+ + Al3+]-substituted tobermorite and ganophyllite showed a selective Cs exchange of 10.77 and 8.71 meq/100 g from 0.02N NaCl and of 11.08 and 9.04 meq/100 g from 0.02N CaCl2, respectively. The [Na+ + Al3+]-substituted tobermorite is structurally similar to ganophyllite: both are roughly analogous to 2:1 layer silicates, both have cross-linking tetrahedra across the interlayer region, and both have exchangeable cations located in zeolite-like sites in the interlayer region. The similarities in cation exchange properties imply that the residual charge configuration of the analogue 2 : 1 layer must be approximately equivalent also.
The Peralkaline Nepheline Syenites of the Junguni Intrusion, Chilwa Province, Malawi
Authors: Woolley Alan R. , Platt R. Garth
Published: December 2006

The mineralogy of the highly peralkaline Junguni nepheline syenite intrusion of the Chilwa alkaline province has been investigated. The rocks comprise alkali feldspar, very abundant nepheline, locally exceptionally abundant sodalite, sodic pyroxenes, scarce biotite, rare amphibole, and an extensive range of accessory minerals. Electron microprobe analyses indicate that the pyroxenes define an evolutionary trend from salite through aegirine-augite to aegirine, which is unusual in its broadness and ill-definition. This is explained by a series of overlapping trends produced by fluctuating Fe3+/Fe2+ ratios caused by variations in alkali content of the magma, probably produced by periodic alkali loss by surface de-gassing. Much of the sodalite occurs as a primary liquidus phase, but the paragenesis of certain ramifying sodalite veins is more problematical. It is possible that such ramifying masses are the product of coupled migration of alkalis and volatiles by diffusion in a gravitation field under pressure gradients generated by eruptive events, or by the formation of immiscible Na- and Cl-rich liquids. Both early calcic and late mangan-fluor eckermannitic/arfvedsonitic amphiboles occur. The micas vary from biotites with Mg: Fe ratios < 0·5 to almost pure annites; they are fluor-micas and characterized by high Mn contents. Analyses of niobian rutile, mangan ilmenite, ferroan pyrophanite, manganese-rich eucolite, låvenite, mangan-titan låtvenite, rosenbuschite, wöhlerite, pyrochlore, eudialyte and what is believed to be only the second occurrence of kupletskite are given. Many of these minerals are rich in Zr, Nb, Na and Mn and thus typical of the assemblages found in extreme agpaitic complexes such as Ilimaussaq and Lovozero.
The Role of Tholeiitic Magmatism in the English Lake District: Evidence from Dykes in Eskdale
Authors: Macdonald R. , Millward D. , Beddoe-Stephens B. , Laybourn-Parry J.
Published: December 2006

Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.
A Sulphur Isotope Study of Iron Sulphides in the Late Precambrian Dalradian Easdale Slate Formation, Argyll, Scotland
Authors: Hall A. J. , Boyce A. J. , Fallick A. E.
Published: December 2006

Pyritic slates from the late Precambrian, Middle Dalradian Argyll Group Easdale Slate Formation, contain mainly quartz, muscovite and chlorite with variable amounts of dolomite, albite and paragonite. Slates from Easdale Island and Cuan Ferry contain pyrite porphyroblasts with δ34S = + 12 to + 16‰. The pyrite grew during a post-tectonic retrogressive event at the expense ofpyrrhotine which formed during the main regional metamorphism of the Grampian orogeny by reduction of diagenetic pyrite. Slate from Oban contains abundant diagenetic framboidal pyrite and small syn-tectonic pyrite porphyroblasts with δ34S = +22‰. This pyrite was not all reduced to pyrrhotine on metamorphism so there was little retrogressive growth of pyrite. Metamorphism appears to have homogenized local (cm scale at least) isotopic inhomogeneities and preserved an average seawater-sulphate-sulphide isotopic fractionation value. Middle Dalradian seawater-sulphate had a δ34S value of about + 35‰, so the small fractionations are appropriate for bacteriogenic reduction in bituminous sediments, the heavier sulphide in the case of the Oban slate indicating more rapid reduction of sulphate. Lower Dalradian Appin Group, Ballachulish slate contains pyrite with δ34S = +15±2‰ and is best interpreted as forming in the same manner as the Easdale slates of Easdale Island and Cuan Ferry; the sharp increase in late Precambrian ocean-sulphate sulphur isotope signature from +15 to > +30‰ therefore occurred by Lower Dalradian Appin Group times.
Zirconium-rich Sodic Pyroxenes in Felsic Volcanics from the Warrumbungle Volcano, Central New South Wales, Australia
Authors: Duggan M. B.
Published: December 2006

