Obtuse bisectrix

Obtuse bisectrix (BXo) n. The angle supplementary to the acute optic axial angle of a biaxial crystal is obtuse and is bisected by the obtuse bisectrix, which may be either a or y. Weast RC (ed) (1978) CRC handbook of chemistry and physics, 59th edn. CRC Press, Boca Raton, FL. 

From the crystallographic point of view selenium benzoylacetone crystallises in elongated, six-sided plates, with a dome termination of 64°. The extinction is straight and a positive bisectrix emerges normal to the plate, whether acute or obtuse has not been determined with certainty. In any case, the optic axial plane contains the morphological direction of elongation. 

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