Iron oxide
[CAS 1332-37-2]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Oxides and peroxides >> Metal oxide
• Name: Iron oxide
• Synonyms: Iron oxide red; AM 125; AX 1000; Ancor FR; Ancor FY; Auvico AX 1000; BUS; EP-A 0014382; Ferroxon 422; Ferroxon 430; Ferroxon 510; Gastromark; JC-CPW; KN-O; Lautamasse; Luxmasse; MAP 514; MIO 2F; MIO 40GN; MIO-KS; MION 46L; Magnet Black S 0045; NAT; NYB 40; Prodorite Filler; R 2899HP; R 8098; SE-DBS; Siferrit; TAN 20A; TMB1120; Tarox LL 50; Tarox R 110; Tenshoin Bengara; Toda Color 100ED-PR101; Toda Color 160ED
• CAS Registry Number: 1332-37-2 (8075-66-9)
• EC Number: 215-570-8

Safety Data

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Specific target organ toxicity - repeated exposure	STOT RE	1	H372
Acute toxicity	Acute Tox.	4	H332
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319

chemBlink Chemical Story

Iron oxide, CAS 1332-37-2, is a general commercial designation associated with iron oxide materials rather than a name that by itself defines one unique crystalline phase. Iron and oxygen form several important solids, including hematite, Fe₂O₃; magnetite, Fe₃O₄; and hydrated or oxyhydroxide-containing materials closely associated with natural ochres. Differences in oxidation state, crystal structure, particle size, hydration, and impurities give iron oxide materials colors ranging from yellow and orange through red and brown to black. This chemical variety has made them some of the oldest and most enduring pigments used by humans.

Long before people understood oxidation states or crystal structures, they discovered colored earth. Natural ochre deposits contain iron-bearing minerals mixed with clay and other geological materials. Yellow ochres commonly owe much of their color to hydrated iron(III) oxide or iron oxyhydroxide phases such as goethite, while red ochres contain more hematite. The minerals could be collected, ground into powder, and mixed with water, animal fat, plant binders, or other materials to produce paints.

Archaeological evidence shows that humans were collecting and processing ochre far back into prehistory. Ochre occurs at Middle Stone Age sites in Africa tens of thousands of years before the famous painted caves of Europe, sometimes with evidence of grinding, scraping, or deliberate preparation. At Blombos Cave in South Africa, materials approximately 100,000 years old have been interpreted as an ochre-processing workshop in which ochre-rich compounds were prepared and stored in abalone shells. Such discoveries show that manipulation of iron-rich pigments was part of human material culture extraordinarily early.

Iron oxide pigments later became prominent in prehistoric cave art. The reds and browns seen in many painted caves could be made from naturally occurring iron-rich earths. Unlike many organic colors, iron oxides are mineral pigments and can be remarkably persistent. The color does not depend on a delicate organic molecule that readily decomposes; it comes from stable inorganic solids. Protected from severe weathering, these pigments can retain recognizable color for thousands of years.

One particularly interesting transformation connects yellow and red ochres. Heating iron oxyhydroxide minerals such as goethite removes structural water and favors formation of hematite. As this happens, a yellowish material can become distinctly redder. Prehistoric people therefore did not necessarily have to find every desired pigment color in nature. Heating an iron-rich earth could alter its mineral composition and color.

Whether all ancient examples of red ochre were produced deliberately by heating remains a subject for archaeological investigation, because naturally red hematite-rich deposits are also widespread. Nevertheless, experimental and mineralogical studies clearly establish the underlying chemistry: dehydration and structural transformation of iron oxyhydroxides can produce red hematite. The phenomenon is an early and intuitive example of materials processing—heat changes structure, and structure changes color.

Why do different iron oxides have different colors in the first place? Color in an inorganic solid arises from the way its electronic structure interacts with visible light. Iron can occur in different oxidation states and coordination environments, while atoms can be arranged in different crystal structures. These factors determine which wavelengths are absorbed and which reach the eye. Particle size and shape further influence scattering and apparent shade. A pigment therefore cannot always be described completely by elemental composition alone.

Magnetite provides an especially striking contrast. Fe₃O₄ contains both Fe(II) and Fe(III) and is typically black. It is also strongly magnetic, giving iron oxide chemistry a property that red and yellow pigments do not immediately suggest. Hematite, although also an iron oxide and historically important as a red pigment and iron ore, has a very different magnetic response and appearance.

Industrial pigment manufacture eventually transformed these naturally variable earth colors into controlled products. Synthetic iron oxide pigments can be prepared with specified phase composition, particle characteristics, purity, and shade. Red, yellow, brown, and black iron oxide pigments are used in coatings, concrete, construction materials, ceramics, plastics, and numerous other products. Their appeal includes chemical stability, lightfastness, relatively low cost, and resistance to weathering.

Iron oxides also illustrate why a familiar word such as "rust" can hide complicated chemistry. Rust on iron exposed to air and moisture is not usually a single pure compound. It can contain various iron oxides, oxyhydroxides, hydrated phases, and poorly crystalline materials whose proportions depend on oxygen, moisture, salts, temperature, and time. The orange-brown corrosion layer on a steel object is therefore chemically more complicated than simply "Fe₂O₃."

This complexity is particularly relevant to CAS 1332-37-2. The broad name "iron oxide" should not automatically be interpreted as specifying one pure mineral phase. Commercial materials described under broad iron oxide terminology may differ in composition and physical form according to grade and intended use. For applications in which crystal phase, oxidation state, particle size, or purity matters, the supplier's specification is therefore more informative than the generic name alone.

Few families of inorganic materials have accompanied humans for as long as iron oxides. They colored earth before humans existed, were collected and processed by prehistoric communities, became pigments in some of humanity's earliest surviving visual expressions, and today are manufactured with controlled particle and crystal properties for modern industry.

The continuity is remarkable. A prehistoric artist grinding red earth and a modern materials scientist engineering an iron oxide pigment are working with the same fundamental relationship: iron, oxygen, crystal structure, and particle form determine how a material interacts with light. Iron oxide is therefore more than a familiar inorganic material. It is one of chemistry's oldest links between geology, color, technology, and human culture.

References

1. Henshilwood, C. S. et al. (2011). "A 100,000-Year-Old Ochre-Processing Workshop at Blombos Cave, South Africa." Science, 334, 219-222.

2. Cornell, R. M.; Schwertmann, U. (2003). The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. Wiley-VCH.

3. Eastaugh, N.; Walsh, V.; Chaplin, T.; Siddall, R. (2008). Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments. Butterworth-Heinemann.

4. Published archaeological and mineralogical studies of ochre pigments, hematite, goethite, and thermal transformation of iron oxyhydroxides.