nontronite

from The Century Dictionary.

• noun Hydrated silicate of iron; a variety of chloropal occurring in small yellow nodules embedded in an ore of manganese. It is found in France in the arrondissement of Nontron, department of Dordogne.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Min.) A greenish yellow or green mineral, consisting chiefly of the hydrous silicate of iron.

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

• noun mineralogy An iron-rich clay mineral of the montmorillonite group