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International audience; This possibly antique cameo (Figure 1) has banding originating from the spectacular zonation of red, globular inclusions of microcrystalline hematite (Figures 2 to 5). It also contains disc-like to crescent-shaped hematite inclusions (Figures 4 and 6). Although its macroscopic appearance and refractive index (RI) are that of chalcedony, it fluoresces green to shortwave utraviolet radiation (Figure 7), when most chalcedonies are inert. Luminescence spectrometry proves that the green emission is similar to that of some common opals, with both intrinsic and uranium-related emissions (Figure 8). Raman spectrometry demonstrates by comparison with a reference chalcedony that there is additional signal similar to that of opal-CT (Figure 9). Hence this cameo is actually an intimate mixture of chalcedony and opal, opal giving its luminescence, and chalcedony its RI.

International audience; Zachery-treated turquoise is turquoise made less porous, with a slightly better brilliance, without the use of resin or polymer (Figures 1 to 3). It cannot be identified through classical gemological observations. So far, only an identification criterion based on chemical analysis using laboratory instrumentation (EDXRF, but other similar techniques could work) has been proposed: Zachery-treated turquoise contains much more potassium (K) than its natural counterpart (Fritsch et al., 1999). We have developed a micro-chemical technique to see this difference in K content without using large instruments. We propose to use a small drop (about 1 mm diameter on the surface of the turquoise) of picric acid (concentrated solution in water) put in reaction with the unprotected surface of turquoise. For natural turquoise, there is no real reaction, only rarely a few yellow flakes, and rarely a few short yellow needles (Figure 6). For Zachery-treated turquoise, numerous long yellow needles or groups of needles form as soon as the drop dries out, due to the formation of a potassium compound (Figure 6; see also Table I). This reaction is proof of Zachery treatment for this gem. The test must be practiced under a binocular microscope, to affect the smallest possible area, in an unexposed zone of the gem. As most fashioned turquoises receive a surface treatment after polishing, this reaction does not work if the surface material is not removed. This is achieved with a droplet of diluted nitric acid (30% volume in water). The picric acid can then be put on the exposed turquoise surface, with the reaction as described above. After clearing the needles on treated material, often a yellow or white stain remains, which can be easily cleaned. For rough, the yellow stain disappears after putting a drop of the nitric acid solution on it, and then rinsing with water. For fashioned material, the white stain can be removed by simply washing with water using for example an old toothbrush. Because this technique requires the use of chemicals that are rather concentrated, this test must be practiced with care, in compliance with local regulations on the relevant chemicals.

