2V90 Biaxial Ellipsoid

sketchfab

In cases where minerals display '2V90', the two circular sections intersect at right angles, resulting in optic axes that are precisely 90 degrees apart - this defines the '2V angle'. The shape of the ellipsoid is neither prolate nor oblate, but rather falls squarely between these two extremes. For further clarification on deriving the biaxial indicatrix, visit this link. Compare and contrast this with optically positive biaxial indicatrixes and their optically negative counterparts.