Crystal chemistry of the basic iron phosphates

A general structural principle is widespread among the basic phosphates of ferrous and ferric iron. It is based on a highly stable polyatomic complex involving ferrous-ferric oxy-hydroxy octahedral face-sharing triplets. The orientation of the associated corner-linked nearest neighbor octahedra and tetrahedra is so similar in several mineral structures that a general hierarchy of structure types has been derived. Crystal structures of the various members are dictated by the ways in which the fundamental polyatomic octahedral three-cluster and its nearest neighborhood of octahedra link along a third "variable" axis. The refined crystal structures of dufrenite and rockbridgeite are revealed. Structure cell and indexed powder data are presented for dufrenite, souzalite, rockbridgeite, beraunite, laubmannite, and mineral A (a new species). Data in the literature are misleading and largely in error for most of these compounds. Members of the basic ferrous-ferric phosphate family known to possess the octahedral face-sharing three-cluster include barbosalite, lipscombite, dufrenite, souzalite, rockbridgeite, beraunite, laubmannite, and mineral A. Other compounds which may also involve the three-cluster include azovskite, cacoxenite, richellite, mitridatite, egueite, tinticite, borickyite, and foucherite. All basic ferrous-ferric phosphates which possess the three-cluster are strongly pleochroic and greenish-black in color. Their completely oxidized equivalents are yellow to reddish-brown and weakly pleochroic. The relative absorption of polarized white light appears to be dictated in a complex way by the orientation of the tbree-cluster in the crystal, the extreme pleochroism resulting from directed charge transfer Fe2++Fe3+reversible arrow Fe3++Fe2+. Parageaetic schemes involving some of these minerals are proposed on the basis of specimen study and crystal chemistry, Typical sequences involve-denser phases followed by higher hydrates. Typical steps in the hydrothermal reworking of the early phosphates involve laubmannite -> dufrenite -> beraunite; rockbridgeite -> beraunite; rockbridgeite -> mineral A. Color banding along the fibers in mammillary aggregates of some rockbridgeites results from the appearance of different phases. Typical species and their colors are: rockbridgeite (greenish-black), lipscombite (bluish-green granular), mineral A (brown to greenish-yellow), and beraunite (orange).