THE CRYSTAL-STRUCTURE OF LUDLOCKITE, PBFE43+AS103+O22, THE MINERAL WITH PENTAMERIC ARSENITE GROUPS AND ORANGE HAIR

The crystal structure of ludlockite, PbFe43+As103+O22, a 10.426(4), b 12.074(5), c 18.349(8) Angstrom, alpha 101.84(3), beta 100.21(3), gamm a 90.60(3)degrees, V 2222(2) Angstrom(3), A (1) over bar, Z = 4, has b een solved by direct methods and refined to an R index of 4.7% for 385 8 observed reflections measured with MoK alpha X-radiation. The chemic al formula of ludlockite, originally given as (Fe,Pb)As2O6, has been r evised, and the results of electron-microprobe analysis are in accord with the revised formula. The orientation of the unit cell has also be en changed from P- to A-centered, such that the major structural featu res are easily related to the principal crystal lographic axes. There is one unique Pb position surrounded by eight oxygen atoms in a triang ular dodecahedral arrangement. There are four unique Fe positions, eac h surrounded by a distorted octahedral arrangement of oxygen atoms; th e <Fe-O> distance of similar to 2.025 Angstrom indicates that all Fe i s in the trivalent state. There are ten unique As positions, each show ing triangular pyramidal coordination with <As-phi> distances of simil ar to 1.79 Angstrom, compatible with all As being in the trivalent sta te 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 linea r [As5O11] groups. This sheet is parallel to (001), and adjacent sheet s stack along [001]. There are no interstitial species, and interactio n between adjacent sheets is very weak, accounting for the perfect cle avage of ludlockite.