Philippe de Parseval

Crystal-chemistry of talc: A near infrared (NIR) spectroscopy study

Abstract The crystal-chemistry of fifteen samples of talc from various localities and origins having a relatively wide range of Fe, Al, and F contents, have been studied mainly by Fourier-transform infrared (FTIR) spectroscopy, in both the near infrared (NIR) and middle infrared (MIR) regions, and by Mössbauer spectroscopy. For the first time, assignments have been made for the 2νOH bands in talc, and Mg/Al substitutions were revealed using NIR. Less than 0.01 atom of Fe2+ or Al per half unit cell can be detected from the 2νOH region, whereas the sensitivity is about half in the νOH region. The amounts of F also can be quantified by NIR measurement, using the F inductive effect on the width of the 2νMg3OH band. The NIR data show that Fe2+ and Mg are distributed randomly in their octahedral sheet. Actual structural formulae of natural talcs can be obtained from NIR and Mössbauer spectroscopies only, even for impure talc samples. NIR spectroscopy is simple to use and is a very powerful tool to study the crystal chemistry of talc.

Gold grain morphology and composition as an exploration tool: application to gold exploration in covered areas

The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; >300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.

Mineralogy and geochemistry of micas from the Pinilla de Fermoselle pegmatite (Zamora, Spain)

The highly fractionated, Li-F-Be-B-P-bearing Pinilla de Fermoselle (PF) pegmatite crops out in the westernmost part of the Zamora province (Spain). This body appears as a cupola over the PF leucogranite, displaying a non-symmetrical internal zonation with a complete sequence from a barren pegmatitic facies near the granite, to a highly evolved zone in the uppermost part of the body. Representative samples of micas from the different pegmatite zones have been studied. Based on textural and chemical criteria, the micas may be grouped into two assemblages: Al-rich micas and Fe-rich micas. In general, Al-rich micas show a continuous evolution from muscovitic to lepidolitic compositions from the leucogranite to the most evolved zone. Fe-rich micas range from Fe-biotite in the leucogranite and in the least evolved pegmatite zones, to an intermediate composition between zinnwaldite and trilithionite in the most evolved pegmatitic facies. The incorporation of Li into micas appears to be controlled by the substitutions Si 2 LiAl -3 ,a nd Li3Al-1-2, AlLiR-2 ,S iLi 2R-3 ,a nd SiLiAl -1R-1, where R = (Fe 2+ + Mg + Mn). Paragenetic relationships and chemical variations in micas from different zones making up the PF pegmatite suggest that the pegmatitic system derived from a granitic melt and evolved upwards by fractionation processes. Evidence in support of this model comes from: (i) the gradual enrichment in Li, Rb, Cs and F, parallel to the decrease in Mg and Ti; (ii) the convergent evolutionary trends towards lepidolite showed by the Al- and Fe-micas; and (iii) the parallel decrease in the K/Rb ratio in micas.

Multimetallic contamination from Zn-ore smelter: solid speciation and potential mobility in riverine floodbank soils of the upper Lot River (SW France)

The former Zn-ore smelting activity in Viviez (Aveyron, France) resulted in multimetallic contamination of the upper Lot River system (SW France). This study addresses for the first time the metals/metalloids mobility in impacted riverside fluvial soils due to reducing conditions during long-term flooding events. Six impacted riverside fluvio-soils were sampled along the Riou Mort and Lot rivers. Their levels of contamination decrease with the distance from the contamination source. Higher enrichment factors (EF) relative to French average soil metallic content occur 1 km far from the contamination source and vary from 3 for Sn, to 5 for As, 9 for Sb, 27 for Pb, 40 for Zn and 63 for Cd. At 20 km downstream from the contamination source, EF still reach a value of 3 for Cd and Zn. A micro-physical mineralogical characterisation of contaminated soils revealed a variety of metal-bearing phases: metallic compounds, oxides and sulphides, Fe oxides, glass and silicates as well as coke. Sequential chemical extraction experiments were conducted on the most contaminated soil in order to identify which mineral phases were the best candidates to release metals/metalloids. At the end of experiments more than 70 % of total Cd and As, 55 % of Zn and Sb, and 40 % of Pb and Sn were extracted from the soil. Only Cd appeared as readily mobilised, with 33 % of its total amount being extracted in the first steps of experiments. Under reducing conditions, iron oxyhydroxides, franklinite and multimetallic oxides are the best candidates, in decreasing order of importance, for metals/metalloids release from the soil.

Fine-probing the crystal-chemistry of talc by MAS-NMR spectroscopy

Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy was used to probe the crystallographic environment of Si, Al, F and H in 14 natural talc samples originating from different localities and containing various, small amounts of iron (Fe2O3 (total) < 2 wt %). We show that iron induces strong variations in NMR spectra and that even very low quantities can be used as an indirect NMR parameter to characterize in detail the crystal-chemistry of talc. Conversely, their correlations with full width at half height (FWHH) in MAS-NMR spectra for 29Si, 27Al, 1H, and 19F obtained from these 14 talc samples allow for rapid estimations of the total iron content. Finally we highlight how, by simply recording Mossbauer, NIR and NMR spectra, one can obtain an accurate structural formula of talc, which is a necessary parameter for uses in specific industrial applications.

