Description of twin in frondelitc -rockbridgeilc ¿issocialed with the pegmatites of the L'Arize Massif (Pirinees, Ariege)

A cruciform intergrowth in a phosphate of the frondelite-rockbrid geite series associated with beryl-columbite pegmatites is studied on the basis of geometric considerations. The crystallographical calculations prove, for the first time, the existence of a reticular pseudomeroedric twinning in frondelite, having as twin plane the (041), an obliquity of ~ 3 and a twin index ~ 7, which may explain the rarity of this twinnin