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

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.     

A PROCESS OF STELLAR NUCLEOSYNTHESIS WHICH MIMICKS MASS FRACTIONATION IN P-XENON

The combination of the O-shell theory of Heymann and Dziczkaniec and the supernova theory of Woosley and Howard clearly identifies the astrophysical sites for the formation of the anomalous light Xe component in carbonaceous chondrites. These sites are the O- and Ne-shells, and possibly C-shell of a massive star. Most of the ¹²⁴Xe and ¹²⁶Xe are formed in the O-shell during hydrostatic core silicon-burning, when a seed of heavy nuclei is exposed to an effective temperature near T₉ = 2.0. ¹²⁸Xe is formed via ¹²⁸Ba in the O-shell, but the amounts appear too small to satisfy the deduced ¹²⁸Xe/¹²⁴Xe and ¹²⁸Xe/¹²⁶Xe yield ratios from the chondrites. However, substantial amounts of ¹²⁸Xe can be formed in the adjacent Ne- and C-shells during the explosion. The formation of ¹²⁸Ba in the O-shell would increase if the (γ, α) photodisintegration rate in ¹²⁸Ba is actually smaller than calculated by Woosley and Howard. Lewis et al. have proposed that the anomalous light Xe component is mass-fractionated normal Xe. It is in this sense that the process of stellar nucleosynthesis of the present paper mimicks mass-fractionation.