Eddavidite, a New Mineral Species, and the Murdochite (Cu12Pb2O15Cl2)-Eddavidite (Cu12Pb2O15Br2) Series

Abstract

Eddavidite is a new mineral species (IMA2018-010) with ideal formula Cu12Pb2O15Br2. It has cubic Fm3m symmetry; a = 9.2407(9) Å; V = 789.1(2) Å3; Z = 2. Eddavidite is the bromide analog of murdochite, with which it forms a solid solution series. The type locality is the Southwest mine, Bisbee, Arizona, U.S.A. Eddavidite also occurs in the Ojuela mine, Mapimí, Durango, Mexico. Eddavidite forms zones within mixed murdochite-eddavidite crystals. Spot analyses of Bisbee samples show up to 67% eddavidite component while Ojuela samples show up to 62%. Eddavidite-murdochite crystals show forms {100} and {111}; the habit grades from simple cubic through cuboctahedral to unmodified octahedral. Eddavidite is black and opaque with submetallic luster, and visually indistinguishable from intergrown murdochite. Its Mohs’ hardness is 4. Eddavidite exhibits good cleavage on {111}. The empirical formula, normalized to 12 Cu apfu is Cu12(Pb1.92Fe0.06Si0.06) (O15.08F0.02) (Br0.99Cl0.89•0.12). dcalc. = 6.33 g/cm3. dmeas. = 6.45 g/cm3. The crystal structure consists of corner sharing square planar CuO4 units, arranged in Cu12O24 metal oxide clusters, which encapsulate Br atoms. PbO8 cubes share edges with Cu12O24 clusters in a continuous framework. Eddavidite is one of only 10 mineral species with essential Br. Eddavidite crystallizes from bromine enriched fluids leftover from desiccation of paleo-seawater at its two known localities.