aphthitalite

from The Century Dictionary.

• noun A native potassium sulphate found on Mount Vesuvius in delicate crystallizations. Also called aphthalose and Vesuvius-salt.

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

• noun mineralogy A saline evaporite consisting of mixed potassium and sodium sulphates; also found in the fumaroles of volcanos.