Recognizing and understanding silica-polymorph microcrystals in ceramic glazes.

• Development of silica microcrystals is inferred from morphology and local probe data. • Cristobalite and tridymite can nucleate from iron oxide not requiring alkali metals. • Morphology of silica microcrystals can hide intergrowths and paramorphic phases. • Cristobalite hexagonal plates and dendrites show a tridymite external growth front. Different methods have been historically used to identify silica-polymorph microcrystals in various contexts. However, in the existing literature there is a substantial lack of correlation between local probe analytical data and the corresponding images showing crystal morphologies. Here we demonstrate that such correlation is attainable through an approach based on the characterization of samples prepared as polished thin sections combining optical microscope images with local probe techniques (synchrotron through-the-substrate X-ray microdiffraction and confocal micro-Raman). We report the characterization of different morphologies of silica microcrystals embedded in lead glazes from glazed pottery produced in Jouques (France) in the mid-19th century. Among the eight identified morphologies, are worthy of note cristobalite-coated quartz, paramorphic quartz after cristobalite, cristobalite-tridymite intergrowths or cristobalite twinned crystals. Elusive tridymite has been identified and mapped using μ-Raman and locates predominantly at the borders of some morphologies suggesting a growth front of tridymite that turns to cristobalite. [ABSTRACT FROM AUTHOR]