Cordierite

A mineral of the silicate family

Cordierite crystal from Madagascar

Cordierite is a silicate mineral that belongs to the cyclosilicate family. It is known for its distinct pleochroism, which means it can display different colors when viewed from different angles. Cordierite is often used as a gemstone and is sometimes referred to as "iolite" in the gem trade.

Properties

Cordierite is a magnesium iron aluminium cyclosilicate with the chemical formula (Mg,Fe)₂Al₄Si₅O₁₈. It typically forms in metamorphic rocks and is known for its ability to withstand high temperatures, making it useful in various industrial applications.

Crystal structure of cordierite

Physical Characteristics

Cordierite is typically blue to violet in color, although it can also appear in shades of gray, yellow, or green. It has a hardness of 7 to 7.5, making it relatively hard and durable. The mineral has a vitreous to pearly luster and a conchoidal fracture.

Optical Properties

One of the most notable features of cordierite is its strong pleochroism. This optical phenomenon allows the mineral to show different colors when viewed from different angles, typically blue, violet, and yellow-brown. This property is particularly pronounced in gem-quality specimens.

Occurrence

Cordierite is commonly found in metamorphic rocks such as gneiss and schist, where it forms under high-temperature conditions. It can also occur in igneous rocks like granite and pegmatite. Major sources of cordierite include Sri Lanka, India, Madagascar, and the United States.

Cordierite sample showing typical coloration

Uses

Cordierite is primarily used as a gemstone, where it is often marketed under the name "iolite." Its unique pleochroism makes it a popular choice for jewelry. In addition to its use in jewelry, cordierite is also used in industrial applications, particularly in the manufacture of heat-resistant ceramics and catalytic converters.

Gemology

In the gem trade, cordierite is valued for its color and pleochroism. It is often cut into faceted stones to enhance its optical properties. The gemstone is sometimes used as a substitute for sapphire due to its similar appearance.

Iolite gemstone, a variety of cordierite

Pleochroism

Cordierite's pleochroism is a result of its crystal structure, which causes light to be absorbed differently along different crystallographic axes. This property is not only a key identifier for the mineral but also a desirable trait in gemstones.

Cordierite showing pleochroism

Related pages

• Silicate minerals
• Metamorphic rock
• Gemstone
• Pleochroism