Hydrothermal Retrogradation from Chlorite to Tosudite: Effect on the Optical Properties Ahmadi, Zahra Nieto García, Fernando Khormali, Farhad Velilla Sánchez, Nicolás Einali, Morteza Maghsoudi, Abbas Amini, Arash Optical properties Tosudite Chlorite he following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15030326/s1, Table S1: Compositions of chlorites, nearly free of smectite layers, determined by EMPA, used for chlorite geothermometry. Projects TED2021-131697B-C22 and PID2023- 147440OB-C22 from the Spanish MICIU/AEI/ 10.13039/501100011033 Project FEDER-UJA Andalucía 2021-2027. M.1.B.B TA_000655 Research Group RNM-179 of the Junta de Andalucía. Z.A. acknowledges the Ministry of Science, Research and Technology of Iran and the Vicerrectorado de Investigación CICODE of the Granada University for the grants for funding her stay in the Granada University In the argillic alteration zone of the SinAbad area of the Urumieh–Dokhtar magmatic belt (Iran), Mg-rich, Fe-poor chlorites, which crystallised at temperatures between 160 ◦C and 260 ◦C, were affected by extensive alteration to smectite mixed-layering at the micro- and nano-scales during the retrograde evolution of the hydrothermal system. Chlorites retain their usual optical aspect and properties, except for the index of refraction perpendicular to the (001) layers, which becomes lower than those parallel to the layers, producing an increase in birefringence and change in the optic and elongation signs, in comparison to the ordinary ones for Mg chlorites. Scanning electron microscopy (SEM) maps and compositions, and electron microprobe (EMP) analyses indicate minor but ubiquitous Ca (and K) content. X-ray diffraction (XRD) of chloritic concentrates allowed the identification of chlorite and tosudite. High-resolution transmission electron microscopy (HRTEM) images show major 14 Å (chlorite), with the frequent presence of 24 Å (contracted tosudite) individual layers and small packets up to five layers thick. Lateral change from 14 Å to 24 Å individual layers has been visualised. High-resolution chemical maps obtained in high-angle annular dark-field (HAADF) mode confirm the existence of areas preferentially dominated by chlorite or tosudite. The overall chemical compositions obtained by SEM, EMP, and transmission electron microscopy (TEM) align from the chlorite to the tosudite end-members, whose pure compositions could be determined from extreme analytical electron microscopy (AEM) analyses. The described intergrowths and interlayers, under the optical resolution, could provide a clue to explain changes in the normal optic properties of chlorite, which are mentioned, but not explained, in the literature. 2025-04-04T10:20:50Z 2025-04-04T10:20:50Z 2025-03-20 journal article Ahmadi, Z.; Nieto, F.; Khormali, F.; Velilla, N.; Einali, M.; Maghsoudi, A.; Amini, A. Hydrothermal Retrogradation from Chlorite to Tosudite: Effect on the Optical Properties. Minerals 2025, 15, 326. [https://doi.org/10.3390/min15030326] https://hdl.handle.net/10481/103451 10.3390/min15030326 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional MDPI