UNUSUAL TWINNING FEATURES IN LARGE PRIMARY GYPSUM CRYSTALS FORMED IN SALT LAKE CONDITIONS, MIDDLE MIOCENE, MADRID BASIN, SPAIN - PALEOENVIRONMENTAL IMPLICATIONS

A gypsum formation in the Middle Miocene saline lake deposits of the M adrid Basin is composed of large twinned crystals displaying an unusua l habit characterised by the twin re-entrant angles systematically ope ning downward. Such a habit is an exception to the commonly accepted M ottura'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 int ersection of the pinacoid {010} and the hemipyramid {111}. Field and p etrographic evidence indicate a primary origin by subaqueous growth on the bottom, ruling out a secondary origin. Brine composition, dominat ed by Ca, Na, SO4, Cl, can be one of the several factors potentially c ontrolling the selective growth of this habit. Further observations co uld indicate that such inverted twinned Crystals may be considered dia gnostic of gypsum growth in continental settings as such a pattern has not been recognised in marine evaporite formations.