arsenian

from The Century Dictionary.

• noun One of a party in the Greek Church, in the thirteenth century, named from its leader Arsenius, patriarch of Constantinople, who excommunicated the emperor Michael Palæologus for putting out the eyes of John Lascaris, a minor and heir to the throne.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective mineralogy Describing minerals that contain pentavalent arsenic