Towards a revisitation of vesuvianite-group nomenclature: The crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley, Pakistan

© 2016 International Union of Crystallography.Vesuvianite containing 5.85 wt% TiO2 from an Alpine-cleft-type assemblage outcropped near Alchuri, Shigar Valley, Northern Areas, Pakistan, has been investigated by means of electron microprobe analyses, gas-chromatographic analysis of H2O, X-ray powder diffraction, single-crystal X-ray structure refinement, 27Al NMR, 57Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are: a = 15.5326 (2), c = 11.8040 (2) Å, space group P4/nnc. The structure was refined to final R1 = 0.031, wR2 = 0.057 for 11247 I > 2σ(I). A general crystal-chemical formula of studied sample can be written as follows (Z = 2): [8-9](Ca17.1Na0.9) [8]Ca1.0[5](Fe2+ 0.44Fe3+ 0.34Mg0.22) [6](Al3.59Mg0.41) [6](Al4.03Ti2.20Fe3+1.37Fe2+ 0.40) (Si18O68) [(OH)5.84O2.83F1.33]. The octahedral site Y2 is Al-dominant and does not contain transition elements. Another octahedral site Y3 is also Al-dominant and contains Fe2+, Fe3+ and Ti. The site Y1 is split into Y1a and Y1b predominantly occupied by Fe2+ and Fe3+, respectively. The role of the Y1 site in the diversity of vesuvianite-group minerals is discussed

Structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides

Structural features of 3,4-diaryl-1H-pyrrol-2,5-diimines and their derivatives have been studied by molecular spectroscopy techniques, single-crystal X-ray diffraction, and DFT calculations. According to the theoretical calculations, the diimino tautomeric form of 3,4-diaryl-1H-pyrrol-2,5-diimines is more stable in solution than the imino-enamino form. We also found that the structurally related 2,3 exist in the solid state in the dimeric diketo form. 3,4-Diary1-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides exhibit fluorescence in the blue region of the visible spectrum. The fluorescence spectra have large Stokes shifts. Aryl substituents at the 3,4-positions of 1H-pyrrol-2,5-diimine do not significantly affect fluorescence properties. The insertion of donor substituents into 2,3diarylmaleimides leads to bathochromic shift of emission bands with hyperchromic effect. (C) 2017 Elsevier B.V. All rights reserved

The Crystal Structure of Sergeysmirnovite, MgZn₂(PO₄)₂·4H₂O, and Complexity of the Hopeite Group and Related Structures

The crystal structure of sergeysmirnovite, MgZn2(PO4)2·4H2O (orthorhombic, Pnma, a = 10.6286(4), b = 18.3700(6), c = 5.02060(15) Å, V = 980.26(6) Å3, Z = 4), a new member of the hopeite group of minerals, was determined and refined to R1 = 0.030 using crystals from the Këster mineral deposit in Sakha-Yakutia, Russia. Similar to other members of the hopeite group, the crystal structure of sergeysmirnovite is based upon [Zn(PO4)]– layers interlinked via interstitial [MO2(H2O)4]2– octahedra, where M = Mg2+. The layers are parallel to the (010) plane. Within the layer, the ZnO4 tetrahedra share common corners to form chains running along [001]. Sergeysmirnovite is a dimorph of reaphookhillite, a mineral from the Reaphook Hill zinc deposit in South Australia. The relations between sergeysmirnovite and reaphookhillite are the same as those between hopeite and parahopeite. Topological and structural complexity analysis using information theory shows that the hopeite (sergeysmirnovite) structure type is more complex, both structurally and topologically, than the parahopeite (reaphookhillite) structure type. Such complexity relations contradict the general observation that more complex polymorphs possess higher physical density and higher stability, since parahopeite is denser than hopeite. It could be hypothesized that hopeite is metastable under ambient conditions and separated from parahopeite by a structural and topological reconstruction that requires an essential energy barrier that is difficult to overcome

Dynamic Disorder of Fe3+ Ions in the Crystal Structure of Natural Barioferrite

A natural barioferrite, BaFe3+12O19, from a larnite–schorlomite–gehlenite vein of paralava within gehlenite hornfels of the Hatrurim Complex at Har Parsa, Negev Desert, Israel, was investigated by Raman spectroscopy, electron probe microanalysis, and single-crystal X-ray analyses acquired over the temperature range of 100–400 K. The crystals are up to 0.3 mm × 0.1 mm in size and form intergrowths with hematite, magnesioferrite, khesinite, and harmunite. The empirical formula of the barioferrite investigated is as follows: (Ba0.85Ca0.12Sr0.03)∑1(Fe3+10.72Al0.46Ti4+0.41Mg0.15Cu2+0.09Ca0.08Zn0.04Mn2+0.03Si0.01)∑11.99O19. The strongest bands in the Raman spectrum are as follows: 712, 682, 617, 515, 406, and 328 cm−1. The structure of natural barioferrite (P63/mmc, a = 5.8901(2) Å, c = 23.1235(6) Å, V = 694.75(4) Å3, Z = 2) is identical with the structure of synthetic barium ferrite and can be described as an interstratification of two fundamental blocks: spinel-like S-modules with a cubic stacking sequence and R-modules that have hexagonal stacking. The displacement ellipsoids of the trigonal bipyramidal site show elongation along the [001] direction during heating. As a function of temperature, the mean apical Fe–O bond lengths increase, whereas the equatorial bond lengths decrease, which indicates dynamic disorder at the Fe2 site

Dmisteinbergite, CaAl2Si2O8, a Metastable Polymorph of Anorthite: Crystal-Structure and Raman Spectroscopic Study of the Holotype Specimen

