Spurrite

Spurrite is a white, yellow or light blue mineral with monoclinic crystals. Its chemical formula is Ca₅(SiO₄)₂CO₃.

Spurrite is generally formed in contact metamorphism zones as mafic magmas are intruded into carbonate rocks. Spurrite's space group is P 2/a. It is biaxial with a birefringence of 0.0390–0.0400, giving second order red interference colors when viewed under crossed polarizers in a petrographic microscope.