Optical and chemical characteristics of the mineral particles found on the face of the Turin Shroud

We have studied optical properties (in optic and petrographic microscopy) of mineral particles (diameter > 1 μm) deposited on the surface of a small (≈ 1.4 mm height, 610 μm wide) sticky tape triangle corresponding to the Face area of the Turin Shroud (TS). All these particles were studied by scanning electronic microscopy (SEM) and their elemental compositions were analysed by X-ray microfluorescence (XRMF). These mineral particles were classified among nine main categories: particles of the limestone fraction, alumine, particles of the silica fraction, clay minerals (including kaolinite, montmorillonite and illites), chlorites, talcum and evaporites. The most important fraction (one for two) of the mineral particles found is clay minerals. Some of them (montmorillonite/illites mixtures) are highly characteristic: they are constituted of potassium-rich clays, with a high calcium content and they contain iron and titanium inclusions. Most of the silica particles found are shaped in an eolian fashion. The presence of gypse and ferric oxide indicates a soil nature corresponding to desertic or semi-desertic climates. Some minerals, rounded in shape, can mimic-by aspect and color-true red blood cells.