Leightonite is a hydrous potassium calcium copper sulfate mineral, K2Ca2Cu(SO4)4·2H2O. It is a secondary mineral that forms in the near-surface oxidized zone of copper-sulfide ore deposits in arid climates. Leightonite has a nonmetallic, somewhat glassy luster, pale blue to greenish blue color, white streak, is moderately soft (H=3), is transparent to translucent, and has no cleavage. Crystals of leightonite vary from slender to nonslender prisms, some with curved surfaces resulting in somewhat hourglass-shaped crystals.
Leightonite from the Chuquicamata Mine, Calama, El Loa Province, Antofagasta region, Chile. (CSM # 89.672, Colorado School of Mines Geology Museum, Golden, Colorado, USA)