The crystal structure of ludlockite, PbFe (super 3+) ₄ As (super 3+) ₁₀ O ₂₂ , the mineral with pentameric arsenite groups and orange hair

University of Manitoba, Department of Geological Sciences, Winnipeg, MB, Canada

The crystal structure of ludlockite, PbFe (super 3+) ₄ As (super 3+) ₁₀ O ₂₂ , a 10.426(4), b 12.074(5), c 18.349(8) Aa, alpha 101.84(3), beta 100.21(3), gamma 90.60(3) degrees , V 2222(2) Aa ³ , A1, Z = 4, has been solved by direct methods and refined to an R index of 4.7% for 3858 observed reflections measured with MoKae X-radiation. The chemical formula of ludlockite, originally given as (Fe,Pb)As ₂ O ₆ , 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 "Fe-O" distance of approximately 2.025 Aa indicates that all Fe is in the trivalent state. There are ten unique As positions, each showing triangular pyramidal coordination with "As-phi " distances of approximately 1.79 Aa, 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 FeO ₆ octahedra and PbO ₈ triangular dodecahedra embraced by convoluted but topologically linear [As ₅ O ₁₁ ] 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.

This record provided courtesy of AGI/GeoRef.