Crystal chemistry and cation ordering in pseudobrookite and armalcolite from Spanish lamproites

Abstract

Pseudobrookite-like minerals from two outcrops of Spanish lamproites (Jumilla, Murcia province, and Cancarix, Albacete province) cover the compositional range: Fe 1.6 3+ Mg 0.2 Ti 1.2 O 5 −Fe 0.6 3+ Mg 0.7 Ti 1.7 O 5 −Fe 0.2 3+ Fe 0.6 2+ Mg 0.3 Ti 1.9 O 5 thus they range between pseudobrookite and armalcolite. Al, Cr, Mn, Ca, Zr and Nb are minor elements. Single crystal X-ray structural refinements, in the space group Bbmm, give R-values between 0.016 and 0.024. The structural refinements show that: (1) M1 site has wider variations in mean bond lengths and is larger and more distorted than M2 site because of the preferential ordering in the M1 site of Mg and Fe 3+ in respect to Ti; (2) the cation ordering is probably controlled both by chemical environment and temperature of crystallization. In particular, ordering in pseudobrookites from rocks of comparable composition increases under decreasing temperature of crystallization; (3) in Jumilla rocks, the highest in oxygen activity among the lamproites analyzed (above the NNO buffer), all the iron in pseudobrookite occurs in the trivalent state, whereas in Cancarix and Las Minas de Helin, close to NNO and QFM buffers respectively, iron is present also in the divalent state