The temperature of transition of synthetic γ-Fe2O3 (maghemite) to α-Fe2O3 (hematite) is lowered by increasing pressure. The transition line is represented by the equation, p=150-0.3T, where p is pressure in bar and T is temperature in °C. The transition of Ti-bearing γ-Fe2O3 (titanomaghemite) needs higher pressure and temperature than that of pure γ-Fe2O3. From the experimental results, it is suggested that maghemite is formed only near the surface of the earth, probably at depths shallower than about 500m of the crust.
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Ikuo Kushiro (1960) studied this question.