Proyer (2017), the Discussion in this issue of American Mineralogist, disagrees with our recent findings indicating that, in some gem corundum, oriented rutile needles grew by epitactic coprecipitation rather than the usual assumption of exsolution. Proyer's concerns are, in part, related to a misunderstanding of our model and, in places, his arguments are based upon claims that are not supported by the cited literature. We welcome the opportunity to clarify the matter. However, before starting, it was pointed out to us that the correct term is “epitactic” and not “epitaxial” (Bailey et al. 1978). The term “epitactic”...

You do not currently have access to this article.