Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 27… (1911)
“ The crystals are usually six-sided prisms terminated by the basal planes, but are sometimes modified by numerous pyramidal planes which exhibit parallel hemihedrism. Rounded crystals and groups also occur. The colour is usually light brown or yellow, but crystals from Arizona are bright red. Owing to isomorphous replacement of the vanadium by phosphorus and arsenic, the specific gravity varies from 6.6 to 7.2; a variety containing much arsenic is called endlichite. The hardness is 3. The mineral is one of secondary formation in veins of lead ore. ”
