Abstract: Composition comprising a highly ferromagnetic CrO.sub.2 having deposited on the surface thereof an insoluble or sparingly soluble compound, thereby rendering the composition essentially non-reactive to water or oxidizable organic substances. Compound comprises either an oxide or hydroxide of Si and/or Ti.

Abstract: Ferromagnetic chromium dioxide is prepared by thermally decomposing, at a temperature of 250.degree.-500.degree.C, and a pressure of at least 80 atmospheres, a mixture of hydrated trivalent chromium chromate of the formula Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O, wherein n is 7-12, and antimony or a compound thereof, the amount of antimony in the mixture being between 0.2 and 1.8% by weight based on the weight of anhydrous Cr.sub.2 (CrO.sub.4).sub.3. In another embodiment, other known modifiers for chromium dioxide, such as lanthanum, lithium, strontium, iron or lead, or a compound thereof may be added to the mixture of chromium chromate and antimony. The amount of said other modifiers is between 0.1 and 2% by weight of the element, based on the weight of anhydrous Cr.sub. 2 (CrO.sub.4).sub.3.