Asymmetrical compositional gap between muscovite and NH 4 -bearing micas – structural constraints: a combined experimental–theoretical study

The usual coexistence in nature and in synthesised samples of NH 4 -bearing micas with NH 4 -free muscovite or illite has brought us to study their structural stability and characteristics. We used a combination of density functional theory methods and updated absorption-corrected nanoscale analyses of previously studied Germunde (Portugal) and Iberian Range (Spain) samples. Energy dispersive X-ray compositional maps were acquired using high-resolution scanning transmission electron microscopy, with high-angle annular dark-field images of ion-milled samples. The maps show the micaceous areas formed by a nano-intergrowth of major ammonium mica and minor muscovite or illite. To avoid mutual contamination between the two types of micas, zones were selected on homogeneous areas of the maps. The computer simulation allowed the structure of tobelite and various NH4/K ratio compositions to be reproduced and their energetic differences to be calculated. The overall range from pure tobelite up to 3K/1NH4 compositions is stable, but the composition 7K/1NH4 has positive cation exchange energy, being unstable. This composition coincides with a compositional gap found in the analysed real samples, which is responsible for the co-existence at the level of samples of NH 4 -free micas with the ammonium micas. The computer calculation has confirmed the structural similarity between the two kinds of micas, with the exception of the interlayer space, where H bonds play a significant role in the stabilisation of the tobelite structure. As a difference compared to K micas, the NH 4 micas mix a variety of polytypes at the sample level. Ammonium micas cover a continuous solid solution from tobelite up to a composition with major K but still some NH 4 ; hence, they should be considered to be a series name, with minor structural differences compared to muscovite, separated by a compositional gap.