The crystal structure of dmisteinbergite has been determined using crystals from the type locality in Kopeisk city, Chelyabinsk area, Southern Urals, Russia. The mineral is trigonal, with the following structure: P312, a = 5.1123(2), c = 14.7420(7) Å, V = 333.67(3) Å3, R1 = 0.045, for 762 unique observed reflections. The most intense bands of the Raman spectra at 327s, 439s, 892s, and 912s cm −1 correspond to different types of tetrahedral stretching vibrations: Si–O, Al–O, O–Si–O, and O–Al–O. The weak bands at 487w, 503w, and 801w cm−1 can be attributed to the valence and deformation modes of Si–O and Al–O bond vibrations in tetrahedra. The weak bands in the range of 70–200 cm−1 can be attributed to Ca–O bond vibrations or lattice modes. The crystal structure of dmisteinbergite is based upon double layers of six-membered rings of corner-sharing AlO4 and SiO4 tetrahedra. The obtained model shows an ordering of Al and Si over four distinct crystallographic sites with tetrahedral coordination, which is evident from the average <T–O> bond lengths (T = Al, Si), equal to 1.666, 1.713, 1.611, and 1.748 Å for T1, T2, T3, and T4, respectively. One of the oxygen sites (O4) is split, suggesting the existence of two possible conformations of the [Al2Si2O8]2− layers, with different systems of ditrigonal distortions in the adjacent single layers. The observed disorder has a direct influence upon the geometry of the interlayer space and the coordination of the Ca2 site. Whereas the coordination of the Ca1 site is not influenced by the disorder and is trigonal antiprismatic (distorted octahedral), the coordination environment of the Ca2 site includes disordered O atoms and is either trigonal prismatic or trigonal antiprismatic. The observed structural features suggest the possible existence of different varieties of dmisteinbergite that may differ in: (i) degree of disorder of the Al/Si tetrahedral sites, with completely disordered structure having the P63/mcm symmetry; (ii) degree of disorder of the O sites, which may have a direct influence on the coordination features of the Ca2+ cations; (iii) polytypic variations (different stacking sequences and layer shifts). The formation of dmisteinbergite is usually associated with metastable crystallization in both natural and synthetic systems, indicating the kinetic nature of this phase. Information-based complexity calculations indicate that the crystal structures of metastable CaAl2Si2O8 polymorphs dmisteinbergite and svyatoslavite are structurally and topologically simpler than that of their stable counterpart, anorthite, which is in good agreement with Goldsmith’s simplexity principle and similar previous observations

An Apatite-Group Praseodymium Carbonate Fluoroxybritholite: hydrothermal synthesis, crystal structure, and implications for natural and synthetic Britholites

Britholites are the lanthanide–silica-rich end-members of the apatite group, commonly studied for their optical properties. Here, we show ∼50–100 μm single crystals synthesized hydrothermally at 650–500 °C and 500–300 MPa composed of a solid solution between Ca2Pr3(SiO4)3F–fluorbritholite and CaPr4(SiO4)3O–oxybritholite, with a significant carbonate component substitution, via C4+ replacing Si4+. Single-crystal X-ray diffraction and density functional theory computations show that a planar carbonate group occupies the face of a now-vacant silica tetrahedron. This modifies Pr–O bond lengths, diversifying lanthanide optical emission wavelengths. Our britholite was synthesized in geologically reasonable conditions and compositions, suggesting that carbonated oxybritholites could exist as yet-unrecognized natural minerals

Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular

Garnet supergroup minerals are in the interest of different applications in geology, mineralogy, 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 microscopy. 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¯d, I41/acd, I41/a, and I4¯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 Mg2+ and Fe3+ in two independent octahedral sites, which cause the symmetry breaking according to the group–subgroup relation Ia3¯d → I41/a. Octahedral crystals of (H4O4)4−-substituted grossular are merohedrally twinned by twofold axis along [110]. The mechanism of twining growth led to the generation of stacking faults on the (110) plane and results in the formation of crystals with a long prismatic habit

Thermal Behavior and Phase Transition of Uric Acid and Its Dihydrate Form, the Common Biominerals Uricite and Tinnunculite

Single crystals and powder samples of uric acid and uric acid dihydrate, known as uricite and tinnunculite biominerals, were extracted from renal stones and studied using single-crystal and powder X-ray diffraction (SC and PXRD) at various temperatures, as well as IR spectroscopy. The results of high-temperature PXRD experiments revealed that the structure of uricite is stable up to 380 °C, and then it loses crystallinity. The crystal structure of tinnunculite is relatively stable up to 40 °C, whereas above this temperature, rapid release of H2O molecules occurs followed by the direct transition to uricite phase without intermediate hydration states. SCXRD studies and IR spectroscopy data confirmed the similarity of uricite and tinnunculite crystal structures. SCXRD at low temperatures allowed us to determine the dynamics of the unit cells induced by temperature variations. The thermal behavior of uricite and tinnunculite is essentially anisotropic; the structures not only expand, but also contract with temperature increase. The maximal expansion occurs along the unit cell parameter of 7 Å (b in uricite and a in tinnunculite) as a result of the shifts of chains of H-bonded uric acid molecules and relaxation of the π-stacking forces, the weakest intermolecular interactions in these structures. The strongest contraction in the structure of uricite occurs perpendicular to the (101) plane, which is due to the orthogonalization of the monoclinic angle. The structure of tinnunculite also contracts along the [010] direction, which is mostly due to the stretching mechanism of the uric acid chains. These phase transitions that occur within the range of physiological temperatures emphasize the particular importance of the structural studies within the urate system, due to their importance in terms of human health. The removal of supersaturation in uric acid in urine at the initial stages of stone formation can occur due to the formation of metastable uric acid dihydrate in accordance with the Ostwald rule, which would serve as a nucleus for the subsequent growth of the stone at further formation stages; afterward, it irreversibly dehydrates into anhydrous uric acid