Silicates & Quartz
Cordierite
What it is
A magnesium-iron-aluminum silicate best known as iolite in its gem form. Vikings reportedly used thin slices of iolite as a polarizing filter to find the sun on cloudy days for navigation.
How it forms
Forms in metamorphic rocks that experienced high temperatures but moderate pressures — typically in contact zones near magma intrusions.
(Mg,Fe)₂Al₄Si₅O₁₈
