Allotropes of Iron: Types, Density, Uses & Facts

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Iron is used in a variety of materials from kitchen tools to buildings. In this lesson, we will learn about the different allotropes of iron and how they are each used.

Iron Allotropes

Iron is used to make steel, cast iron pots, tools, and many other things all around you. Iron can be used in so many different applications by using its different allotropes. There are 4 allotropes of iron:

  • Alpha
  • Gamma
  • Delta
  • Epsilon

Of these, alpha, gamma, and delta iron are all found at atmospheric pressure, while epsilon is only found at high pressures.

Alpha Iron

Alpha iron is a body-centered cubic allotrope that forms when molten iron is cooled to 912°C. It is only able to dissolve small concentrations of carbon. Since steel is an iron-carbon alloy, then steel that is made from alpha-iron is a low-carbon steel. This type of steel is ductile (the shape can be manipulated) and is used in wires. Alpha iron contains the highest volume and is the least dense of the three atmospheric allotropes.

If the iron is cooled to below 770°C then it becomes magnetized (so it is magnetic). The first magnetic metal used and discovered was iron, thus it is the metal that was used in compasses.

We mentioned that alpha iron is a body-centered cubic allotrope. What does this mean? This refers to the shape that the iron forms when atoms arrange. A body-centered cubic shape is one where we form a cube of iron atoms surrounding a single iron atom in the center of the cube.

Alpha and Delta iron allotropes have a body-centered cubic crystalline structure
Body-centered cubic crystalline structure

Alpha iron is also the most stable form of iron found at room temperature. This means that most iron that is mined or found naturally, would be in the alpha iron form.

Gamma Iron

Gamma iron is a face-centered cubic allotrope that forms when molten iron is cooled to 1394°C. This form has the lowest volume and is the densest.

Steel was probably made accidentally at first, with carbon coming from the coals in the fire used to melt the iron. Today we are able to control how much carbon is dissolved in the iron, changing the strength and ductility of steel. Gamma iron can dissolve the highest amount of carbon, thus forming the high-carbon steels and cast iron.

The high-carbon steels also commonly contain other compounds with the alloy such as manganese. These are used for their strength.

The face-centered cubic crystalline structure has a cube of iron atoms connected together and there are iron atoms on each face of the cube connected to each other.

Gamma iron allotropes have a face-centered cubic crystalline structure
Face-centered cubic crystalline structure

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