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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.