What is Cordierite?

Paul Scott

Cordierite is a naturally occurring mineral belonging to the cyclosilicate or ring silicate family of minerals. One of the three six-member rings, cordierite has a magnesium, iron, and aluminum base with a magnesium being the strongest component. The mineral is often included in ceramic compounds, thereby lending them excellent unidirectional thermal expansion characteristics. These ceramics are of particular value in the manufacture of insulators, catalytic converters, and refractory linings. Cordierite is also encountered in transparent, violet hued gem grade varieties commonly known as iolite that are prized for their color, density, and pleochroism.

Scientist with beakers
Scientist with beakers

Silicate minerals represent the largest single group of rock building minerals and make up over 90% of the Earth's crust. The ring silicates group to which cordierite belongs are silica- and oxygen-based with secondary ring member elements such as iron, magnesium, copper, and aluminum. Cordierite is one of the three six-member rings and is made up of magnesium, iron, and aluminum elements. The mineral is fairly hard with values of 7 to 7.5 and exhibits good unilateral cleavage and thermal expansion resistance. The mineral is found in contact with or in the region of metamorphic rock formations; India, Burma, Madagascar, Connecticut in the U.S., and Canada possess good deposits.

The cleavage and thermal conductivity characteristics of cordierite make the mineral a popular additive in specialized ceramics. These ceramics have excellent resistance to expansion when exposed to extreme heat levels and are extensively used as insulators on high temperature ignitors and electric arc electrodes. They are also used to make heater cores, welding tapes, thermocouples, and catalytic converter parts. Linings and shapes used in metal refractory installations are frequently made from these ceramics due to their good thermal shock resistance qualities. The ceramics can also be produced as self-glazing parts to increase their dielectric and structural strength.

There are a variety of grades of cordierite which range from translucent industrial grade varieties to highly attractive and sought after transparent gem grades. Known as iolite, gem grade cordierite is a hard and dense stone with a color range which extends from blue-violet through russet to colorless. The most noteworthy characteristic of iolite is perhaps its pleochroism, i.e., tendency to change color when viewed from different angles. While many pleochroic minerals show two colors when rotated, iolite frequently displays three. Due to their attractive qualities and similarity in color to some sapphire variants, these gems make popular low-cost alternatives to those more expensive precious stones.

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