International audience

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International audience

Despite its large area, the Armorican Massif provided only very few new mineral species: plombogummite, laumontite, bertrandite, natrodufrenite and lulzacite. This last species was recently discovered by one of us (Y. L.) in a sandstone quarry at Saint-Aubin-des-Châteaux (near Châteaubriant, Loire-Atlantique department) (Moëlo et al., 2000), and various studies permitted to reveal original mineralogical features, related to the superimposition of hydrothermal processes on an Ordovician oolitic ironstone interstratified in the sandstone sequence. This ironstone belongs to a very large sedimentary Fe deposit lying at the East margin of the Armorican Massif. At Saint-Aubin, ooliths are constituted essentially by siderite and chlorite, with abundant Srrich fluorapatite and organic matter. During Hercynian orogenesis, faulting controlled hydrothermal processes, which induced pronounced mineralogical changes, especially massive sulphidation of the ironstone (Gloaguen, 2002). Due to the abundance of primitive apatite and other peculiar geochemical features, it permitted the crystallisation of various Sr or lanthanoide phosphates. Lulzacite, Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10, is very well crystallised in small quartz veins crosscutting the ironstone, together with siderite and pyrite. It is isostructural with jamesite (Léone et al., 2000). It occurs as massive aggregates (up to some cm3), as well as euhedral crystals up to 1 cm in size. It was formed by short-range remobilisation of synsedimentary apatite (process of lateral secretion). Later, its decomposition lead to the formation of goyazite (with amethyst colour), together with late fluorapatite, and some berthierine. Lanthanoide phosphates were formed directly within the ironstone. The very rare scandium phosphate pretulite, ScPO4, has grown together with xenotime-(Y) in epitaxy on detrital zircon crystals (Moëlo et al., 2002). SEM as well EPMA revealed concentric zonation of pretulite and xenotime crystals, indicative of a multi-stage process. Monazite-(Ce) is present as minute anhedral neoformed aggregates, while detrital crystals are rare. Detrital zircon shows growth zones enriched with Sc-, Y- and HREE- phosphate components. EPMA of zircon and pretulite suggested a complete solid solution according to the heterovalent substitution rule: Zr + Si -> Sc + P. This solid solution has been confirmed experimentally in high temperature conditions (Dubost, 2003). A general metallogenic study is in progress (Gloaguen, 2002; Gloaguen et al., in prep.). After the main Fe-S-(As) stage, a Zn-Pb-(Cu) stage presents some Sb sulfosalts (boulangerite, bournonite and tetrahedrite), as well as traces of electrum. While the formation of lulzacite and subordinated goyazite is directly related to the hydrothermal process acting in the deposit of Saint-Aubin, that of xenotime and monazite begins very early within the ironstone formation, as observed in the neighbouring Fe deposit of Rougé, what may also permit the discovery of new pretulite occurrences. Thus, it seems that hydrothermalism of such ironstones is a major key and control when studying rare minerals associated with ironstones. Moreover, others phosphates have been previously described within ironstones: wolfeite (Fe2+,Mn2+)2(PO4)(OH) (Brousse and Chauvel, 1969), lazulite (Chauvel, 1968) and are not yet recognized in the Saint-Aubin-des-Châteaux quarry. We propose that differences between these phosphates minerals parageneses are controlled by ironstone initial chemistry and hydrothermalism. Such hydrothermalism is favoured and associated with late orogenic context. In this way, the large European palaeozoic ironstone belt is probably an important target for potential new and rare minerals species. Actually, all armorican iron mines are closed and a re-examination of ironstones samples preserved in museums could probably allow the discover of new species. Brousse, R. and Chauvel, J.-J. Bull. Soc. fr. Minéral. Cristallogr. 92, 1969, pp 93-94. Chauvel, J.-J. Mém. Soc. géol. Minéral. Bretagne, 16, 1968, 243 p. Dubost, V. Unpublished Research report, Magistère de Physico-Chimie Moléculaire, Paris XI-ENS Cachan, Institut des Matériaux de Nantes, 2003, 32 p. Gloaguen, E. Unpublished Research report, DEA Géosystèmes, Université d'Orléans, 2002, 41 p. Léone, P, Palvadeau, P. and Moëlo, Y. C. R. Acad. Sci. Paris, IIc, 2000, 301-308. Moëlo, Y., Lasnier, B., Palvadeau, P., Léone, P. and Fontan, F. C. R. Acad. Sci. Paris, 330, 2000, 317-324. Moëlo, Y., Lulzac, Y., Rouer, O., Palvadeau, P., Gloaguen, E. and Léone, P. Can. Mineral., 40, 2002, 1657-1673.

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 µm 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: Prl0.973Xnt0.020Zrn0.007, Prl0.907Xnt0.088Zrn0.005, Prl0.873Xnt0.042Zrn0.085, Prl0.718Xnt0.024Zrn0.258 and Prl0.453Xnt0.042Zrn0.505. A single-crystal X-ray refinement of the structure in space group I41/amd (R = 0.0389) gives a 6.5870(9), c 5.809(1) Å, for the formula (Sc0.904Y0.032HREE0.016Zr0.048)(P0.952Si0.048)O4. The Raman spectrum is presented. Detrital zircon shows phosphate-rich metamict zones containing HREE and Sc (up to 3.2 wt.% Sc2O3). 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.% Sc2O3). 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.

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National audience

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