Articles | Volume 38, issue 6
https://doi.org/10.5194/ejm-38-655-2026
https://doi.org/10.5194/ejm-38-655-2026
Research article
 | 
05 Nov 2026
Research article |  | 05 Nov 2026

Asymmetrical compositional gap between muscovite and NH4-bearing micas – structural constraints: a combined experimental–theoretical study

Fernando Nieto, Elizabeth Escamilla-Roa, Vicente Timón, Josefina Pedrajas, María del Mar Abad, and Blanca Bauluz

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Cited articles

Bauluz, B. and Nieto, F.: Ammonium-bearing micas in very low-grade metapelites: micro- and nano-texture and composition, Clay Miner., 53, 105–116, https://doi.org/10.1180/clm.2018.8, 2018. 
Bentabol, M. and Ruiz Cruz, M. D.: Characterization of tobelite formed from kaolinite under hydrothermal conditions (200 °C), Appl. Clay Sci., 126, 160–172, https://doi.org/10.1016/j.clay.2016.01.053, 2016a. 
Bentabol, M. and Ruiz Cruz, M. D.: NH4 for K substitution in dioctahedral mica synthesized at 200 °C, Appl. Clay Sci., 126, 268–277, https://doi.org/10.1016/j.clay.2016.03.025, 2016b. 
Bobos, I.: Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania, Am. Mineral., 97, https://doi.org/10.2138/am.2012.3801, 2012. 
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Short summary
Differences in the interlayer between NH₄ and K micas, caused by H bonds in the former, explain forbidden mixed compositions near the K endmember. The areas of the samples with these compositions resolve their energetically unfavourable nature through the coexistence, at the nanoscale, of muscovite (and/or illite) with NH₄ micas containing varying proportions of K. For all other NH4/K ratios, continuous solid-solution series exits as these compositions are energetically favourable.
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