Soda-rich pyroxenes in felsic rocks from the Warrumbungle Volcano, central New South Wales, contain up to 14.5 wt. % ZrO2, which is more than double the previously reported maximum ZrO2in pyroxene. Zr is believed to enter aegirine as the component Na(Fe2+,Mn,Mg)0.5Zr0.5Si2O6via the coupled substitution: (Fe2+,Mn,Mg)VI+ZrVI= 2(Fe3+)VI. This component exceeds 50 mol. % in some analyses.

Pronounced pyroxene Zr-enrichment is restricted to rocks in which sodic amphibole is the major ferromagnesian mineral, with pyroxene only a minor late-stage phase. The Zr-rich pyroxenes resulted from a combination of host lava peralkalinity, low oxygen fugacity, rapid disequilibrium crystallization and low mobility of the Zr ion. These factors collectively led to the development of interstitial Zr-enriched microdomains in the felsic hosts during their final stages of crystallization.
Compositional Variation in Wolframites from the Barroca Grande Mine, Portugal: Evidence for Fault-Controlled Ore Formation
Authors: Polya D. A.
Published: December 2006

The major and trace element composition of over 40 wolframites from the Barroca Grande mine of the Panasqueira W-Sn-Cu quartz-vein hosted orebody have been determined by electron probe microanalysis.

The hübnerite content of wolframite is largely uniform (12 to 20 mol. %) throughout most of the mine, suggesting relative uniformity in composition of the main stage ore-forming fluids. However, wolframites show a striking pattern of enrichment in niobium adjacent to major sub-vertical fault zones. Typical wolframite Nb2O5 contents are below 1000 ppm, whilst values greater than 1000, 2000 and 4000 ppm are generally only found in wolframites less than 50, 30, and 10 metres respectively from major sub-vertical faults. This strongly suggests that some of these fault zones acted as major pathways for hydrothermal fluid migration during ore formation. The idea that the greisened cupola at Panasqueira was the sole major pathway for fluid movement is discounted.
Xenocrysts in Eocene Camptonites from Taourirt, Northern Morocco
Authors: Mokhtari Abdelkader , Velde Danielle
Published: December 2006

A study of xenocrystic material included in 57 Ma igneous rocks that outcrop in the Taourirt area of north-east Morocco has been made. The mineralogy of the host rocks is essentially clinopyroxene, rare olivine, titanomagnetite and either kaersutite or biotite. Feldspars (plagioclase and alkali feldspar) and nepheline are usually altered. Accessories may include haüyne, perovskite and ferriannite-rich annite. The complex xenocrystic assemblage includes various types of clinopyroxene, amphiboles, micas of differing compositions and various types of spinel. These inclusions did not dissolve in the melt and, as a consequence, the residual (alkali-rich) mineralogical assemblage is not modified. Their abundance is such that they may not significantly affect the major element compositions, but they may strongly influence the levels of certain trace elements such as Cr and Ni.
Mineral Chemistry of Leucitites from Visoke Volcano (Virunga Range, Rwanda): Petrogenetic Implications
Authors: Marcelot G. , Rançon J. Ph.
Published: December 2006

