MS25-P04 Electron diffraction tomography of modulated minerals: the crystal structure of daliraniteDaliranite is a mineral originally discovered in 2003 at Zarshouran, northwest Iran, and approved by IMA in 2007 with the formula PbHgAs2S6. It occurs as matted nests of acicular crystals usually associated with quartz and orpiment. Single-crystal needles are just several hundreds of nanometers in size. On the basis of zone-axis electron diffraction patterns, daliranite was recognized as monoclinic, 19 Å x 4 Å x 23 Å and b =115°, with possible space groups P2, Pm or P2/m. The unit cell was confirmed by powder X-ray diffraction, however no structure solution was achieved and its crystal structure remained unknown .
The daliranite structure problem is a perfect candidate to be tackled using electron diffraction tomography (EDT). This method allows collecting 3D electron diffraction data on coherent domains having size of few hundreds of nanometers, like a single crystal X-ray diffractometer equipped with an area detector .
EDT data collected on isolated squared pieces of 200 nm of broken acicular crystals reveal
s that daliranite is exhibits a modulated structure with the main reflections that can be indexed with an orthorhombic cell having a = 9.5 Å b = 4.3 Å c = 21 Å and extinction symbol Pc_n. The modulation is along a (q = α 0 0) with α varying from crystal to crystal in the range between 0.33 and 0.25. The average structure of daliranite can be solved in space group Pcmn. The resulting structure shows a chemical formula that differs from that originally reported: PbHgAs2S5, which does not require the presence of (S2)2- in the structure. The average structure is formed by zig-zag chains of PbS8 bicapped trigonal prisms running along a, laterally connected by linear HgS2 and As2S6 dimers. In order to discover the mechanism behind the modulation we integrated the superstructure reflections from a crystal with q = 0.25 0 0 and analyzed them with a superpace approach using JANA2006. The modulated structural model obtained in the 4D space group Pcmn (α00)0s0 can be refined and indicates that the modulation is due to shifts of the As atoms of the dimers along b, forming alternatively dimers parallel or inclined with respect to the ac plane. It is highly remarkable that EDT can furnish data reliable for structure investigation of modulated structures on such small crystal grains, where in fact zone-axis electron diffraction and powder X-ray diffraction even failed in the unit cell determination.
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Keywords: Electron diffraction, modulated structure, daliranite