Raoul Caruba

Les zircons des pegmatites de Narssârssuk (Groënland) — L'eau et les groupements OH dans les zircons métamictes

Abstract Using chemical analysis, X-ray, infrared (IR) and thermoluminescence (TL), a study was made of the famous pegmatitic zircons of Narssârssuk (south Greenland). The results indicate that these crystals have important variations in their major- and trace-element composition and water concentration (48.5% 2 O a = 661.4 ± 0.2 pm; c = 599 ± 0.2 pm) to late zircons ( a = 673 ± 2 pm; c = 603 ± 2 pm). From comparison of the zircons of Narssârssuk with normal, ZrSiO 4 , and hydroxylated, Zr(SiO 4 ) 1− x (OH) 4 x , synthetic zircons we suggest that the following replacements occur: Zr 4+ ⇌ M n+ + (4 − n) H + (SiO 4 ) 4− ⇌ 4(OH) − Zr 4 + (SiO 4 ) 4− ⇌ M n+ + (OH) − n + (4 − n) H 2 O Moreover, hydrogen may associate with (OH) 4 groups to form H 2 O.

Zircon in charnockitic rocks from Rogaland (southwest Norway)" Petrogenetic implications

Zircons separated from monzonoritic rocks and quartz mangerites (igneous and metamorphic) have been studied for various mineralogical, chemical and physical properties (colour, zoning, presence of core, U distribution, morphology, lattice parameters, thermoluminescence) to determine the relationships with chemical composition of the host rocks and geological setting. The zircons grade from nearly perfectly crystalline to slightly metamict types. The crystal morphology shows prismatic habits and a striking lack of well-developed pyramidal faces, possibly indicating growth in a dry formation environment. The zircons can be divided into two groups. The first one comprises zircons from monzonoritic rocks. They are unzoned, devoid of brownish core, with a uniform U distribution, and low U content (except in Hidra). The second group comprises zircons from igneous and metamorphic quartz mangerites; they contain brownish, zoned, U-rich cores. In rocks where temperature of crystallization can be determined, Watson and Harrisons (1983) experimental data indicate strong zircon saturation. The cores are considered as inherited, a fact that can be accounted for either by dry anatexis or bulk contamination. In the Bjerkreim-Sokndal layered lopolith, previous isotopic data are confirmed, which indicate that the acidic upper part of the massif is not comagmatic with the anorthosito-noritic lower part and the monzonoritic rocks.

Geochemistry of clay dunes and associated pan in the Senegal Delta (Mauritania)

Since the construction of the anti - salting Diama dam , a resumption of aeolian deflation has been observed in the Senegal delta and particularly in Diawling National Park . It results in the formation of many clay dunes with high salt concentrations . The soft powdery structure observed on the soil surface , affected by aeolian defla tion is due to rapid salt crystallization . In order to identify the mineralogy of the salt involved in this environment , the chemistry of soil and groundwater was studied along a transect from a pan to the associated clay dune . Equilibria computation and field and scanning electron microscope (SEM) observations indicated that the changes observed in the chemical facies of the solution were due to the following precipitation sequence: calcite > gypsum > halite . A progressive but significant fixing of Na+ onto the clay particles was observed . The calcite deposition in the soil was limited by exhibition of the potential acidity arising from the former man grove site soils...

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.

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.

Influence de la maturation des sols de mangrove sur la déflation éolienne et la formation des dunes argileuses dans le delta du fleuve Sénégal

Abstract The study of clay dunes is frequently used for the reconstitution of the palaeoenviron ment. A stop in the sedimentation is generally attributed to changes in the climatic, or hydrological environment. Our study in the Senegal delta shows that transformations in the superficial part of the soil can result in a stop of the aeolian deflation and clay dune formation, without changes in the environmental conditions.