Natural Iron Oxide and Hematite Pigments

Natural Iron Oxides are earth pigments which range from rich red-brown to dark red-violet and have a very high iron oxide content making them both high tinting and opaque. Modern Iron Oxides were invented to replace the natural ones but the unique qualities of the natural iron oxides cannot be matched. Caput Mortuum and many Indian Reds are the names of colors assigned to natural iron oxides.

Hematite starts out as a heavy gray metallic ore. Upon inspection you might even consider it to be a piece of metal. After hundreds of years mother nature breaks it down into the pigment commonly known as Indian Red, Persian Red, or English Red. Hematite can be found through out the world with large deposits in Spain and the Southwest of the United States.

Caput Mortuum is Latin for "Dead Head" the color can range from a rich red-violet to a dark deep earthy purple. This is color is often underestimated and time after time will surprise many artists as to its beauty and usefulness. Many painters quickly fall in love with this marvelous pigment.

Class of chemical compounds composed of iron and oxygen

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Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.[1]

Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.

Stoichiometries

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Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3. Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe2+ and Fe3+.

Iron oxides feature as ferrous (Fe(II)) or ferric (Fe(III)) or both. They adopt octahedral or tetrahedral coordination geometry. Only a few oxides are significant at the earth's surface, particularly wüstite, magnetite, and hematite.

Thermal expansion

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Iron oxide CTE (× 10−6 °C−1) Fe2O3 14.9[6] Fe3O4 >9.2[6] FeO 12.1[6]

• goethite (α-FeOOH),
• akaganéite (β-FeOOH),
• lepidocrocite (γ-FeOOH),
• feroxyhyte (δ-FeOOH),
• ferrihydrite (Fe5HO8 · 4 H2O approx., or 5 Fe2O3 · 9 H2O, better recast as FeOOH · 0.4 H2O)
• high-pressure pyrite-structured FeOOH.[7] Once dehydration is triggered, this phase may form FeO2Hx (0 < x < 1).[8]
• green rust (Fe

  III
  x

  Fe

  II
  y

  OH3x + y − z (A−)z where A− is Cl− or 0.5

  SO

  2−

  4

  )

Reactions

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In blast furnaces and related factories, iron oxides are converted to the metal. Typical reducing agents are various forms of carbon. A representative reaction starts with ferric oxide:[9]

2 Fe2O3 + 3 C → 4 Fe + 3 CO2

In nature

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Iron is stored in many organisms in the form of ferritin, which is a ferrous oxide encased in a solubilizing protein sheath.[10]

Species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, use iron oxides as terminal electron acceptors.[11]

Uses

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Almost all iron ores are oxides, so in that sense these materials are important precursors to iron metal and its many alloys.

Iron oxides are important pigments, coming in a variety of colors (black, red, yellow). Among their many advantages, they are inexpensive, strongly colored, and nontoxic.[12]

Magnetite is a component of magnetic recording tapes.

See also

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References

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