New data for metamorphic vermiculite

Vermiculite-like minerals from a very low-grade metamorphic sequence (Malag uide Complex, Betic Cordillera, Spain) were studied by optical microscopy, X-ray diffraction (XRD), differential thermal analysis /thermogravimetry (D TA-TG), electron microprobe (EMPA), and transmission/analytical electron mi croscopy (TEM/AEM). They occur as transformation products of chlorite, the conversion sequence being: chlorite --> chlorite/vermiculite mixed-layers - -> vermiculite. This sequence is characterized by a continuous chemical evo lution that includes Fe+Mg loss and an increase in Si/Al ratio. These chemi cal changes are accompanied by a variable increase in alkali (K and Na) and Ca. AEM data of homogeneous packets have permitted the chemical characteri zation of high-temperature or metamorphic vermiculite, which shows a narrow compositional range. This mineral sequence probably developed during the A lpine metamorphic event at less than 400 degrees C, and the phases formed m ay be considered, in the light of the available data, as intermediate in th e chlorite to biotite transformation, their development being probably depe ndent on both temperature and cation availability.