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
Al6.74(Ti,Fe,Mg,Ca)0.14(Si2.92Al0.08)BO17.60(OH)0.40
with variation expected in all subscripts, except perhaps for B.

All of our specimens were blue, pale red, or red violet and contained less Fe and Ti than brown dumortierites reported in the literature. Optical properties are linearly related to minor-element composition, such that refractive indices and birefringence increase with increasing transition-metal concentrations, but decrease as the proportion of Fe to Ti increases. Optic axial angle varies in exactly the opposite way. Color and pleochroism (blue or pale red with Ec, and colorless with Ec) in these dumortierites are attributed to charge transfer among Fe2+, Fe3+, and Ti4+ replacing Al in the Al(1) face-sharing octahedral chains. It appears from the correlation between color and composition that the blue color can be attributed to Fe2+-Fe3+ charge transfer, and the pale red to Fe2+-Ti4+ charge transfer. The Fe:Ti ratio 1:4 separates pale-red dumortierites (poorer in Fe) from blue ones (richer in Fe).

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

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.