Mimetite: Gemstone Information
Mimetite is Pb5(AsO4)3Cl and forms transparent and very attractive prismatic crystals of the hexagonal system in yellow to orange-yellow to brown, sometimes colourless specimens with a characteristic resinous to subadamantine lustre. The hardness is 3.5–4 and the SG 7.24–7.26. The RI for the ordinary and extraordinary rays is 2.147 and 2.128, uniaxial (commonly anomalously biaxial) negative. The birefringence is 0.019.Mimetite occurs as a secondary mineral in the oxidized zone of arsenicbearing lead deposits. Especially fine examples are found at Tsumeb,Namibia (one specimen reported to show an orange-red fluorescence under LWUV), and globular masses at Mapimi, Durango, Mexico.