Yurii V. Seryotkin

A new mechanism of anionic substitution in fluoride borates

A comprehensive study of the BaF2-Ba3(BO 3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba 7(BO3)4-y F2+3y. The latter exhibited (BO 3)3- ↔ 3F- anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4-y F2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3- ↔ 3F- isomorphism, allows for the disclosure of the mechanism of (BO3)3- ↔ 3F - heterovalent anionic substitution in fluoride borates via [(BO 3)F]4- tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3- ↔ 3F- substitution.

Isoenergetic Polymorphism: The Puzzle of Tolazamide as a Case Study.

In the present case study of tolazamide we illustrate how many seemingly contradictory results that have been obtained from experimental observations and theoretical calculations can finally start forming a consistent picture: a puzzle put together. For many years, tolazamide was considered to have no polymorphs. This made this drug substance unique among the large family of sulfonylureas, which was known to be significantly more prone to polymorphism than many other organic compounds. The present work employs a broad and in-depth analysis that includes the use of optical microscopy, single-crystal and powder X-ray diffraction, IR and Raman spectroscopies, DSC, semiempirical PIXEL calculations and DFT of three polymorphs of tolazamide. This case study shows how the polymorphs of a molecular crystal can be overlooked even if discovered serendipitously on one of numerous crystallizations, and how very different molecular packings can be practically isoenergetic but still crystallize quite selectively and transform one into another irreversibly upon heating.

The crystal structure of paranatrolite

Paranatrolite from the Khibiny massif, Kola Peninsula, Russia, Na 1.88 K 0.22 Ca 0.06 [Al 2.24 Si 2.76 O 10 ].3.1H 2 O, is monoclinic (space group Cc, Z = 4). For better comparison of this mineral with the related structure of natrolite, we have selected a pseudoorthorhombic setting in F1d1 ( a = 18.971(4), b = 19.204(3), c = 6.5952(12) A, β = 91.601(18)°, Z = 8). The dominant Na+ cations are situated near the sodium positions in the natrolite structure. Additional positions occupied by K+ are located in the eight-membered rings. H 2 O molecules are located in four independent positions, two being occupied statistically. The Na-polyhedra correspond to distorted NaO 3 (H 2 O) 3 octahedra forming chains along the c axis by sharing common H 2 O-H 2 O edges and H 2 O vertices. The availability of potassium in the structure results in two configurations for the coordination environment of Na+.

The reversibility of the paranatrolitete-tranatrolite transformation

The problem of the reversibility of paranatrolite-tetranatrolite transformation is a key problem in understanding the paragenesis of the natrolite group zeolites. Two paranatrolite samples of different chemical composition were studied by X-ray powder diffraction and thermogravimetry. It was found that the existence of a particular phase depends on the water vapor pressure in an ambient atmosphere. High-potassium paranatrolite from the Khibiny massif, Kola Peninsula, Na 1.90 K 0.22 Ca 0.06 [Al 2.24 Si 2.76 O 10 ]·3.1H 2 O, is stable at 25 °C and air humidity of about 70 %. Upon heating, the sample loses some of the water content and transforms into tetranatrolite. At 38 °C it consists of a pure tetranatrolite phase. The reverse tetranatrolite-paranatrolite transformation occurs upon cooling the sample to room temperature. The recovery of the paranatrolite phase proceeds even after heating to 300 °C with a 60 % water loss. A high-calcium sample from Mont Saint-Hilaire, Quebec, with approximate formula Na 1.59 Ca 0.32 Sr 0.02 [Al 2.35 Si 2.65 O 10 ]·nH 2 O, had significantly lower stability. It consisted of a mixture of paranatrolite and tetranatrolite in ambient conditions. Upon heating, the sample already consisted of a pure tetranatrolite at 31 °C. After keeping the sample for one day under normal conditions a two-phase mixture close to the initial sample was restored. The sample wetted by water immediately transformed into paranatrolite. A lower stability of high-calcium paranatrolite as compared with the high-potassium sample may be explained by the difference in the configuration of ionic-molecular assemblage and, presumably, by a higher water content.

