Single crystal refinement of the structure of baghdadite from Fuka (Okayama Prefecture, Japan)

the crystal structure of baghdadite, ideally Ca3zr(si2o7)o2, was refined using 1986 reflections to R = 0.034. Baghdadite is monoclinic, space group P21/a, with cell parameters a 10.432(3), b 10.163(2), c 7.356(2) Å, b 90.96(2)°. the refined crystal structure is in agreement with the structure of synthetic Ca3zrsi2o9. the structure is characterized by the presence of walls of cation polyhedra, four columns large, linked together by both direct connection as well as by disilicate groups. Baghdadite is the first phase in the cuspidine group in which the Pauling’s fourth rule is violated. riassUnto la struttura cristallina della baghdadite, formula ideale Ca3zr(si2o7)o2, è stata raffinata fino ad un indice di accordo R = 0.034, utilizzando 1986 riflessi. la baghdadite è monoclina, gruppo spaziale P21/a, con parametri di cella a 10.432(3), b 10.163(2), c 7.356(2) Å, b 90.96(2)°. la struttura raffinata è sostanzialmente identica a quella del composto sintetico Ca3zrsi2o9. È caratterizzata dalla presenza di muri di poliedri, connessi tra loro sia per condivisione di vertici sia attraverso gruppi disilicato. la baghdadite è l’unica fase del gruppo della cuspidina in cui non è rispettata la quarta regola di Pauling. KEy worDs: baghdadite; cuspidine group; crystal structure. introDUCtion Baghdadite is a quite rare calcium zirconium disilicate belonging to the cuspidine group, a series of silicates with general formula M4(si2o7)X2, where M denotes cations with various charges and ionic radii, characterized by an octahedral to roughly octahedral coordination. as described by Merlino and Perchiazzi (1985a), the crystal structure of the minerals in the cuspidine family is formed by two kinds of modules whose connection gives rise to the structures of the various phases: four columns wide ‘octahedral’ walls, extending along [001], and disilicate groups. the octahedral walls are interconnected by corner-sharing, forming the framework common to all the minerals in this family. the different ways in which the diorthosilicate groups may be connected to the octahedral walls give rise to the observed different unit cells and symmetries. another difference is obviously related to their crystal chemistry, linked to the various possible cationic distributions within the polyhedra. Baghdadite was found in melilite skarn in contact with banded diorite from the Dupezeh Mountains, Qala-Dizeh region, nE iraq, and PeriOdicO di MineralOgia established in 1930 Period. Mineral. (2010), 79, 3, 1-9 doi: 10.2451/2010PM0013 http://go.to/permin An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archeometry and Cultural Heritage Single crystal refinement of the structure of baghdadite from Fuka (Okayama Prefecture, Japan) Cristian Biagioni*, ElEna BonaCCorsi, natalE PErChiazzi and stEfano MErlino Dipartimento di scienze della terra, Università di Pisa, Via santa Maria 53, i-56126 Pisa, italy Submitted, June 2010 Accepted, September 2010 * Corresponding author, E-mail: biagioni@dst.unipi.it biagioni:periodico 30/11/2010 13:26 Pagina 1