Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 13… (1911)
“ Crystals are monoclinic, and have a characteristic coffin-shaped habit. They have a perfect cleavage parallel to the plane of symmetry (M in the figure) , on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or white, sometimes brick-red, and varies from transparent to translucent. ”
