A new Al-rich hydroxylian pseudorutile from Kalimantan, Indonesia

Abstract

A new type of Al-rich hydroxylian pseudorutile (HPR), containing 64.6 wt% TiO 2 , 12.1 wt% Al 2 O 3 , 10.9 wt% Fe 2 O 3 , and 12.4 wt% H 2 O, has been identified in detrital heavy mineral concentrates from Central Kalimantan, Indonesia. The unusual translucent orange-colored grains have been characterized using electron microprobe analyses, thermal analyses, 27 Al MAS NMR, and synchrotron XRD studies including the use of differential pair distribution function analysis, to determine how the Al is incorporated. The results support a model in which diaspore-form AlOOH is incorporated into nanopores in the solid hydrogel-like mineral. The HPR matrix structure comprises disordered unit-cell scale intergrowths of diaspore-type and rutile-type structure elements, which have a close dimensional match to the diaspore structure, thus providing a suitable template for epitactic nucleation and growth of the diaspore. The average composition of the grains is [FeTi 6 O 10.8 (OH) 5.4 ]·1.8AlOOH·H 2 O, where the formula within the square brackets represents the HPR matrix, and the AlOOH and H 2 O occupy the intragrain pore volume.