Please download to get full document.

View again

of 17
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.


Publish on:

Views: 13 | Pages: 17

Extension: PDF | Download: 0

1657 The Canadian Mineralogist Vol. 40, pp (2002) SCANDIUM MINERALOGY: PRETULITE WITH SCANDIAN ZIRCON AND XENOTIME-(Y) WITHIN AN APATITE-RICH OOLITIC IRONSTONE FROM SAINT-AUBIN-DES-CHÂTEAUX, ARMORICAN MASSIF, FRANCE YVES MOËLO Laboratoire de Chimie des Solides, Institut des Matériaux Jean Rouxel (UMR CNRS 6502), 2, rue de la Houssinière, BP 32229, F Nantes Cedex 3, France YVES LULZAC Laboratoire de Gemmologie, Université de Nantes, 2, rue de la Houssinière, BP 9208, F Nantes Cedex 3, France OLIVIER ROUER Institut des Sciences de la Terre d Orléans (UMR CNRS-Université C 6113), 1A, rue de la Férollerie, F Orléans Cedex 2, France PIERRE PALVADEAU Laboratoire de Chimie des Solides, Institut des Matériaux Jean Rouxel (UMR CNRS 6502), 2, rue de la Houssinière, BP 32229, F Nantes Cedex 3, France ÉRIC GLOAGUEN* Laboratoire de Minéralogie, Muséum National d Histoire Naturelle, Unité CNRS FREE 2456, 61 rue Buffon, F Paris, France PHILIPPE LÉONE Laboratoire de Chimie des Solides, Institut des Matériaux Jean Rouxel (UMR CNRS 6502), 2, rue de la Houssinière, BP 32229, F Nantes Cedex 3, France ABSTRACT The scandium phosphate pretulite has been identified with scandian zircon and xenotime-(y) in an apatite-rich oolitic Ordovician ironstone at Saint-Aubin-des-Châteaux, Armorican Massif, France. Pseudo-octahedral crystals of pretulite, up to 400 m across, have grown epitactically on detrital zircon. They reveal complex zoning due to incorporation of Y and HREE, as well as to an extended solid-solution toward the zircon end-member. Characteristic compositions in the pretulite xenotime-(y) zircon system are: Prl Xnt Zrn 0.007, Prl Xnt Zrn 0.005, Prl Xnt Zrn 0.085, Prl Xnt Zrn and Prl Xnt Zrn A single-crystal X-ray refinement of the structure in space group I4 1 /amd (R = ) gives a (9), c 5.809(1) Å, for the formula (Sc Y HREE Zr )(P Si )O 4. The Raman spectrum is presented. Detrital zircon shows phosphate-rich metamict zones containing HREE and Sc (up to 3.2 wt.% Sc 2 O 3 ). Analytical and crystallographic data suggest a complete solid-solution between zircon and pretulite. Xenotime-(Y), also epitactic on zircon, shows distinct stages of crystallization, with a decrease in Y together with an enrichment in the lighter REE and Sc (up to 0.7 wt.% Sc 2 O 3 ). The scandium minerals at Saint-Aubin reflect the evolution of the iron ore, from sedimentation to diagenesis and metamorphism, followed by multistage hydrothermal leaching and recrystallization. Despite the high concentration of Fe in the environment, this quite unique occurrence of Sc minerals illustrates the high capacity of the phosphate ion to extract scandium and precipitate it as a specific phase, at relatively low-temperature conditions. Keywords: scandium, phosphate, pretulite, zircon, xenotime-(y), iron ore, Ordovician, Armorican Massif, France. address: * Present address: Institut des Sciences de la Terre d Orléans (UMR CNRS-Université C 6113), 1A, rue de la Férollerie, F Orléans Cedex 2, France 1658 THE CANADIAN MINERALOGIST SOMMAIRE Un phosphate de scandium, la prétulite, a été identifié, en association avec du zircon et du xénotime-(y) scandifères, dans un minerai de fer oolithique ordovicien riche en apatite à Saint-Aubin-des-Châteaux, Massif Armoricain, en France. Les cristaux pseudo-octaédriques de prétulite (jusqu à 400 m) sont en surcroissance épitaxique sur du zircon détritique. Ils montrent une zonation complexe due à l incorporation du Y et des terres rares lourdes, ainsi qu à une solution solide étendue vers le pôle zircon. Des compositions-types dans le système prétulite xénotime-(y) zircon sont: Prl Xnt Zrn 0.007, Prl Xnt Zrn 0.005, Prl Xnt Zrn 0.085, Prl Xnt Zrn et Prl Xnt Zrn L affinement de la structure cristalline aux rayons X sur monocristal (groupe spatial I4 1 /amd, R = 0,0389) a été faite avec a 6,5870(9), c 