Titre du document / Document title

The crystal structure of orlandiite, Pb³Cl⁴(SeO³)H²O, a complex case of twinning and disorder

Auteur(s) / Author(s)

DEMARTIN Francesco ; GRAMACCIOLI Carlo Maria ; PILATI Tullio ;

Résumé / Abstract

The crystal structure of orlandiite, Pb³Cl⁴(SeO³)H²O, has been determined and refined to R = 0.042. Orlandiite is triclinic, space group P1 (no. 2), with a 8.136(3), b 8.430(6), c 9.233(7) Å, a 62.58(7), β 71.84(4), γ 75.13(4)°, and Z = 2. The unit cell can be referred to a pseudomonoclinic C-centered setting using the transformation [-2 ½ = 0 / 0 -½ 0 / 0 -½ 1], leading to a 15.727, b 4.215, c 8.196 Å, β 103.03°. The crystals so far examined are twinned on the {0 1 0} plane in the pseudomonoclinic reference-frame, in cases closely simulating monoclinic symmetry. Besides twinning-induced apparent disorder, especially involving the position of a lead atom, there is also actual disorder involving the selenite anion. Owing to disorder of the selenite group, two independent Pb atoms in the structure can be either nine-fold or eight-fold coordinated. In the first case, they are surrounded by six chlorine and three oxygen atoms of the selenite anion (tricapped trigonal prismatic coordination); in the second case, one of the bonded oxygen atoms is missing. These coordination polyhedra are linked together to form sheets normal to [1 0 0]. A third nine-fold coordinated Pb atom, showing apparent signs of disorder (actually caused by twinning) is located between these sheets, together with the SeO³2- anion and the H²O molecule.

Revue / Journal Title

Canadian mineralogist   ISSN 0008-4476   CODEN CAMIA6 

Source / Source

2003, vol. 41 (5), pp. 1147-1153 [7 page(s) (article)]

Langue / Language

Anglais
Revue : Multilingue

Editeur / Publisher

Mineralogical Association of Canada, Ottawa, On, CANADA  (1957) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 12015, 35400011341428.0040