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What is α-Fe2O3, γ-Fe2O3?

wallpapers Industry 2020-09-30

α and γ are two crystal structures, and at a specific temperature, α-Fe2O3 can be transformed into γ-Fe2O3.

In nature, there are known two kinds of homogeneous and polymorphic variants of Fe2O3, namely α-Fe2O3 and γ-Fe2O3. The former is stable under natural conditions and is called hematite; the latter is not as durable as α-Fe2O3 under natural conditions and is in a metastable state and is called maghemite.

Hematite: Fe 69.94%, O 30.06%, often containing Ti, Al, Mn, Fe2+, Ca, Mg, and a small amount of Ga and Co. Trigonal crystal system, intact crystals are rare. The crystalline hematite is steel grey and cryptocrystalline; the earthy hematite is red. The streaks are cherry red or fresh pork liver colour. Metal to semi-metallic lustre. Sometimes the gloss is dim—hardness 5~6. The proportion is 5~5.3.

The aggregates of hematite have various forms, forming some mineral subspecies, namely:

(1) Spiegelite is an aggregate of rosette or flake hematite with a metallic lustre.

(2) Mica hematite, crystalline fine scaly hematite with a metallic sheen.

(3) Oolitic or kidney-shaped hematite Hematite in the form of oolitic or kidney-shaped.

Hematite is one of the iron minerals widely distributed in nature. It can be formed in various geological processes but is mainly hydrothermal, sedimentary and regional metamorphism. In the oxidation zone, hematite can be created by dehydration of limonite, lepidocrocite, and goethite. But it can also become goethite and water hematite. Under reducing conditions, hematite can be transformed into magnetite, called pseudomagnetite.

3. Maghemite

γ-Fe2O3, it's chemical composition often contains mixed substances such as Mg, Ti and Mn. Equiaxed crystal system, pentagonal tritetrahedral crystals, mostly granular aggregates, dense masses, often with the illusion of magnetite. The colour and streaks are brown, hardness 5, specific gravity 4.88, and strong magnetism.

Maghemite is mainly formed by secondary changes of magnetite under oxidizing conditions. Fe2+ ​​in magnetite is completely replaced by Fe3+ (3Fe2+→2Fe3+), so octahedral positions occupied by 1/3Fe2+ create vacancies. Besides, maghemite can be formed by the loss of water from lepidocrocite, or by organic action of iron oxides.


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Tag: Fe2O3