Crystal chemistry of Fe- and Cr-rich warwichite.

Warwickite is an orthorhombic oxyborate mineral with the general formula t6lM2Ot3lBOj (M : Mg2+, Mn2+, Fe3+, Ti4+, Al3+). The studied warwickite samples are from lamproitic rocks (umillites) and associated carbonatite-like veins (Jumilla, Murcia province, southeast Spain). Their chemical compositions are quite variable and different from those of warwickite described in the literature: in particular, CrrO, varies between 0.0 and 13.5 wto/0,t otal wto/oF e as FerO, is as high as 50.6 Mo/oa nd total wto/oT iO, as high as 10.2 wto/0T. wo main substitutions occur in Jumilla warwickite: Mg'* + Tia+ = 2Fe3+a nd Fe3* + Cr3+. The structures of seven crystals of different compositions were refined in space group Pnma, giving R : 0.018-0.038. The structural results show that: (l) The Ml octahedron is smaller and more distorted than the M2 octahedron. (2) Distortion of the octahedral layer is greater in Fe-rich warwickite. (3) Fe preferentially enters the Ml site. (4) The M2- 04 distance decreasesw ith increasingT i, suggestingp referential ordering at the M2 site. TEM studies show planar (100) defects. These defects cause variations in the octahedral chain length.