Ferrimolybdite is a scarce, hydrous iron molybdate mineral, Fe2(MoO4)3·7-8H2O. It has an earthy to silky, nonmetallic luster, bright yellow color, pale yellow streak, is soft (H = 1 to 2 to 3), and has cleavage. Ferrimolybdite tends to form small, fibrous, radiating masses and powdery coatings. This mineral forms in the oxidized zone of molybdenum-bearing deposits.
Ferrimolybdite (yellow) on molybdenitic garnetite (field of view ~1.2 cm across) from near Brownes Lake, northeastern Beaverhead County, southwestern Montana, USA.