Calcitic radial palisadic fabric in freshwater stromatolites: diagenetic and recrystallized feature or physicochemical sinter crust?

When favorable conditions occur for carbonate precipitation (physicochemica l processes) in tufas and travertines, organisms intervene either directly (the framework created by organisms acts as a crystal support) or indirectl y (organic products influencing the mineralogy, nucleation, growth and habi t of primary crystals). In the literature, fluvial and travertine crystalli ne sparite crusts with palisadic or prismatic crystals are generally interp reted as the result of early recrystallization of micrite in algal buildups (mostly composed of Schizothrix). Therefore, biological mediation is essen tial for the construction of continental bioherms and biostromes, even if o rganic remains are frequent in micrite and primary sparites, and rare or to tally absent in secondary sparites. (C) 1999 Elsevier Science B.V. All righ ts reserved.