M. P. Atherton, B. P. Atkin, and M. H. Naggar. “Kyanite in the Hercynian Metamorphic Rocks of the Oporto-Viseu Belt, North Portugal”. Netherlands Journal of Geosciences, Jan. 1974, pp. 189-92, https://njgjournal.nl/index.php/njg/article/view/14450.