5,809(1) Å, sur la base de la formule (Sc 0,904 Y 0,032 TRL 0,016 Zr 0,048 )(P 0,952 Si 0,048 )O 4. Le spectre Raman est présenté. Le zircon détritique montre des zones métamictes riches en phosphate, avec terres rares lourdes (TRL) et scandium (jusqu à 3,2% pds Sc 2 O 3 ). Les données tant analytiques que cristallographiques suggèrent une solution solide complète entre prétulite et zircon. Le xénotime-(y), également en épitaxie sur le zircon, montre plusieurs stades de cristallisation, traduisant un appauvrissement en Y corrélatif d un enrichissement en faveur de terres rares plus légères, ainsi qu en scandium (jusqu à 0,7% pds Sc 2 O 3 ). La minéralogie du scandium à Saint-Aubin reflète l évolution du minerai de fer: sédimentation, diagenèse et métamorphisme, et enfin lessivage et recristallisation par des venues hydrothermales polyphasées. Malgré la haute concentration en fer de l environnement, cette occurrence très particulière illustre la forte capacité de l ion phosphate à se combiner au scandium et à le précipiter sous forme d une phase minérale spécifique à relativement basse température. Mots-clés: scandium, phosphate, prétulite, zircon, xénotime-(y), minerai de fer, Ordovicien, Massif Armoricain, France. INTRODUCTION Scandium is rarely expressed as specific mineral species, owing to its dilution in common silicates, where it substitutes for Fe, Mg and Al. There exist at present only nine approved species of scandium minerals: six silicates (bazzite, thortveitite, cascandite, jervisite, scandiobabingtonite and kristiansenite), and three phosphates (kolbeckite, pretulite and juonniite) (Mellini et al. 1982, Orlandi et al. 1998, Hey et al. 1982, Bernhard et al. 1998b, Liferovich et al. 1997, Raade et al. 2002). Pretulite was discovered at Höllkogel, in eastern Austria, by Bernhard et al. (1998b), who described numerous occurrences within quartz lazulite veins in the Lower Austro-alpine Grobneis complex. Another probable occurrence of pretulite was incompletely described as an unnamed Sc phosphate by Novák & Srein (1989) in the Dolní Bory pegmatites of western Moravia, Czech Republic. The present study deals with a new occurrence of pretulite, identified in a sandstone quarry located at Saint-Aubin-des-Châteaux, Loire-Atlantique, in western France. Here, pretulite is closely associated with scandian zircon and xenotime-(y) within a sedimentary iron ore showing a complex paragenetic evolution. This occurrence presents new insight concerning aspects of the geochemistry and crystal chemistry of scandium. (1974), they are essentially composed of iron oxides (magnetite, hematite, ilmenite), silicates (chlorite, stilpnomelane) and quartz, siderite, pyrite and apatite. The phosphate is ubiquitous (mean concentration over 2 wt.%; up to 6 wt.% in the B horizon). In the Saint-Aubin quarry, only the A horizon is well developed. It is mainly composed of siderite and chlorite, but locally very enriched in Sr-bearing fluorapatite (4 wt.% SrO: Chauvel & Phan 1965). It was affected by diagenesis and very low grade metamorphism. The primary sedimentary features, where preserved, consist of millimetric layers of oolites. Some of these layers are enriched in fluorapatite (abundant), or minor detrital GEOLOGICAL SETTING AND PETROLOGY Figure 1 gives the geographic location of the quarry of Saint-Aubin-des-Châteaux. This quarry is situated in the lower member of the Grès armoricain Formation, of Arenigian age. This member is mainly composed of sandstone; at a regional scale, it includes oolitic ironstones (Chauvel 1974) at four main horizons, A to D, from top to bottom. These ironstones have been mined in the past (Puzenat 1939). According to Chauvel FIG. 1. Geographic location of the Bois-de-la-Roche quarry at Saint-Aubin-des-Châteaux (arrow on the inset map). SCANDIUM MINERALS IN OOLITIC IRONSTONE, SAINT-AUBIN, FRANCE 1659 titanium oxides and zircon. Carbonaceous phases (organic matter and late graphite) are disseminated throughout the rock, giving it a characteristic