Crystal chemistry of Fe- and Cr-rich warwickite

Warwickite is an orthorhombic oxyborate mineral with the general formula ^[6]M₂O^[3]BO₃ (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, Cr₂O₃ varies between 0.0 and 13.5 wt%, total wt% Fe as Fe₂O₃ is as high as 50.6 wt% and total wt% TiO₂ as high as 10.2 wt%. Two main substitutions occur in Jumilla warwickite: Mg²⁺ + Ti⁴⁺ ⇌ 2Fe³⁺ and 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: (1) The M1 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 M1 site. (4) The M2-04 distance decreases with increasing Ti, suggesting preferential ordering at the M2 site. TEM studies show planar (100) defects. These defects cause variations in the octahedral chain length.