Crystal chemistry of some Fe- and Ti-poor dumortierites

Of twelve specimens of dumortierite from various localities, we have analyzed all by electron microprobe, determined lattice parameters for all, measured optical properties for six, performed X-ray crystal-structure refinements on four, and obtained infrared absorption spectra for three. Aside from major Si and Al, the only cations detected above the ∼0.02 wt% concentration level were minor Ti, Fe, Mg, and Ca. Infrared spectra confirmed the presence of similar amounts of (OH) in the three specimens checked, and we assume that (OH) is present in all twelve. From rough charge-balance considerations, based on the numbers of vacancies at the partially occupied Al(1) site, it appears that there are ∼0.4 (OH) per formula unit. X-ray site refinements showed the B site to be fully occupied in all four refined structures, and there are no crystallochemically reasonable substituents for three-coordinated B. All but one specimen showed a deficiency of Si, which is supplemented by substituent Al, yielding the observed full occupancy of the Si(l) and Si(2) sites. For the twelve specimens investigated, an approximate average formula is with variation expected in all subscripts, except perhaps for B.