Wüstite General Category Oxide mineral Formula (repeating unit) FeO Strunz classification 4.AB.25 Crystal system Cubic Crystal class Hexoctahedral (m3m) H–M symbol: (4/m 3 2/m) Space group Fm3m (no. 225) Identification Color Greyish white to yellow or brown; colorless in thin section Crystal habit Pyramidic, prismatic Cleavage {001} perfect Fracture Subconchoidal to rough Mohs scale hardness 5–5.5 Specific gravity 5.88 Density 5.7 g/cm3 Refractive index 1.735–2.32 in synthetic crystals Pleochroism None Solubility Soluble in dilute HCl Other characteristics Forms solid solution with periclase