a greenish silicate of iron and potassium, a kind of mica found in greensand
See glauconite in American Heritage Dictionary 4
(glôˈkə-nītˌ)
noun
A greenish mineral of the mica group, a hydrous silicate of potassium, iron, aluminum, or magnesium, (K,Na)(Al,Fe,Mg)2(Al,Si)4O10(OH)2, found in greensand and used as a fertilizer and water softener.