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Published online by Cambridge University Press: 14 March 2018
Now that the measurement of extinction-angles in cleavage-flakes is frequently used to assist in the identification of minerals, the question of the relation of angles so measured to the orientation of the optic axes is one that must frequently arise. I confine myself here to the case of monoclinic minerals, such as hornblende and augxte, in which the optic axes are parallel to the clinopinacoid and the cleavage is prismatic.
1 See (e.g.) Fletcher, Min. Mag. (1891), vol. IX., p. 341.