Subsolidus Transformation of Amorphous Interstellar Dusts after Injection in a Protoplanetary Disk

Introduction: Infrared observations indicate that a dominant fraction of the condensed matter found in the interstellar medium (ISM) is amorphous (silicates, organic matter, ices). On the other hand, similar spectroscopic studies show that silicates are mainly crystalline in protoplanetary disks [1]. Furthermore, cometary material appears also essentially crystalline [2, 3]. Because the ISM dust is the precursor material of protosolar cloud, it appears that structures and compositions of the silicate dust is strongly thermally processed in such environments (e.g. [4]). Recent dynamical models of accretion disks indicate that turbulent radial mixing probably occurred and that thermally processed silicates from the inner part of the disk were likely transported toward outer regions [5]. With this scheme, reheating and crystallization of amorphous material below solidus temperatures of typical minerals were the first transformations that occurred during the transportation of the interstellar silicate in the protosolar disk. In this contribution we explore experimentally this subsolidus route of crystallization for amorphous silicates in the CaO-MgO-Al2O3-SiO2 (CMAS) composition system. We found an unexpected diversity of minerals which show that a large redistribution of elements occurs via metastable steps during the crystallization scheme. We finally discuss basic physicochemical mechanisms responsible for the microstructures, compositions and mineralogy of recovered crystallized products.