black color. SEM examinations reveal rare minute crystals of galena, sphalerite and monazite-(ce). Pretulite and xenotime-(y) are only present as epitactic overgrowths on zircon crystals. The primary texture was later altered by hydrothermal processes (at least three stages of hydrothermal activity), as indicated notably by the presence of massive pyrite, together with minor base-metal sulfides like marcasite, galena, and sphalerite (Herrouin et al. 1989, Moëlo et al. 2000, Gloaguen 2002). Centimetric to decimetric veinlets of quartz, siderite, pyrite and lulzacite (a recently described Sr Al Fe phosphate: Moëlo et al. 2000, Léone et al. 2000) were formed within the ironstone (incorrectly called limestone in Moëlo et al. 2000). Hydrothermal processes have also transformed the ironstone itself locally by recrystallization of siderite, dissolution and recrystallization of Srrich fluorapatite, crystallization of graphite lamellae at the expense of organic matter, and formation of a redbrown variety of chlorite. Descriptive aspects PRETULITE Pretulite was first discovered in an apatite-rich fragment from the A horizon, showing numerous submillimetric hexagonal platelets of red-brown chlorite (a Mg-poor, Al-rich chamosite). This fragment was dissolved in HCl; the whole residue contains a dozen crystals of pretulite with a flattened pseudo-octahedral habit (Fig. 2). The main form is the bipyramid {011}, with subordinate basal faces {001}; faces of the prismatic form {110} are rare. The size of these crystals varies from 150 to 400 m across; they are translucent, yellowish white, with an adamantine luster. In polished section, one of these crystal appears to be an overgrowth on a rounded crystal of zircon (Fig. 3); xenotime-(y) also is present. Thin sections made of samples of the A horizon show other crystals of pretulite, invariably as an epitactic overgrowth on zircon crystals (Fig. 4); gangue minerals are chamosite with fluorapatite and graphite. Chemical characterization Imaging by scanning electron microscopy (SEM) and with back-scattered electrons (BSE) invariably reveals the presence of chemical zoning in the crystals of pretulite. Figure 3 reveals a complex pattern of growthinduced zoning, showing schematically first a dark grey core (A zone), secondly, a narrow intermediate rim (B zone, light grey), then a wide grey outer zone (C zone), and finally (at the opposite side) a thinner white layer located close to the zircon crystal (D zone). These zones were characterized chemically by elemental mapping and quantitative electron-probe micro-analysis (EPMA; Figs. 5a-d, Table 1). In the A zone (anal. 1 7), the pretulite is the richest in Sc, with low contents of yttrium (1.2 to 3.0 wt.% Y 2 O 3 ) and heavy rare-earth elements (HREE) (mostly FIG. 2. SEM BSE image of a euhedral crystal of pretulite (left), and proposed crystal forms (right): combination of main {011} bipyramid with minor {001} and rare {110}. 1660 THE CANADIAN MINERALOGIST FIG. 3. Overgrowth of a zoned crystal of pretulite (~ m), together with a crystal of xenotime-(y) (Xnt) over a detrital zircon crystal. SEM-BSE image of a polished section. Central zone A (dark grey): REE- and (Zr, Si)-poor pretulite; intermediate zone B (light grey) enriched in REE and (Zr, Si); zone C (medium grey, upper and right border): REE-rich pretulite; zone D (white rim close to zircon): (Zr, Si)-rich pretulite. FIG. 4. Epitactic overgrowth of euhedral pretulite (Ptl, greyblack), with an external Zr-rich rim [Ptl(Zr), light grey) at the two opposite ends of a detrital crystal of zircon (Zrn, light grey). The white zones correspond to an epitactic overgrowth of xenotime-(y) (Xnt). Chlorite (Chl) with graphite lamellae (within black area) is the matrix mineral. A smaller zircon xenotime-(y) pretulite aggregate is visible at right. SEM BSE image; for clarity, the contrast between the central part [zircon and xenotime-(y)] and the rest of the image has been attenuated. FIG. 5. Chemical zoning of a section of a