THE CRYSTAL STRUCTURE OF LUDLOCKITE, PbFe3+4As3+10O22, THE MINERAL WITH PENTAMERIC ARSENITE GROUPS AND ORANGE HAIR

MARK A. COOPER and FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2

Abstract

The crystal structure of ludlockite, PbFe3+4As3+10O22, a 10.426(4), b 12.074(5), c 18.349(8) Å, 101.84(3), 100.21(3), 90.60(3) , V 2222(2) Å3, A, Z = 4, has been solved by direct methods and refined to an R index of 4.7% for 3858 observed reflections measured with MoK X-radiation. The chemical formula of ludlockite, originally given as (Fe,Pb)As2O6, has been revised, and the results of electron-microprobe analysis are in accord with the revised formula. The orientation of the unit cell has also been changed from P- to A-centered, such that the major structural features are easily related to the principal crystallographic axes. There is one unique Pb position surrounded by eight oxygen atoms in a triangular dodecahedral arrangement. There are four unique Fe positions, each surrounded by a distorted octahedral arrangement of oxygen atoms; the distance of 2.025 Å indicates that all Fe is in the trivalent state. There are ten unique As positions, each showing triangular pyramidal coordination with distances of 1.79 Å, compatible with all As being in the trivalent state with a stereoactive lone-pair of electrons. The structural unit of ludlockite consists of a sheet of edge-sharing FeO6 octahedra and PbO8 triangular dodecahedra embraced by convoluted but topologically linear [As5O11] groups. This sheet is parallel to (001), and adjacent sheets stack along [001]. There are no interstitial species and interaction between adjacent sheets is very weak, accounting for the perfect cleavage of ludlockite.