Fig. 2

Fragment of primary anhydrite with signs of thermal outgassing in the high-fired gypsum mortar constituting the Pietà in the Abbey Church Marienberg (transmitted light, parallel/crossed Nicols). In thin sections, anhydrite is clearly distinguishable from gypsum due to its higher light refraction and birefringence, i.e. in this case grey interference colours of the gypsum matrix versus up to blue interference colours of the anhydrite grains having higher relief