Occurrence and late re-equilibration of pollucite from the Koktokay no. 3 pegmatite, Altai, northwestern China

Abstract The Koktokay no. 3 pegmatite, Altai, NW China, is a strongly zoned spodumene-subtype pegmatite. Pollucite is the unique ore mineral exploited for Cs in this pegmatite. It occurs in internal textural zones of the pegmatite in different abundances. Primary pollucite is largely homogeneous, but it displays a broad range of composition between different textural zones with CRK [=100(Cs + Rb + K)/(Na + K + Rb + Cs + Mg + Ca)] = 74.86 and Si/Al = 2.20.2.51. Four principal types of pollucite were distinguished. (1) First is pollucite with blebby mosaic texture consisting of Na-enriched and Cs-enriched phases, which clearly resulted from local exsolution of primary pollucite in the sub-solidus state. (2) The second type is nearly end-member pollucite (CRK > 90) that occurs as an aureole of primary pollucite in contact with lepidolite or feldspar clusters. This is formed by dissolution/ re-precipitation of primary pollucite. (3) Symplectic pollucite is associated with quartz, observed in sub-parallel veinlets penetrating surrounding albite crystals; compositionally, this pollucite attains a CRK ratio of up to 96 and resulted from replacement of albite by Cs-rich fluids. (4) Oscillatoryzoned pollucite is the fourth type, typically restricted to the contact of pollucite with small miarolitic cavities. The zonation compositionally oscillates about the Cs/Na variations, and is simply related to locally changing fluid composition.

Monazite from mountain to ocean: a case study from Trolognaro (Fort-Dauphin), Madagascar

We report results for 18 samples of sediments containing detrital monazite, collected along the valley of the Ifaho River, near Trolognaro (Fort Dauphin), Madagascar. This valley, approximately 50 km long, connects the Manangotry Massif, which contains monazite-rich plutonic rocks with several very large monazite occurrences, in the Anosyan mountain belt, to the Indian Ocean where sands are unusually rich in heavy minerals, including monazite. Monazite grains were extracted from the sediments using chemical, gravitational, and magnetic separation techniques. Grains were studied for morphology, chemical composition, age, and structural state. From mountain to ocean, grains become smaller and more rounded. The chemical composition (more than 400 electron probe micro-analysis, EPMA) remains constant, and always highly radioactive. All ages (55 EPMA measurements) are Pan-African. X-ray diffraction shows that the monazite grains studied are well crystallized, despite the high self-irradiation doses they have experienced. In all aspects, except shape and size, detrital monazites are identical to monazites from the source region, and are transported essentially unchanged from mountain to ocean. This study demonstrates that monazite can survive the erosion-transport-deposition sequence, and supports the proposition to use this mineral as actinide waste-form for long-term storage of nuclear materials.

Structure of the {001} talc surface as seen by atomic force microscopy comparison with X-ray and electron diffraction results

In this study the surface structure of a centimetre sized crystal of talc from the Trimouns deposit (Ariege, France) was imaged by atomic force microscopy. The direct image shows detailed characteristics of clay tetrahedral surfaces. The unit-cell dimensions obtained 30 using atomic force microscopy (aor = 5.47±0.28 and bor = 9.48±0.28 A) are found to be slightly higher, with an increased uncertainty, than those obtained using X-ray diffraction (aor = 5.288±0.007 and bor = 9.159±0.010 A) and selected-area electron diffraction (aor = 5.32±0.03 and bor = 9.22±0.05 A). Talc has a quasi-ideal surface, clean of strong 34 structural distortion as compared to most of other clay minerals and unlikely surface 35 relaxation. The observation on the obtained image of apparent cell dimension enlargement is 36 then more likely attributed to instrumental artefacts, also responsible for scattered values of unit-cell parameters rather than related to any surface structural features.

Succession of Permian and Mesozoic metasomatic events in the eastern Pyrenees with emphasis on the Trimouns talc–chlorite deposit

Abstract Recent studies proposing pre-orogenic mantle exhumation models have helped renew the interest of the geosciences community in the Pyrenees, which should be now interpreted as a hyper-extended passive margin before the convergence between Iberia and Eurasia occurred. Unresolved questions of the Pyrenean geology, as well as the understanding of the formation of hyper-extended passive margins, are how the crust was thinned, and when, where and how the crustal breakoff occurred. The study of the Variscan and pre-Variscan Pyrenean basement is thus critical to document and understand this Cretaceous crustal thinning. In order to specify the timing of Mesozoic metasomatism and the associated deformation in the pre-Mesozoic basement of the Pyrenees, we carried out a U–Th–Pb laser ablation ICP–MS study on a large panel of REE and titanium-rich minerals (titanite and rutile) from talc–chlorite ores from the eastern Pyrenees, with a special emphasis on the Trimouns deposit, the world’s largest talc quarry. Our results suggest that the Trimouns talc formation was restricted to the upper Aptian–Cenomanian time, while the talc and chlorite formation in the eastern Pyrenees occurred during several distinct Permian, Jurassic and Cretaceous episodes. These results give strong constraints on the tectonic setting of the Pyrenean domain during the transition between the Variscan and Alpine orogenic cycles, and particularly on when and how the upper crust was thinned before the crustal breakoff and the final mantle exhumation.

Study of the Eutectoid Transformation in Nodular Cast Irons in Relation to Solidification Microsegregation

Eutectoid transformation in cast irons may proceed in the stable or the metastable systems giving ferrite and graphite for the former and pearlite for the latter. The present work demonstrates that composition profiles across ferrite/pearlite boundaries are smooth and similar to those issued from the solidification step. No trace of long-range diffusion of substitutional solutes due to austenite decomposition could be observed. In turn, this ascertains that both stable and metastable transformations proceed with the product matrix—either ferrite or pearlite—inheriting the parent austenite content in substitutional solutes. This result sustains a physical model for eutectoid transformation based on the so-called local para-equilibrium which is commonly used for describing solid-state transformation in steels.