Beraunite: Refinement, comparative crystal chemistry, and selected bond valences

Abstract

Abstract Deep green beraunite from Campanian age marls of Mullica Hill, N. J. (USA) was investigated owing to its unique crytal chemistry (two previous refinements involved orange crystals). Our crystal gave nearly 4[Fe2+Fe5 3+(OH)5(H2O)4(PO4)4 · 2H2O], a = 20.953(5), b = 5.171(1), c = 19.266(4) Å, β = 93.34(2)°, C2/c, R = 0.043 (isotropic) for 2065 unique F o and 91 variable parameters. Common to several known “ferrosoferric” phosphate structures, the underlying principle is a linear face-sharing oxygen octahedral trimer whose cations are M(4) – M(1) – M(4). For our beraunite, ca. 83% Fe2+ resides in the central M(1) site with mean M(1) – O = 2.11 Å. The other octahedral cation sites M(2), M(3), and M(4) (mean M – O distances 2.01 to 2.02 Å) are Fe3+. The orange crystals of the other studies have predominant Fe3+ at all M(1) to M(4) octahedral sites (mean M – O distances 1.97 to 2.04 Å). Distortion of the octahedral face-trimer is pronounced, with M(1) – M(4) distance lengthening of 19% compared with the perfect octahedral model. This results from cation-cation repulsion, which distorts the entire immediate environment from the perfect model. The OH(1) site on the trimer is coordinated by M(1), M(3), M(4), and H(1). Bond valence computations for the trimer suggest that Fe2+ Fe3+ (green) → Fe3+ Fe3+ (orange) may involve (1) O – H(1) distance response to bond valence deviation or (2) an averaged model 2OH−→2(OH½O½)1.5−, but independent spectroscopic study is required to unravel the mechanism.