Alain Baumer

Systematic Cathodoluminescence Spectral Analysis of Synthetic Doped Minerals: Anhydrite, Apatite, Calcite, Fluorite, Scheelite and Zircon

As the geochemical behavior of the rare earth elements (REE) is of increasing interest in geology (see the reviews edited by Lipin and McKay 1989 and Jones et al. 1996), the main REE minerals in most igneous, metamorphic and sedimentary rocks are now being studied in detail. REE are major or trace constituents in many minerals (Burt 1989). Besides the specific REE mineral assemblages which occur in alkaline, peralkaline and carbonatitic rocks (Vlassov 1966; Burt 1989; Mariano 1989; Larsen 1996; Taylor and Pollard 1996; Wall and Mariano 1996; Khomyakov 1996), more common accessory minerals such as zircon, apatite, anhydrite, carbonates and fluorites are also REE carriers and play an important role in petrologic processes. The major application of the REE studies is the melt mineral partition coefficient, used to model igneous petrogenetic processes. This will depend on the compatibility of the REE in major minerals occurring in late differentiated stages such as apatite and zircon (McKay 1989). Most of the REE are well known to be luminescence activators (Pringsheim 1949; Levrenz 1950; Monod-Herzen 1966; Diecke 1968; Marfunin 1979; Marshall 1988, Waychunas 1988). In order to interpret the cathodoluminescence (CL) emissions of natural REE bearing minerals, it is essential to compare their CL spectra to those of synthetic minerals.

Cathodoluminescence of synthetic (doped with rare-earth elements) and natural anhydrites

Abstract In order to interpret cathodoluminescence (CL) spectra of natural anhydrites, synthetic crystals were indirectly obtained by precipitation of gypsum from solutions containing doping rare-earth elements (REE), and by transformation into anhydrite by dehydration at 800°C during 3 h. Analyses by ICP-MS of several anhydrites obtained from solutions containing 130 ppm of REE clearly showed that light REE (LREE) can be concentrated to up to 2000–3000 ppm in the solid; heavy REE (HREE) are less prone to enter the structure and only 255 ppm were found for Lu. REE such as Ce3+, Pr3+, Sm3+, Eu2+, Gd3+, Tb3+, Dy3+ and Tm3+ display strong CL emissions and spectra usually are complex except for three of them with intense emissions in the near UV domain: Ce3+ (306–327 nm), Gd3+ (314 nm) and Eu2+ (386 nm). In the near IR domain, only Nd3+ (874 nm) gives an important luminescence. Several natural anhydrites were examined for REE CL emissions: a sample from the Faraday uranium mine, Bancroft, Ontario, presents mainly a strong Ce3+ emission with traces of Eu2+, Dy3+, Sm3+ and Nd3+; in the anhydrite from Balazuc, France, Gd3+, Eu2+ and Dy3+ are the more abundant, followed by Ce3+ and traces of Tb3+ and Sm3+. A crystal, probably from Hall, Tyrol, displays Gd3+, Ce3+, Er3+, Tb3+, Dy3+ and Sm3+: its analysis by ICP-MS showed that the CL can be induced by only some ppm of these elements, even less than 1 ppm for Er3+ and Tb3+.

Synthese et etude morphologique, radiocristallographique et spectrometrique I.R. de la wadeite K2 ZrSi3 O9; domaine de formation; remplacements isomorphiques; application a la geochimie du zirconium

Abstract The first synthesis of wadeite, K2ZrSi3O9, was realised by the authors in 1970 using a hydrothermal technique. These new data were the basis for further research works about the formation of this mineral by way of dry and hydrothermal synthesis, making use of various potassic compounds. The conditions for the formation of wadeite were explained as a function of the chemical environment. The pressure—temperature range of formation was investigated from 100 to 900°C and 100 to 2000 bar. The morphology of the wadeite crystals that were obtained is discussed. Starting from wadeite structure, various isomorphic substitutions were determined (Zr/Hf; K/Rb, Cs; Si/Ge); their study resulted in an interesting diagram of crystallographic parameters as a function of the ionic radius of the substituted alkaline elements: cryst. param. = f(R+alk. element); the bands are linear. The infra-red investigations enabled us to establish the spectrum of (Si3O9)6− and (Ge3O9)6− in wadeite structure as well as the substitutions caused by alkalis and transition metals. Finally a comparative experimental investigation of the ZrSiO4-K2ZrSi3O9-K2ZrSi6O15 system brought to light new data regarding the geochemical behaviour of zirconium in igneous rocks.

Réflexions à propos d'apatites des phosphates sédimentaires

Abstract Sedimentary phosphate apatite is a phosphocalcium carbo-apatite. For this kind of structure, two possibilities exist for PO 3− 4 CO 2− 3 replacement: one corresponds to PO 3− 4 ( CO 2− 3 , F − ) with substitution ratio x , the other one to PO 3− 4 ( CO 2− 3 , Ca 2+ , F − ) with substitution ratio y . The structural formula relating to these two models is: Ca 10− y ( PO 4 ) 6− x − y ( CO 3 , F ) x ( CO 3 ) y F 2− y For the marine sedimentary apatites, the fluorine content increases when: (a) OH − and Cl − are missing in the molecule; and (b) x -value is high. The fluorine content decreases when: (a) Na + is present in the molecule with Ca 2+ ( Na + , F − ) substitution; and (b) y -value is high. When the replacement OH − F − is missing, the structural formula relating to the all substitution models and permitting the x and y calculation is: Ca 10− y − z − ϵ Na z Mg ϵ ( PO 4 ) 6− x − y ( CO 3 , F ) x ( CO 3 ) y F 2− y − z The IR spectrometrical and thermogravimetrical experimental study confirms the expressed assumptions and permits a better understanding of sedimentary apatites.

Remplacements isomorphiques par synthèses hydrothermales dans les silicates fluores: Topaze et zunyite

Abstract The substituted topaz Al 2 GeO 4 (OH,F) 2 , Al 2 (Si x Ge 1- x )O 4 (OH,F) 2 and Ga 2 GeO 4 (OH,F) 2 , as well as the substituted zunyite Al 13 Ge 5 O 20 (OH,F) 16 F 2 Cl have been synthetised. The unit-cell parameters of these artificial minerals, isomorphs of natural compounds, have been determined; the corresponding lists of d hkl are given. The study by I.R. spectroscopy of these compounds with a total or a partial replacement of Si by Ge in the same way as Ga-Ge topaz suggests the assignment of certain absorption bands to characteristic structural groups. The thermal decomposition of AlSi and AlGe topaz into mullite has been studied.

L'instabilité des ions Eu3+ provoquée par le bombardement électronique dans les minéraux synthétiques observée par luminescence

Abstract Comparative study of luminescence properties of doped synthetic minerals (anhydrite, calcite, fluorite and zircon) by cathodoluminescence and fluorescence methods is presented. The Eu 3+ doped minerals (anhydrite; other sulfates; fluorite) exhibit partial reduction of Eu 3+ to Eu 2+ when they are studied by cathodoluminescence method. On the other hand, in the spectra of Eu 3+ doped calcite, strontianite and zircon, no redox reaction is observed.