olivenite

Olivenite is a copper arsenate mineral, formula Cu₂AsO₄OH. It crystallizes in the monoclinic system (pseudo-orthorhombic), and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces. More commonly, it occurs as globular aggregates of acicular crystals, these fibrous forms often having a velvety lustre; sometimes it is lamellar in structure, or soft and earthy.