THE SYMMETRY AND CRYSTAL STRUCTURE OF GORCEIXITE, BaAl3[PO3(O,OH)]2(OH)6, A MEMBER OF THE ALUNITE SUPERGROUP

Abstract

The crystal structure of the Ba-Al phosphate mineral gorceixite, a 7.0538(3), c 17.2746(6) Å, V 744.4(2) Å 3 , space group R3m, Z = 3, has been refi ned to an R 1 index of 2.3% on the basis of 253 unique refl ections measured using MoK␣ radiation on an automated single-crystal diffractometer. The specimen, from the Rapid Creek area, Yukon Territory, has rhombohedral rather than monoclinic Cm symmetry previously reported for the species. The crystal-structure refi nement shows that the atomic arrangement of gorceixite is similar to that of other members of the alunite supergroup. The Ba-O and Ba-OH distances are 2.825 and 2.859 Å (both ϫ6), respectively, and the volume of the Ba(O,OH) 12 polyhedron is 55.64 Å 3. Energy-dispersion spectra and electronmicroprobe analyses indicate that the site is completely occupied by Ba, but the site occupancy refi nes to 88% occupancy; the reason for this is unclear. The average Al-O,OH and P-O distances are 1.906 and 1.544 Å, respectively, and the OH-H distance is 0.980 Å. Interatomic distances [H...O(2) = 1.904 Å and OH...O(2) = 2.884 Å] and a bond-valence analysis suggest that each O(2) atom is involved in hydrogen bonding (as an acceptor) with three different OH groups. However, the low bond-valence sum for O(2) suggests that it not only acts as an acceptor but functions as a donor as well. No hydrogen-atom sites could be identifi ed from the difference-Fourier map, but the presence of OH groups at the O(2) site would help resolve the problem of charge balance. Assuming equal occupancy of the O(2) site by O and OH, the general formula of gorceixite is BaAl 3 [PO 3 (O,OH)] 2 (OH) 6. Although R3m is the most appropriate space-group for the gorceixite sample studied here, we do not claim that all gorceixite samples crystallize in this space group. Different conditions of formation and compositions might be responsible for the lower symmetry observed previously, but the mechanism to achieve the symmetry lowering remains unclear.

Rare earth element stripping from kaolin sands via mild acid treatment

“…Natural kaolinite sources usually contain kaolinite as a major phase and also several other side phases. For raw material from AKW, the most likely ones are α‐quartz, orthoclase feldspar, muscovite, and gorceixite 25–28 . Multiple separation steps are required to separate the different phases and to purify the single phases prior to the usage.…”

“…Beside Al 2 Si 2 O 5 [OH] 4 (kaolinite) 24 itself, major expected side phases like SiO 2 (α‐quartz), 25 KAlSi 3 O 8 (orthoclase feldspar), 26 and KAl 2 (AlSi 3 O 10 )(F,OH) 2 (muscovite) 27 are denoted. Due to an XRF analysis provided by AKW (see Tables S3–S6) that contained Ba and phosphate as minor components we added BaAl 3 (PO 4 )(PO 3 OH)(OH) 6 (gorceixite) 28 as an additional and most‐probable phase in the XRD phase analysis. Calculated reflection intensities based on published structure data of those compounds are drawn with negative intensity for better visibility and comparison.…”

“…The program package STOE WinXPOW was used to process the raw data for phase analysis performed in Jana2006 22,23 . Literature data for kaolinite, 24 quartz, 25 orthoclase feldspar, 26 muscovite, 27 and gorceixite 28 were used to identify the most probable phases occurring in RK1 and RK2. This selection of phases is based on the knowledge provided by AKW.…”

“…For raw material from AKW, the most likely ones are α-quartz, orthoclase feldspar, muscovite, and gorceixite. [25][26][27][28] Multiple separation steps are required to separate the different phases and to purify the single phases prior to the usage. The raw material at AKW is treated in a magnetic separation process, whereby the purer stream is separated from the magnetic impure side stream.…”

Synthesis of Purine‐based Triazoles by Copper (I)‐catalyzed Huisgen Azide–Alkyne Cycloaddition Reaction

“…Anomalous dispersion effects were included in Fcalc; the values for Δf′ and Δf″ were those of Creagh and McAuley. The values for the mass attenuation coefficients are those of Creagh and Hubbell . All calculations were performed using the crystal structure crystallographic software package except for refinement, which was performed using SHELXL‐97 (Table ).…”