Cordierite
Image: Shannon Heinle (CC0)
Mineral NameCordierite
Chemical FormulaMg2Al4Si5O18
Mohs Hardness7
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavagedistinct on {010}, poor on {100} and {001}
Fractureconchoidal to uneven
Specific Gravity2.65
Colorsblue-violet, light blue, purplish
Transparencytransparent to translucent
Type LocalityOntario
Mineral GroupSilicates
Usesjewelry, collector
Rarityuncommon
Description
This sample of Cordierite displays an unusual blue-violet color, often compared to a light blue sapphire with a purplish tint. It is strongly pleochroic, meaning it shows different colors when viewed from different angles, and the sample is approximately 7 cm in size.
Geological Context
Cordierite typically forms in metamorphic rocks such as gneiss, schist, and hornfels, resulting from the regional or contact metamorphism of argillaceous (clay-rich) sediments. It can also occur in some igneous rocks.