The Visoke complex is one of the main Quaternary volcanic centres of the Virunga Range, located north of lake Kivu. Mineralogical (microprobe) data are given for two representative leucitite lavas; one sample contains a complex coarse-grained xenolith (phlogopite, diopside, leucite, titanomagnetite, perovskite and apatite) and megacrysts of pyroxene, phlogopite and olivine scattered in fine-grained leucite-rich host lava. Compared with the typical leucite-dominated, low-pressure phenocryst assemblage of the two samples studied, the chemical trends of ferromagnesian crystals suggest an earlier igneous event (high-pressure phenomenon) strongly related to the leucite-bearing magma suite.
Metamorphism of the Palaeozoic Cinco Villas Massif (Basque Pyrenees): Illite Crystallinity and Graphitization Degree
Authors: Pesquera A. , Velasco F.
Published: December 2006

The degree of metamorphism affecting the Devonian-Carboniferous rocks of the Palaeozoic Cinco Villas massif has been studied by (a) petrographic techniques, (b) illite crystallinity, and (c) degree of graphitization of the carbonaceous material. Some mineralogical differences have been found between the Devonian and Carboniferous rocks; paragonite and mixed-layer paragonite/muscovite, typical of anchimetamorphic areas, appear in the Devonian but are not found in the Carboniferous rocks. These are characterized by the local appearance of chloritoid, garnet, amphibole, epidote, andalusite and biotite, and the generalized presence of muscovite/chlorite. Illite crystallinity shows a metamorphic zonation (anchizone epizone) towards the granitic Aya massif, and a concentric pattern around the intrusive body.

The optical analysis shows that the first effects of the regional metamorphism began before the D2deformation, reaching its paroxysm during this phase and continuing afterwards. The degree of graphitization evolves progressively with metamorphism, and this fact is reflected in an exponential relationship betweend(002)and crystallinity Lc(002). The similarity in the degree of graphitization between homologous materials within and outside the Aya aureole suggests a similar kinetic factor for both the thermal and regional metamorphisms, or a similar duration time. This fact, as well as the distribution and relationship between the deformation and recrystallization of the minerals, suggests a syn-plutonic regional metamorphism developed at low pressures, in accordance with the value of theb0parameter for the white micas, and probably a maximum temperature of about 500°C.
The Significance of Almandine Garnets in the Lundy and Dartmoor Granites
Authors: Stone Maurice
Published: December 2006

Almandine garnets in the cordierite-bearing granite of Sweltor Quarry, Dartmoor, contain < 10 mol. % of the spessartine end-member, whilst those in the Lundy granite have c. 10 mol. % spessartine. Experimental work indicates that such compositions can grow in equilibrium with siliceous melts at depths of 18–25 km. This evidence, reaction rims, lack of marked zoning and comparison with garnets in other siliceous calc-alkaline siliceous rocks point to a genesis involving partial melting of the ‘local’ lower crust. A restite origin rather than direct crystallization from magma is favoured but the evidence is equivocal. The Dartmoor granite (Hercynian) is a typical peraluminous late- to post-tectonic S-type granite. The S-type character of the Lundy (Tertiary) granite is revealed by the occurrence of garnet and topaz together with biotite enriched in Rb, Cs and F, despite its close association with Tertiary basic magmatism in an anorogenic setting.
Magmatic Garnets in the Cairngorm Granite, Scotland
Authors: Harrison T. N.
Published: December 2006

Small, euhedral Mn-rich garnets (32–52 mol. % spessartine) from the Cairngorm granite, Eastern Grampian Highlands, Scotland, are considered to be of magmatic origin and have not been derived from the assimilation of metasedimentary material, despite their occurrence largely at the margins of the pluton. Similar garnets also occur in a late cross-cutting aplite sheet. The garnets in the granite crystallized early in the sequence and are thought to have formed in response to the ponding of Mn-rich fluids against the wall of the pluton. This Mn enrichment of the fluid phase continued throughout the evolution of the pluton, resulting in Mn-rich biotites and opaque oxides and the localized crystallization of Mn-rich garnets in aplite. Garnet contains up to 1.67 wt. % Y, but has not played a major role in the geochemical evolution of the Cairngorm granite, which has high SiO2 (72–77%) and is enriched in Y and HREE. Chemical analyses of garnets, biotites and rocks are given.
Cathodoluminescence Colours of α-Quartz
Authors: Ramseyer K. , Baumann J. , Matter A. , Mullis J.
Published: December 2006