Transformation of pyrite to pyrrhotite in the presence of Au-Ag alloys at 500 °C

Abstract Dry annealing of AuxAg1–x alloys (x = 0.19, 0.35, 0.56 or of fineness 300, 500, 700‰) and pyrite was used to reveal the solubility of Au (Ag) in FeS2 and study phase equilibria in the FeS2-AuxAgi1–x system at 500 °C. Pyrrhotite, acanthite, and uytenbogaardtite and Au-Ag alloys with increased fineness were established at the contacts of pyrite blocks with Au-Ag plates. The obtained results evidence the absence of solubility between FeS2 and Au (Ag) at 500 °C. The Ag content in alloys influences the stability of pyrite and contributes to its transformation in pyrrhotite and sulfidation and ennobling of Au-Ag alloys. Au-Ag sulfides and pyrrhotite may be present in the sulfide ores of metamorphogene deposits as annealing products of Au-Ag-pyrite-bearing ores.

Influences of extraframework cations on features of natrolite group zeolites: the crystal structure of partly dehydrated K-containing paranatrolite

Abstract The partly dehydrated paranatrolite from the Khibiny massif, Russia, Na1.88K0.22Ca0.06[Al2.24Si2.76O10]·2.5H2O, has a space group F1d1 and the unit cell parameters a = 18.6586(5), b = 18.6536(5), c = 6.64119(13) Å, β = 90.199(2)°, and Z = 8. The dominant Na+ cations are located nearby the sodium positions in the natrolite structure. The potassium cations occupy two individual positions. Their different occupancies define the retention of the monoclinic distortion of the structure. H2O molecules statistically occupy eight positions, six being generated with a triple splitting of two positions. Three kinds of main local cation-water assemblages were distinguished, two being similar to the assemblages in the initial high-water paranatrolite. The comparison with the gonnardite and paranatrolite structures determined previously was carried out. Common structural characteristics of natrolite group minerals are suggested from the local water-cation assemblages.

Correction: Na4Ca(CO3)3: a novel carbonate analog of borate optical materials

Correction for ‘Na4Ca(CO3)3: a novel carbonate analog of borate optical materials’ by Sergey V. Rashchenko et al., CrystEngComm, 2018, 20, 5228–5232.

Na4Ca(CO3)3: a novel carbonate analog of borate optical materials

Carbonate optical materials currently attract a lot of interest as an unexplored alternative to borate ones. However, these two classes of compounds were believed to have quite different crystal chemistry, so that no systematic attempts were made to reproduce crystal structures of known borate optical materials in carbonates. Here we describe a novel Na4Ca(CO3)3 cubic carbonate (Ia3d, a = 15.5770(5) A), isostructural to borates of the NaBa4(BO3)3 family known for their phosphor properties, and discuss further perspectives in the investigation of ‘borate-structured’ carbonate materials.

Kuliginite, a new hydroxychloride mineral from the Udachnaya kimberlite pipe, Yakutia: Implications for low-temperature hydrothermal alteration of the kimberlites

Abstract Kuliginite is a new iron-magnesium hydroxychloride mineral with the ideal formula Fe3Mg(OH)6Cl2 from the Udachnaya East kimberlite, Yakutia, Russia. It occurs as green prismatic-bipyramidal crystals (0.2–0.5 mm) and fills cavities and veins in several units of kimberlites together with iowaite, gypsum, calcite, halite, barite, and celestine. It is trigonal, with R3̅ space group. Kuliginite has imperfect cleavage on {101̅1}. The spinel-like crystal structure of kuliginite is also typical for several copper minerals of the atacamite group with common formula Cu3M(OH)6Cl2; kuliginite can be regarded as a Fe2+ analog of tondiite [Cu3Mg(OH)6Cl2. The occurrence of the kuliginite + iowaite + gypsum assemblage has implications for the interpretation of low-temperature (below 100°C) hydrothermal processes and alteration of kimberlite by hydrothermal fluids/brines, as well as for transport of metals in Cl-bearing solutions. This secondary hydrothermal mineral assemblage formed much later than the kimberlite groundmass minerals. Kuliginite contains inclusions of iowaite indicating their simultaneous crystallization.