yanite (Al₂SiO₅) is a common accessory mineral in metamorphic quartzites exposed in the central Virginia Piedmont. At Willis Mountain in Buckingham County kyanite is pale blue-gray to white, whereas kyanite at Baker Mountain in Prince Edward County has a deep blue-gray to blue-green color.

Brent Owens and Susan Dickerson from the College of William & Mary report that these color differences are a function of contrasting element compositions in kyanite from the two localities. Electron microprobe analyses of bluish kyanite from Baker Mountain reveal up to 0.4% of both chromium oxide (Cr₂O₃) and iron oxide (FeO), while Willis Mountain kyanite is almost pure Al₂SiO₅.

Differences in trace elements are commonly responsible for color differences in the same mineral species (for instance: blue vs. gray quartz, pink vs. greenish microcline). Additional elements in the crystal lattice can significantly affect both the adsorption and reflection of visible light. Owens and Dickerson suggest that the different levels of chromium, in particular, at Baker and Willis Mountains require different igneous protoliths (parent rocks) for these deposits prior to metamorphism.

Left: Blue-green kyanite from Baker Mountain. Right: Gray-white kyanite from Willis Mountain.

Owens, B. E., and Dickerson, S. E., 2001, Kyanite color as a clue to contrasting protolith compositions for kyanite quartzites in the Piedmont Province of Virginia, Southeastern Geology, v. 40, in press. Owens, B. E., and Dickerson, S. E., 2001, Kyanite color as a clue to contrasting protolith compositions for kyanite quartzites in the Piedmont Province of Virginia, Geological Society of America Abstracts with Programs, vol., 33, no. 2, pg. 70. (11/4/98)

APPALACHIAN PLATEAU | VALLEY & RIDGE | BLUE RIDGE | PIEDMONT | COASTAL PLAIN