A new cathodoluminescence-microscope has been developed with a considerably improved detection limit. Time-dependent luminescence intensity changes observed during electron bombardment enabled the recognition of short-lived, long-lived, and brown luminescence colour types in α-quartz.

Short-lived bottle-green or blue luminescence colours with zones of non-luminescing bands are very common in authigenic quartz overgrowths, fracture fillings or idiomorphic vein crystals. Dark brown, short-lived yellow or pink colours are often found in quartz replacing sulphate minerals. Quartz from tectonically active regions commonly exhibits a brown luminescence colour. A red luminescence colour is typical for quartz crystallized close to a volcanic dyke or sill.

The causes of these different and previously poorly understood luminescence colours were investigated using heat treatment, electron bombardment and electrodiffusion. Both natural and induced brown luminescence colours reflect the presence of lattice defects (nonbonding Si-O) due to twinning, mechanical deformation, particle bombardment or extremely rapid growth. The bottle-green and blue linearly polarized luminescence colour, characterized by a plane of polarization parallel to the c-axis, both depend on the presence of interstitial cations. The yellow and red luminescence colours in α-quartz both exhibit a plane of polarization perpendicular to the c-axis and appear to be related to the presence of trace elements in an oxidizing solution and to ferric iron respectively.
The Chemistry of Formation of Some Secondary Arsenate Minerals of Cu(II), Zn(II) and Pb(II)
Authors: Magalhães M. Clara F. , Pedrosa de Jesus Julio D. , Williams Peter A.
Published: December 2006

Solution studies have been carried out on natural and synthetic arsenate minerals, which are often found in the oxide zones of base metal orebodies. Solubility products and free energy of formation data have been derived for the minerals olivenite, cornubite, clinoclase, adamite, legrandite, euchroite, duftite, conichalcite, austinite, bayldonite, and schultenite at 298.2 K (25 °C). The data have been used in turn to construct stability field diagrams illustrating the chemical conditions under which the various species may crystallize from aqueous solution. This equilibrium model is then compared with several natural occurrences of the arsenate suites and it is demonstrated that it can be used to explain a number of observed paragenetic sequences. Descriptions of solution conditions which describe the stabilities of the arsenate minerals with respect to more commonly found secondary minerals of Pb(II), Cu(II), and Zn(II) involve more complex calculations and estimates of the likely levels of various dissolved species, but it is shown how these may be taken into account in the development of a more complex equilibrium model.
Ion microprobe analysis—a review of geological applications
Authors: Reed S. J. B.
Published: December 2006

In ion microprobe analysis the specimen is bombarded with a focussed ion beam a few µm in diameter and the secondary ions produced are accelerated into the entrance slit of a mass spectrometer. An outline of the salient features of the instrument is given here, together with an account of the methods used for quantitative elemental and isotopic analysis.

The major part of this paper consists of a comprehensive account of the geological applications of ion microprobe analysis. These include elemental analysis, especially for trace elements (down to sub-ppm levels in many cases) and light elements (H-F) which are beyond the scope of the electron microprobe. The other main area of geological interest is isotopic analysis, where the ion microprobe has the advantage over conventional mass spectrometry of being capable of in situ analysis of selected points on polished sections, obviating the need for laborious specimen preparation, and enabling spatially-resolved data to be obtained, with a resolution of a few µm. The ion microprobe has been especially successful in U-Pb zircon dating and the study of isotope anomalies in meteorites. Other significant applications include diffusion and stable isotope studies.
A Pan-African zincian staurolite imprint on Namaqua quartz–gahnite–sillimanite assemblages
Authors: Moore J. M. , Reid A. M.
Published: December 2006

