Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 8… (1911)
“ The specific gravity and optical constants also vary with the chemical composition; the sp. gr. of diopside is 3·2, increasing to 3·6 in hedenbergite, and the angle of optical extinction in the plane of symmetry varies between 38° and 47° in the two extremes of the series. Crystals are usually prismatic in habit with a rectangular cross-section as shown in the figure: the angle between the prism faces m, parallel to which there are perfect cleavages, is 92° 50′. ”
