The crystal structure refinement of cacoxenite

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Marco E. Ciriotti
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The crystal structure refinement of cacoxenite

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 13:29

▪ Avdontseva, M. & Krivovichev, S. (2018): The crystal structure refinement of cacoxenite. IMA2018 Abstract book. Poster presentations. Modular aspects of mineral structures, 481.

Cacoxenite, Fe3+24AlO6(PO4)17(OH)12·nH2O, is a unique natural material with giant pores of 14 Å in diameter. Its natural existence inspired the searchof novel microporous iron phosphates similar to those observed in aluminophosphates. The mineral was first described from the Hrbek mine, Bohemia, Czech Republic, in 1825, and is widespread in oxidized magnetite, «limonite» ores and in Fe- and Mn-bearing novaculites and gossans. Thecrystal structure of cacoxenite was solved Moore and Shen in 1983. We have studied the sample of cacoxenite from the Těškov quarry, Bohemia,Czech Republic. A suitable crystal of the mineral was fixed on a micro mount and placed on a XtaLab Supernova diffractometer (micro-focused monochromated CuKα radiation). The data were collected at 100 K using HyPix-U1 detector with Hybrid Photon Counting. The unit-cell parameters were refined by least square techniques using 4621 reflections in the 2θ range of 6.44–140.00o. The structure was solved by direct methods and refined to R1 = 0.057 (wR2 = 0.137) for 3637 unique reflections with |Fo| ≥ 4σF. Cacoxenite is hexagonal, P63/m, a = 27.4904(5), b = 10.5307(2) Å, V = 6892.1(2) Å3. The structure is based on an open three-dimensional framework of corner-sharing FeO6, Fe,AlO6, AlO5 and PO4 polyhedra. According to the information-based complexity measures, cacoxenite has a very complex structure, possessing (without H-correction) 2656.355 bits per unit cell. This makes cacoxenite one of the ten most complex minerals known so far.
X-ray diffraction studies were carried out in the XRD Research Centre of St. Petersburg State University.
Marco E. Ciriotti

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