Anatase in Brazilian Quartz
Anatase crystals in quartz can be a fascinating sight. Their dipyramidal or acute pyramidal crystallographic habit and adamantine shine often create interference colors due to the inclusion’s high refractive index compared to the host crystal. One notable example was recently observed in a 27.21 ct Brazilian quartz cabochon (see above). The blue interference color visible from every angle was remarkable, since isolated anatase crystals typically have a black, brown, reddish brown, or brownish yellow color. Anatase (along with rutile, brookite, akaogiite, and riesite) is one of the five polymorphic forms of titanium dioxide (TiO2) found in nature and is almost always associated with rutile as an inclusion in quartz. However, rutile forms at high pressures and temperatures and is often found in igneous and metamorphic rocks, while anatase forms in hydrothermal phases at pressures and temperatures much lower than rutile (D.A.H. Hanaor and C.S. Sorrell, “Review of the anatase to rutile phase transformation,” Journal of Materials Science, Vol. 46, 2011, pp. 855–874). The disordered orientation of the acicular rutile crystals and the relative distance from the euhedral anatase crystals suggest the protogenetic formation of rutile and the formation of anatase in a subsequent hydrothermal phase.
