What is Cordierite (Iolite)?
Cordierite is an orthorhombic magnesium aluminium silicate; its transparent blue to violet gem variety is called iolite.
Mineralogy and structure
The reference formula is Mg2Al4Si5O18. Cordierite (Iolite) belongs to the silicates and has a orthorhombic crystal system. Cordierite is strongly pleochroic, so its colour can change from blue-violet to grey or yellowish when viewed in different directions. Iolite is the gem trade name, not a separate mineral species.
Colour and appearance
Typical colours include blue, violet-blue, grey, yellow-brown. Crystal habit, lustre, transparency and matrix association are often as diagnostic as colour.
Formation and localities
Cordierite (Iolite) forms in or is associated with regional and contact metamorphism of aluminous rocks, often with mica, garnet and sillimanite. Important documented localities include Sri Lanka, India, Madagascar, Brazil, Norway and Tanzania.
Collecting and care
Cordierite (Iolite) is best kept as a labelled collector or reference specimen. Protect fragile crystals from impact and prolonged humidity changes. Lead-, arsenic-, chromium- and cobalt-bearing minerals should not be cut, ground or used in jewellery.