pretulite crystal (see also Fig. 3), observed with EPMA elemental mapping (a to d: Sc, Zr, Y and Yb, respectively). Colors range from violet to red, and indicate an increasing concentration of the element. In a, red areas correspond to zone A of Figure 3; zone D (black) is not visible. In b, well-defined red areas correspond to Zr-rich parts of zone B (center) and to zone D (bottom, with zircon). Individual maps do not allow us to distinguish zone C. TABLE 1. RESULTS OF ELECTRON-PROBE MICRO-ANALYSES OF PRETULITE FROM SAINT-AUBIN-DES-CHÂTEAUX SCANDIUM MINERALS IN OOLITIC IRONSTONE, SAINT-AUBIN, FRANCE 1661 1662 THE CANADIAN MINERALOGIST Yb, then Er, Dy and Lu, with a total below 1.3 wt.% of the respective oxides); zirconium and silicon contents are very low, below 0.6 wt.% ZrO 2 and 0.4% SiO 2. The composition richest in Sc (no. 1) corresponds to the formula: [Sc (Y Yb Er ) Zr ](P Si Al )O 3.989, simplified as Prt Xnt Zrn In the B zone (nos. 8 9), the pretulite shows a significant enrichment in Zr and Si, up to 6.9 wt.% ZrO 2 and 3.4% SiO 2. Composition 9 corresponds to [Sc (Y Yb Er Lu ) Zr ](P Si Al )O 4.018, or Prt Xnt Zrn In the C zone (nos ), the pretulite shows minor amounts of Zr and Si (below 2.8% ZrO 2 and 1.4% SiO 2 ), with an enrichment in Y (up to 4.3% Y 2 O 3 ) and HREE (especially Yb, up to 2.5% Yb 2 O 3 ). The composition richest in (Y,HREE), no. 10, corresponds to [Sc (Y Yb Er Lu Dy ) Zr ] (P Si Al )O 3.994, or Prt Xnt Zrn In the D zone (nos ), the pretulite reveals high Zr and Si contents (17 20% ZrO 2 and 9 11% SiO 2 ); minor quantities of Y and Yb are present in similar proportions (about 1 wt.% of the respective oxides). Composition 19 corresponds to [Sc (Y Yb Er Lu Dy ) Zr Hf ](P Si Al ) O 3.997, or Prt Xnt Zrn In all cases enriched in Zr and Si (over 0.02 atoms per formula unit, apfu), the atomic ratio Zr/Si is invariably close to 1, in agreement with a solid-solution scheme according to the coupled heterovalent substitution Sc 3+ + P 5+ Zr 4+ + Si 4+ ; the molar ratio ScPO 4 / ZrSiO 4 attains 2.8. Electron-microprobe data on pretulite from a thin section confirm these results. In the areas with the higher Zr and Si contents, in the D zone (Table 1, nos ), the molar ratio ScPO 4 / ZrSiO 4 generally varies between 3.5 and 2.1, but was found to be as low as 0.90 for one composition (no. 30), thus corresponding to a scandian zircon with formula [Sc (Y Yb Er Dy ) Zr Hf ] (P Si Fe )O 4.000, or Prt Xnt Zrn The pretulite from Saint-Aubin is much richer in Y and HREE (Yb, Er and Dy, and Lu) than the sample from Höllkogel, the type locality (Bernhard et al. 1998b). It is also distinguished by its complex growthinduced zoning, and by the development of compositions intermediate between ideal pretulite and zircon. Diffraction data were used to solve the average structure of this sample of pretulite. Table 4 gives the coordinates of the atoms in the unit cell. Owing to its complex chemical composition, the structure was refined by 1) adjusting the Sc/Zr value, and, accordingly, the P/Si value, and 2) adjusting the Sc/(Y,HREE) value, considering all heavy rare-earth elements (HREE) as Yb, with Sc/Y and Yb/Y values close to the mean of Sc/Y and HREE/Y values indicated by the electron-microprobe data (Table 1). The best R value (0.039) was thus obtained for the simplified structural formula [Sc Y (Yb,HREE) Zr ](P Si )O 4, close to a composition of the C zone. This solution represents an average structure of a relatively inhomogeneous crystal, which explains the lower accuracy of these data compared to those of Bernhard et al. (1998b) (R = 0.019) for the Austrian pretulite. Nevertheless, the relatively low R value, as well as the homogeneity of U factors, indicate that this solution is a good approximation of the real structure. A listing of observed and calculated structure-factors is available from the Depository of Unpublished Data, CISTI, National Research Council, Ottawa, Ontario K1A 0S2, Canada. According to Table 3, the a and c parameters