magnesioferrite

from The Century Dictionary.

• noun An oxid of magnesium and iron, belonging to the spinel group, which has been observed at Vesuvius. Also magnoferrite.

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

• noun mineralogy A magnesium iron oxide mineral, a member of the magnetite series of spinels, which forms black metallic octahedral crystals.