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Cervandonite
Cervandonite is a rare arsenosilicate mineral. It has a chemical formula (Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3SiAs(Si,As)O13 or (Ce,Nd,La)(Fe3+,Fe2+,Ti,Al)3O2(Si2O7)(As3+O3)(OH).〔 It has a monoclinic crustal structure with supercell (Z=6), the crystal structure was established as a trigonal subcell, with space group R3m and a = 6.508(1)Ǻ, c = 18.520(3) Ǻ, V 679.4(2) Ǻ3, and Z=3.〔 It was first described by Buhler Armbruster in 1988, but it has proven to be problem due to the extreme scarcity of single crystals and its unusual replacement of silicon and arsenic.〔Poirier, G., Ercit, T., Tait, K., Piilonen, P., and Rowe, R. (2009) New mineral names. American Mineralogist 94, 399-408.〕 Cervandonite is named after the location where it was first described, Pizzo Cervandone (Scherbadung), Italy in the Central Alps.〔
==Structure==
The current work is based on a single- crystal fragment of cervandonite measuring 0.08 X 0.03 X 0.02 mm.〔 The exact fragment was re-examined with MoKα radiation using BRUKER Apex II diffractometer equipped with a 2K CCD detector. Through the use of X-ray diffraction, the unusual nature of the As to Si substitution found arsenic to be present as As3+ instead of As5+, with the presence of sorosilicate Si2076− anions were also established. The original description of the cervandonite mineral,(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3SiAs(Si,As)O13, was rewritten to,(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3 O2(Si2As7)1-x+y(As03)1+x-y(OH)3x-3y. The x and y values are 0.47 and 0.31. Even though the values were derived from a refined and disordered crystal, the new formula matches the charge balance (negative versus 12.75 positive charges ) for the average chemical composition for the M(1)+M(2) sites.〔Anthony, J., Bideaux, R., Bladh, K., and Nichols, M. (2003) Cervandonite. Handbook of Mineralogy 2, 1〕 The As-containing silicates, none of which contained the Si2As7 composition, or arsenic in the 3+ state, where considered to be examples of As to Si diadochy. However, after accurate determination of the crystal structure the As and Si atoms where shown to occupy distinct cell sites. So far, the As-Si disorder is unique for the cervandonite mineral, and accounts for the As:Si ratio. Due to the unusual diadochy, the variable composition of cervandonite might consist of an assembly of more or less twinned microdomains. Since cervandonite contains different values of As/Si and unit- cell parameters, has observed one type of superstructure.〔Demartin, F., Gramaccioli, C., and Graeser, S. (2008) The crystal structure of cervandonite-(Ce),an interesting example of As (super 3+) -->Si diadochy. Canadian Mineralogist 46, 423-430〕
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