Quartz-gahnite-sillimanite asssemblages are described from a supracrustal enclave at Kraaifontein, 45 km south-west of Springbok in the Namaqualand Metamorphic Complex, South Africa. The assemblage formed by prograde reaction in the granulite facies zone during the 1100 Ma Namaqua event, possibly as the result of desulphidation of sphalerite. Subsequent lower-amphibolite-facies retrogression occurred in close proximity to shear zones during an early Pan-African metamorphic event at 700 Ma. Zincian staurolite formed as overgrowths on gahnite in a hydration reaction involving the consumption of gahnite, quartz, and sillimanite. Compositional zoning to more zinc-rich rims in gahnite at Kraaifontein is unrelated to the retrograde reaction, but is interpreted as a result of changing conditions during the prograde Namaqua event.
A barian bannisterite from Japan
Authors: Matsubara S. , Kato A.
Published: December 2006

Chemical analyses of bannisterite from the Kamo mine, Toba City, Mie Prefecture, Japan, give Ba : Ca = 69:31–55:45, yielding the ideal formula with Ba > Ca, i.e. (Ba,Ca)(K,H3O)(Mn2+ · Mg,Fe2+)21 (Si,Al)32(O,OH)92 · nH2O, where Ba > Ca, Mn2+ > Mg, Fe2+,Si ≫ Al, and O > OH. The unit cell parameters calculated after the indexing of the X-ray powder pattern are: a = 22.95, b = 16.52, c = 25.66 Å, β = 94.2°. It occurs as dark brown veinlets cutting massive caryopilite-rhodochrosite ore, which is also cut by veinlets of manganoan chlorite (Mn/(Mg + Mn + Fe) = c. 0.30–0.39) with minor barian orthoclase (Ba/(K+ Ba) = 0.05 ∼ 0.06).
Zirconium-bearing amphiboles from the Igaliko Dyke Swarm, South Greenland
Authors: Pearce N. J. G.
Published: December 2006

Sodic-calcic and alkali amphiboles from benmoreitic members of the Igaliko Dyke Swarm contain up to 4.13 wt. % ZrO2. It is proposed that Zr enters the amphiboles by a coupled substitution of

(where C = octahedral site and T = tetrahedral site) to produce the richest Zr-bearing amphiboles so far identified, with compositions ranging up to

These amphiboles crystallize at a late stage from magmas which were Zr-rich, highly peralkaline and hydrous, with an fo2 close to the synthetic QMF buffer. The incorporation of Zr in to the amphibole is a consequence of the failure of other Zr-bearing phases (such as zircon, baddeleyite, eudialyte) to crystallize.
Monazite end members and solid solutions: synthesis, unit-cell characteristics, and utilization as microprobe standards
Authors: Montel J. M. , Lhote F. , Claude J. M.
Published: December 2006

The synthesis of monazite was first reported by Radominsky (1875). Since then various methods have been used to synthesize various end members of the monazite solid solution series, mainly CePO4 and LaPO4 (e.g. Anthony, 1957, 1965). As part of an experimental study dealing with the solubility of monazite in granitic melts (Montel, 1986, 1987, and in prep.), the synthesis of some of the end members, as well as solid solutions, was achieved.
Dahllite and whitlockite from Amatuku islet, Tuvalu
Authors: Rodgers K. A.
Published: December 2006

Phosphatic limestones and a related soil were described from collections made by the first Royal Society coral-reef-boring expedition to Funafuti in 1896 (Cooksey, 1896; David and Sweet, 1904, Judd, 1904; Sollas, 1904; cf. Cullis, 1904). Although there is some geographic uncertainty about the source of some of Judd's samples, the specimens came from three localities on Funafuti, the largest being on Amatuku.