Crystals 2022, 12, 1638. https://doi.org/10.3390/cryst12111638 www.mdpi.com/journal/crystals Article Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular Taras L. Panikorovskii 1,2, *, Irina O. Galuskina 3 , Vladimir N. Bocharov 4 , Vladimir V. Shilovskikh 4 and Evgeny V. Galuskin 3 1 Laboratory of Nature-Inspired Technologies and Environmental Safety of the Arctic, Nanomaterials Research Centre, Kola Science Centre, Russian Academy of Sciences, Fersmana str. 14, 184209 Apatity, Russia 2 Department of Crystallography, Institute of Earth Sciences, Saint–Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia 3 Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland 4 Geo Environmental Centre “Geomodel”, Saint–Petersburg State University, Ul’yanovskaya Str. 1, 198504 St. Petersburg, Russia * Correspondence: t.panikorovskii@ksc.ru; Tel.: +7-81555-79628 Abstract: Garnet supergroup minerals are in the interest of different applications in geology, min- eralogy, and petrology and as optical material for material science. The growth twins of natural tetragonal grossular from the Wiluy River, Yakutia, Russia, were investigated using single-crystal X-ray diffraction, optical studies, Raman spectroscopy, microprobe, and scanning electron micros- copy. The studied grossular is pseudo-cubic (a = 11.9390 (4), c = 11.9469 (6) Å) and birefringent (0.01). Its structure was refined in the Ia3 ത d, I41/acd, I41/a, and I4 ത 2d space groups. The I41/a space group was chosen as the most possible one due to the absence of violating reflections and ordering of Mg 2+ and Fe 3+ in two independent octahedral sites, which cause the symmetry breaking according to the group–subgroup relation Ia3 ത d → I41/a. Octahedral crystals of (H4O4) 4− -substituted grossular are mer- ohedrally twinned by twofold axis along [110]. The mechanism of twining growth led to the gener- ation of stacking faults on the (110) plane and results in the formation of crystals with a long pris- matic habit. Keywords: grossular; twin; merohedral twinning; hydrogarnet defect; cation ordering; crystal structure; garnet 1. Introduction Minerals of the garnet supergroup are one of the most widely occurring minerals in the earth crust and upper mantle. They are stable in a wide range of temperatures (T) up to 2000 °C and pressures (P) of ~25 GPa and occur in different PT conditions, including rocks from lower greenschist facies to ultra-high-temperature granulites and ultra-high- pressure eclogites [1]. Owing to the flexibility of their structure, garnets demonstrate great chemical capacity and cationic diversity, which intensively are used for the determination of the PT evolution and oxygen fugacity of rocks [2–4], determining the absolute timing and rates of geological processes [5,6]. Their synthetic counterparts, YAl-garnet, Y3Al5O12, doped with Nd, are the most famous kinds of oxide crystals widely used as the active medium in solid-state lasers [7,8]. The general formula for garnet is X3Y2Z3ϕ12, where X, Y, and Z refer to dodecahedral, octahedral, and tetrahedral sites, respectively, and ϕ is O, OH, or F [9]. In natural garnets, the tetrahedral sites are usually populated by Si 4+ [1]. Low-temperature garnets are some- times characterized by the hydrogarnet-type (H4O4) 4− ↔ (SiO4) 4− substitution, which leads to the formation of hydrogarnet katoite, Ca3Al2(OH)12 [10]. Katoite is cubic, Ia3 ത d; however, Citation: Panikorovskii, T.L.; Galuskina, I.O.; Bocharov, V.N.; Shilovskikh, V.V.; Galuskin, E.V. Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular. Crystals 2022, 12, 1638. https://doi.org/10.3390/ cryst12111638 Academic Editor: Antonio Frontera Received: 17 October 2022 Accepted: 11 November 2022 Published: 14 November 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://crea- tivecommons.org/licenses/by/4.0/).