The crystal structure determination and redefinition of eztlite, Pb2+2Fe3+3(Te4+O3)3(SO4)O2Cl

Owen P. Missen, Stuart J. Mills, John Spratt, Mark D. Welch, William D. Birch, Michael S. Rumsey, Jan Vylita
2018 Mineralogical magazine  
The crystal structure of eztlite has been determined using single-crystal synchrotron X-ray diffraction and supported using electron microprobe analysis and powder diffraction. Eztlite, a secondary tellurium mineral from Moctezuma mine, Mexico, is monoclinic, space group Cm, with a = 11.466(2) Å, b = 19.775(4) Å, c = 10.497(2) Å, β = 102.62(3)° and V = 2322.6(9) Å 3 . The chemical formula of eztlite has been revised to Pb 2+ 2 Fe 3+ 3 (Te 4+ O 3 ) 3 (SO 4 )O 2 Cl from that stated by Williams
more » ... 82) as Fe 3+ 6 Pb 2+ 2 (Te 4+ O 3 ) 3 (Te 6+ O 6 )(OH) 10 ·nH 2 O. This change has been accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, Proposal 18-A. Eztlite was reported to be a mixed-valence Te oxysalt; however the crystal structure, bond valence analysis and charge balance considerations clearly show that all Te is tetravalent. Eztlite contains a unique combination of elements and is only the second Te oxysalt to contain both sulphate and chloride. The crystal structure of eztlite contains mitridatite-like layers, with a repeating triangular nonameric [Fe 3+ 9 O 36 ] 45arrangement formed by nine edge-sharing Fe 3+ O 6 octahedra, decorated by four trigonal pyramidal Te 4+ O 3 groups, compared with PO 4 or AsO 4 tetrahedra in mitridatite-type minerals. In eztlite, all four tellurite groups associated with one nonamer are orientated with the lone pair of the Te atoms pointing in the same direction, whereas in mitridatite the central tetrahedron is orientated in the opposite direction to the others. In mitridatite-type structures, interlayer connections are formed exclusively via Ca 2+ and water molecules, whereas the eztlite interlayer contains Pb 2+ , sulphate tetrahedra and Cl -. Interlayer connectivity in eztlite is achieved primarily by connections via the long bonds of Pbφ 8 and Pbφ 9 groups to sulphate tetrahedra and to Cl -. Secondary connectivity is via Te-O and Te-Cl bonds.
doi:10.1180/mgm.2018.108 fatcat:5736tfa4bzaapfiay5dn3gi7wy