Mineralogical Crystallography Volume III

The United Nations and UNESCO designated 2014 as the International Year of Crystallography, in which the scientific community celebrated the centenary of the discovery of X-ray diffraction. Eight years later, in 2022, UNESCO and mineralogical societies around the world celebrated another milestone—the Year of Mineralogy. These events emphasize the general importance of mineralogy and crystallography in modern human life, as these scientific branches, which originated in ancient times, are still at the forefront of scientific research [1,2,3,4,5].

The first two volumes of the Special Issue “Mineralogical Crystallography” [6,7] were highly fruitful and comprised a diverse range of topics, including the discovery of new mineral species; the crystal chemistry of minerals and their synthetic analogs; the behavior of minerals in non-ambient conditions; biomineralogy; gemology; natural-based cement materials; and crystal growth techniques. The papers included in the third volume can be classified more succinctly, falling under the scope of crystal chemistry and properties of minerals and their synthetic analogs; gemology; and crystal growth techniques and cultural heritage.

We hope that this third volume will be as successful as its predecessors, and that this new collection of papers will arouse genuine interest among readers, encouraging them to pursue their own successful research. The papers that comprise this Special Issue are briefly summarized below.

1. Crystal Chemistry and Properties of Minerals and Their Synthetic Analogs

Kuporev et al. [8] reviewed the crystal chemistry features of the family of natural and synthetic uranyl molybdate compounds. A rather significant discrepancy in the topological types of uranyl molybdate substructural units for natural and synthetic phases was observed. Despite topological similarities to compounds of other U-bearing families, completely isotypic compounds were absent. Additionally, uranyl molybdate building blocks were proven to have a greater influence on the entire structural architecture than interstitial species.

Pekov et al. [9] discussed the new mercury sulphohalide mineral hanauerite, AgHgSI, which was discovered in the oxidation zone of the abandoned mines in Rhineland-Palatinate, Germany. This mineral represents a very specific family of natural compounds from both geochemical and crystal chemical perspectives. It is named in honor of the German mineral collector Dr. Alfred Hanauer (1912–1988).

The crystal structures of synthetic analogs of two natural transition metals—chloride hydrates eriochalcite (CuCl₂·2H₂O) and nickelbischofite (NiCl₂·6H₂O)—were described by R.T. Boeré [10] using combined density functional theory and non-spherical atomic scattering factor calculations. Compared to CuKα, the MoKα source demonstrated a 50% improvement in XRD data modeling using Hirshfeld atom refinement, both in terms of overall refinement parameters and more precise H-atom ellipsoids assignment.

The crystal structure of layered double-hydroxide quintinite, Mg₄Al₂(OH)₁₂(CO₃)·3H₂O from the Jacupiranga (São Paulo, Brazil) was refined in [11], and was thus identified as the 2T polytype of quintinite from Kovdor (Kola peninsula, Russia). This example demonstrates the high probability of known quintinite polytypes being found in other localities, and it shows that despite the differing conditions, the crystal structures of quintinites form in the same way.

Zhitova et al. [12] describe the crystal structure of highly hydrated sulfate mineral alunogen, Al₂(SO₄)₃·17H₂O, from Verkhne–Koshelevsky geothermal field (Kamcatka, Russia). The authors successfully localized H-atoms sites, which should assist with the detailed band assignment in vibrational spectroscopy studies. The studied alunogen is isotypic to previously reported structures but is quite pure in terms of its chemical composition and contains a lower amount of H₂O compared to previous findings.

Pyrochlore mineral and its synthetic analogs are of great interest due to their practical and promising properties. Zhuk and coauthors [13] synthesized multielement niobate with a pyrochlore structure using a solid-phase reaction method and depicted a phase evolution starting from the mixture of simple oxides to pure pyrochlore in the temperature range of 400–1050 °C.

A group of authors from Clemson University (USA) used a hydrothermal technique to synthesize two novel analogs of the vesuvianite mineral Ca₁₉Al₁₃Si₁₈O₇₁(OH)₇ and Sr₁₉Fe₁₂Ge₁₉O₇₂(OH)₆ [14]. Both compounds crystallize in tetragonal unit cells of P4/nnc symmetry, demonstrating that end members without substitutional impurities can be obtained under a strictly controlled synthetic protocol.

2. Gemology

Gao et al. [15] studied Jedi spinel from the Man Sin (Myanmar) using a series of experimental techniques. The amazing neon pink–red color of these samples, in addition to their strong fluorescence, can be attributed to their high Cr and low Fe content. Man Sin spinels can be chemically characterized; they are easily distinguished from Mogok from other Myanmar localities, but less easily distinguished from samples obtained in Namya.

Jade weathering rinds and pristine cores of samples from the Jilin Province (China) were studied by Xue et al. [16] to understand their specific alteration processes. It was shown that Ji’an serpentinite exhibits marine sedimentation genesis, and weathering processes reduce their content of certain major and impure elements, such as Si, Mg, Fe, K, Ca, etc., with the formation of chlorite.

A comprehensive geochemical investigation of nephrite from Chuncheon (South Korea) was reported by Li et al. [17]. The study nephrite is dolomite-related, consists mainly of tremolite, has a unique pseudomorphic metasomatic texture, and can be distinguished from other dolomite-related nephrites by its reduced δ¹⁸O and δD isotopes.

3. Crystal Growth Techniques and Cultural Heritage

Song et al. [18] examined painted stone carvings excavated from the Ming tombs in Chengdu (China), which were damaged by groundwater salts. They discovered that the main component of salt efflorescence is gypsum, which forms in two stages. First, less aggressive damage occurs during underground conservation of tombs in a stable environment with a low amount of water seepage. More intense damage was sustained after the excavation and prior to relocation.

Marin-Troya and coauthors demonstrated that the presence of various ions in the aqueous solution significantly affects the formation, transformation, and evolution of carbonate mineral phases [19], knowledge of which is essential in various industrial processes. For instance, they revealed that Li⁺ ions initiate vaterite precipitation and reduce calcite formation.

By studying ultra-high-temperature granulites from various localities, Zhang et al. [20] proved that accurate analyses of Ti concentrations in quartz can efficiently evaluate the cooling behavior of a metamorphic rock. Thus, granulites from the Mogok Metamorphic Belt (Myanmar) underwent rapid cooling, while those from Khondalite Belt (China) endured a slow cooling process.