A gypsum formation in the Middle Miocene saline lake deposits of the Madrid Basin is composed of large twinned crystals displaying an unusual habit characterised by the twin re-entrant angles systematically opening downward. Such a habit is an exception to the commonly accepted Mottura's rule for the growth of large twinned crystals which have been observed in marine-derived evaporitic settings. The crystals, twinned along (100), are composed of elongated sub-crystals formed by the intersection of the pinacoid {010} and the hemipyramid {111}. Field and petrographic evidence indicate a primary origin by subaqueous growth on the bottom, ruling out a secondary origin. Brine composition, dominated by Ca, Na, SO 4 , Cl, can be one of the several factors potentially controlling the selective growth of this habit. Further observations could indicate that such inverted twinned crystals may be considered diagnostic of gypsum growth in continental settings as such a pattern has not been recognised in marine evaporite formations.