Megaphenocrysts of ferrifayalite occur in biotite monzonitic granite in Jixi county, Anhui province. The rock shows finegrained granitic texture and consists of perthite, oligoclase. quartz and. biotite. The megaphenocrysts can reach 5cm in grain size, and contain small euhedral magnetite crystals in the form of dissemination or discontinuous veins. Under hypergene conditions, ferrifayalite has been partly converted into limonite through oxidation, and the megaphenocryst is quite similar to wolframite in shape, color and luster. The megaphenocrysts were formed in upper mantle under the condition of high temperature and pressure. The high-pressure metacrysts of fayalite were first separated out from the granitic magma through crystallization; during the upward migration of magma along the deep fault, these metacrysts were oxidized into ferrifayalite with the increasing oxygen partial pressure.