Cordierite is a hard mineral composed of magnesium, iron, and aluminum that is often used in ceramics due to its resistance to thermal expansion. It is found in metamorphic rock formations and is used in insulators, catalytic converters, and refractory linings. Gem-quality cordierite, known as iolite, is a popular low-cost alternative to sapphire due to its attractive color and pleochroism.
Cordierite is a natural mineral belonging to the cyclosilicate or ring silicate family of minerals. One of three six-membered rings, cordierite has a base of magnesium, iron, and aluminum with a magnesium being the strongest component. The mineral is often included in ceramic compounds, giving them excellent unidirectional thermal expansion characteristics. These ceramics are of particular value in the production of insulators, catalytic converters and refractory linings. Cordierite also occurs in transparent purple varieties, commonly known as iolite, which are prized for their color, density, and pleochroism.
Silicate minerals represent the largest single group of rock building minerals and make up over 90% of the earth’s crust. The group of ring silicates to which cordierite belongs is based on silica and oxygen with secondary ring elements such as iron, magnesium, copper and aluminum. Cordierite is one of three six-membered rings and is composed of the elements magnesium, iron, and aluminum. The mineral is quite hard with values from 7 to 7.5 and shows good unilateral cleavage and resistance to thermal expansion. The mineral is found in contact with or in the region of metamorphic rock formations; India, Burma, Madagascar, Connecticut in the USA and Canada have good deposits.
The cleavage characteristics and thermal conductivity of cordierite make the mineral a popular additive in specialty ceramics. These ceramics have excellent resistance to expansion when exposed to extreme heat levels and are used extensively as insulators on high temperature igniters and electric arc electrodes. They are also used to make heater cores, solder tape, thermocouples, and catalytic converter parts. Liners and molds used in metallic refractory plants are often made with these ceramics due to their good thermal shock resistance qualities. Ceramics can also be produced as automotive parts to increase their dielectric and structural strength.
There are a variety of cordierite grades ranging from industrial grade translucent varieties to highly attractive and sought after transparent gem grades. Known as iolite, gem-quality cordierite is a hard, dense stone with a range of colors from blue-violet to rusty to colorless. Perhaps the most notable feature of iolite is its pleochroism, i.e. the tendency to change color when viewed from different angles. While many pleochroic minerals show two colors when rotated, iolite often shows three. Because of their attractive qualities and similarity in color to some sapphire variants, these gemstones are popular low-cost alternatives to those more expensive gemstones.
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