Chemical, mineragraphic and X-ray investigations are carried out on minerals belonging to the system Fe 2 O 3 -Fe 3 O 4 . The existence of minerals intermediate in composition between magnetite and maghemite is proved with certainty and a brief discussion is given on the oxidation of natural magnetite. Similar data are presented on titanomaghemites and an attempt is made to explain the great variations in their chemical composition. It is suggested that titanomaghemites are formed mainly by the oxidation of titanomagnetite Fe 3 O 4 -gamma -FeTiO 3 and that they represent a series varying in composition from gamma --Fe 2 O 3 .Fe 3 O 4 --- gamma -FeTiO 3 to gamma -Fe 2 O 3 -TiO 2 .

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