nontronite
/ˈnɒntrənʌɪt/Mineralogy
noun
A pale yellow or greenish crystalline clay mineral, Na0.33Fe2 3+(Si,Al)4O10(OH)2, belonging to the smectite or montmorillonite group and sometimes resembling opal.
Origin
Mid 19th century; earliest use found in Charles Shepard (1804–1886). From French nontronite from Nontron, the name of the town in Dordogne, France, where the mineral was first found + -ite.