Ammonium vermiculite in schists from the Betic Cordillera (Spain)

NH4-bearing phases from pre-Permian schists of the Internal Zones of the Betic Cordillera (Spain) have been characterized by X-ray diffraction, differential thermal analysis and thermogravimetry, infrared spectroscopy, and transmission-analytical electron microscopy. The content and distribution of NH4 in these phases have been estimated by combining elemental analysis of N with mineral quantification based on X-ray diffraction, whole-rock composition, and electron microprobe data. The obtained results indicate that the NH4 content in vermiculite ranges between similar to 0.5 and similar to 1.0 (wt%), leading to terms with NH4 > K in the interlayer. The NH4 content is <0.30 (wt%) in biotite from underlying schists. Muscovite shows very low NH4 contents (0.04 wt%) through the complete sequence. We interpret that, initially contained in low-grade dioctahedral mica, NH4 was incorporated in the structure of the trioctahedral phases formed at increasing metamorphic conditions. The entry of NH4 in the interlayer was favored in chlorite:vermiculite mixed-layered structures and transitional phases with vacancies in the A site (vermiculite) and weak cohesion between layers. NH4 appears to favor the formation of intermediate phases instead the direct transformation of chlorite to biotite.