Rondeau B. auteur aut IRD Chamard-Bois S. auteur aut IRD Fritsch Emmanuel auteur aut IRD Notari F. auteur aut IRD Chauvire B. auteur aut IRD text journalArticle eng text/pdf born digital access We document a dichromatism effect in gem andalusite that shifts from light brownish pink at low thickness to medium green at high thickness. This is roughly reverse to that usually observed in other dichromatic materials. We show that this is due to a very strong pleochroism in andalusite, when dichromatism in other materials is due to an alexandrite effect. Here, colors varies from a strongly absorbing orangey-brown in the c direction to much less absorbing medium green in a and b directions. The brown absorption is so strong that it becomes opaque (total absorption) for thickness above about 1.5 mm (this value may vary according to concentration of the chromophore and intensity of the light source), and hence does not contribute to coloration anymore. For such high thicknesses, only the a and b directions remain transparent and hence only the green component contributes to the color: the sample appears green. For lower thicknesses, the green color combines with the orangey-brown color that becomes transparent along the c direction, resulting in a light brownish pink. From trace elements analysis in zoned samples, we also confirm that the dark orangey-brown color is due to Fe2+-Ti4+, and bring evidences that the light green color is mostly due to Fe, either in the form of isolated Fe3+ or Fe2+-Fe3+ IVCT, with possibly some contributions from isolated Mn3+. specialized Andalusite Reverse dichromatism Total absorption Pleochroism 064 204 611-619 2018 1386-1425 https://www.documentation.ird.fr/hor/fdi:010073776 10.1016/j.saa.2018.06.053 1386-1425 [F B010073776] https://www.documentation.ird.fr/hor/fdi:010073776 https://www.documentation.ird.fr/intranet/publi/2018/08/010073776.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2018-09-12 2023-07-11 fdi:010073776 fre