increase in going from pure synthetic ScPO 4 to the pretulite from Saint-Aubin owing to the substitution of larger cations, Y and HREE, for Sc, as well as to the presence of the zircon component. The two ratural examples of pretulite have very close unit-cell volumes, but a higher density (3.83 g/cm 3 ) was calculated for the Saint-Aubin material owing to its enrichment in the heavier elements. Crystallography Despite its relatively large size, the pretulite crystal shown in Figure 2 was used for a single-crystal X-ray study, with an imaging plate system. Operating conditions and related data are given in Table 2. On the basis of its tetragonal symmetry, the unit-cell parameters of the Saint-Aubin material are a (9), c 5.809(1) Å. Table 3 and Figure 6 compare these new data with those of pretulite from the type locality, pure synthetic ScPO 4, and related isotypic compounds (synthetic YPO 4, zircon and various HREE phosphates). SCANDIUM MINERALS IN OOLITIC IRONSTONE, SAINT-AUBIN, FRANCE 1663 Raman spectroscopy The Raman spectrum of pretulite is presented for the first time; it was obtained on the largest crystal shown in Figure 4, as well as on a sample (HK1A) from the type locality, provided by F. Bernhard. As the presence of REE in pretulite induces strong fluorescence lines, pure synthetic ScPO 4 (provided by Eugene Jarosewich), used as Sc standard for the electron-probe micro-analyses, also was studied for comparison. Data were obtained with a DILOR XY800 Raman microprobe (BRGM CNRS Université d Orléans, Dr. J.-M. Bény, ISTO CNRS, Orléans, analyst). Operating conditions were: Ar + laser, of the exciting radiations 488 and nm, 25 mw (~3 mw on the sample), recording window cm 1, objective 100, and acquisition time 120 s. The spectra obtained on pretulite from Saint-Aubin with the two laser sources allow us to distinguish complex groups of strong REE-fluorescence-induced bands from fine specific Raman bands (Fig. 7a). A comparison of the Raman spectrum of pretulite from the type locality with that of REE-free synthetic ScPO 4 (Fig. 7b) confirms the discrimination between fluorescence and Raman bands. All three samples present eleven welldefined Raman bands (Raman shift in cm 1 ): a very strong pair at and , with an intermediate doublet at and , then seven medium to weak bands at 595, , , 326, , 234 and This Raman spectrum is similar to that obtained for xenotime-(y) (C. Bény, unpubl. data), but with an increase of about 30 cm 1 of the Raman shift of the two strongest bands. Zoned detrital zircon SCANDIAN ZIRCON The finely stratified ironstone shows numerous detrital crystals of zircon, generally with titanium oxides in peculiar millimetric layers. In reflected light and SEM BSE images, these zircon crystals commonly display concentric zoning (Fig. 8) indicative of a primary oscillatory growth. The usually anhedral to subhedral morphology of the zircon crystals, which cuts this growth zoning, clearly indicates their detrital origin, without recrystallization. This kind of zircon was described in iron ores of Lower Ordovician age by Chauvel (1968, 1974), as well as in rutile- and zircon-rich sandstones from the Armorican Massif by Faure (1978). 1664 THE CANADIAN MINERALOGIST FIG. 6. Variation of the unit cell parameters a (Å) and V (Å 3 ) within the series of HREE phosphates related to the zircon structure-type, compared to that of zircon (according to data of Table 3). Nevertheless, in the A horizon at Saint-Aubin, the proportion of such zoned crystals (about one third) is particularly high. Results of the electron-probe micro-analyses are given in Table 5. The dark zones correspond to a phosphate enrichment, together with Sc, Y, Yb, Er, Fe, Ca and Al (and, in some cases, Lu and Th; U is below detection limit). These dark zones give low analytical totals, from 95 down to 92 wt.%, whereas the light zones (pure zircon) give totals close to 100 wt.%. Such a deficit is not strictly correlated with the cum
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks