CHARACTERIZATION OF GYPSUM FROM ZARAGOZA (SPAIN)

A new type of gypsum crystals from the region near Zaragoza (Spain) ha s been studied by different techniques (optical microscopy, microprobe , X-ray diffraction methods and X-ray transmission topography). The pe culiarity of the samples is the relative development of the different crystallographic forms: large lateral {111BAR}, poor developed {010} a nd terminal {120}. The other peculiarity is the mechanism of twinning: the crystals are twinned following (101BAR) by contact at the beginni ng and by penetration at the last stages of growth. Moreover, the two individuals of the twin show a different development, therefore a twin ning by a bidimensional nucleation mechanism was proposed. The crystal defects were characterized by X-ray topography - Lang method - and co mpared with the defects observed on gypsum twinned following (100).