Difference Between Red and Black Iron Oxide

The key difference between red and black iron oxide is that the red iron oxide occurs as a red-brown solid whereas the black iron oxide occurs as a solid black powder. Furthermore, the red iron oxide is ferromagnetic while the black iron oxide is ferrimagnetic.

Red and black iron oxides are oxides of the chemical element iron having different oxidation numbers in each. In other words, red iron oxide has an iron with the oxidation number +3 and black iron oxide has both +2 and +3 oxidation states. These are naturally occurring minerals and are very important components in the chemical industry.

CONTENTS

1. Overview and Key Difference
2. What is Red Iron Oxide
3. What is Black Iron Oxide
4. Side by Side Comparison – Red vs Black Iron Oxide in Tabular Form
5. Summary

What is Red Iron Oxide?

The red iron oxide is ferric oxide that has the chemical formula Fe₂O₃. Its chemical name is iron(III) oxide. Moreover, it is a major oxide of iron, and in mineralogy, we call this compound “hematite”. It is the main source of iron for the steel industry and is ferromagnetic. The molar mass is 159.69 g/mol while the melting point of this compound is around 1,539–1,565 °C and at higher temperatures it easily decomposes. Therefore, this compound is insoluble in water.

Figure 01: Red Iron Oxide Powder

Furthermore, there are different structures of this compound; we call them “polymorphs”. Ex: alpha phase, gamma phase, etc. In every structure, one iron cation binds with six oxygen ligands (around the iron cation). Moreover, there are some hydrated forms of this compound as well. More importantly, red iron oxide occurs as a red-brown solid. Hence, it is a good indicator for us to recognize this compound from other iron oxides.

What is Black Iron Oxide?

Black iron oxide is the inorganic compound that has the chemical formula Fe₃O₄. Its chemical name is iron(II) iron(III) oxide. This has both stable oxidation states of iron (+2 and +3). In mineralogy, we call this compound “magnetite”. Unlike hematite, it contains both Fe²⁺ and Fe³⁺ ions. More importantly, this compound occurs as a black powder.

Figure 02: Naturally Occurring Black Iron Oxide

Moreover, it exhibits ferrimagnetism. The molar mass of the compound is 231.53 g/mol. The melting point is 1,597 °C, and the boiling point is 2,623 °C. In addition, the chemical structure of this compound is cubic inverse spinel group structure; it has cubic, closely packed oxide ions, with all of the Fe²⁺ ions occupy half of the octahedral sites and Fe³⁺ are split evenly across the remaining octahedral and tetrahedral sites.

What is the Difference Between Red and Black Iron Oxide?

Red iron oxide is ferric oxide that has the chemical formula Fe₂O₃ while black iron oxide is the inorganic compound that has the chemical formula Fe₃O₄. The chemical name of red iron oxide is iron(III) oxide while the chemical name of black iron oxide is iron(II) iron(III) oxide. Moreover, Red iron oxide is ferromagnetic whereas black iron oxide is ferrimagnetic. The below infographic gives more details on the difference between red and black iron oxide.

Summary – Red vs Black Iron Oxide

The key difference between red and black iron oxide is that the red iron oxide occurs as a red-brown solid whereas the black iron oxide occurs as a solid black powder. It is a main fact to distinguish two samples of hematite and magnetite. Hematite is red iron oxide while magnetite is black iron oxide.

Reference:

1. “Iron(III) Oxide.” Wikipedia, Wikimedia Foundation, 15 July 2018. Available here
2. User, Super. “Magnetite / Black Iron Oxide (Fe304) Powder.” Cerium Oxide Powder (CeO2). Available here

Image Courtesy:

1.’Iron(III)-oxide-sample’By Benjah-bmm27 – Own work, (Public Domain) via Commons Wikimedia 
2.’Magnetite’By Archaeodontosaurus – Own work, (CC BY-SA 3.0) via Commons Wikimedia 

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