Minyulite is a rare phosphate mineral with a chemical formula of KAl2(OH,F)(PO4)2·4(H2O).
It occurs as groups of radiating fine fibrous crystals within rock cracks of phosphatic ironstone. Minyulite belongs to the orthorhombic crystal system. This indicates that it has three axes of unequal length yet all are perpendicular to each other. Its cell constants are a=9.35, b=9.74 c=5.52.
As for its optical properties, Minyulite is an anisotropic mineral which means the velocity of light differs when traveling through it depending on the cut of its cross-section which gives it more than one refractive index. The mineral is optically biaxial. Its birefringence value is 0.007. It has three refractive indices which are nα=1.531 nβ=1.534 nγ=1.538. Refractive indices are a ratio of the speed of light in a median with respect to the speed of light passing through the mineral.