Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 21… (1911)
“ The crystals found in schistose rocks have the form of cubes, which are sometimes modified on the edges and corners by numerous small planes; on the other hand, the crystals occurring as an accessory constituent of eruptive rocks are octahedral in form and microscopic in size. Although geometrically cubic, the crystals are always doubly refracting, and they sometimes show evidence of complex mimetic twinning ”
