Cumengeite

Amelia Mine, Santa Rosalía (El Boleo), Boleo District, Mun. de Mulegé, Baja California Sur (BC Sur), Mexico © Vincent Bourgoin

Chemical Formula: Pb21Cu20Cl42(OH)40 · 6H2O
Locality: Boleo, near Santa Rosalia, Baja California Sur.
Name Origin: Named for Edouard Cumenge (1828-1902), French mining engineer for the Boleo mines.
Cumengite is a rare mineral It shares a close relationship with another rare halide, boleite. Boleite and cumengite both come from the same type locality at Boleo, Baja California, Mexico; both resulted from the oxidation of igneous copper ore bodies; both have similar chemistries, although cumengite lacks silver; both have an attractive indigo blue color and both have interesting crystal forms. But all that is not the reason for the close relationship. Cumengite and boleite have about as close a relationship as two minerals can have since cumengite actually grows on the cube faces of boleite crystals.

Physical Properties of Cumengite

Cleavage: {101} Distinct, {110} Poor
Color: Indigo blue.
Density: 4.67
Diaphaneity: Transparent
Fracture: Brittle – Conchoidal – Very brittle fracture producing small, conchoidal fragments.
Hardness: 2.5 – Finger Nail
Luster: Adamantine
Streak: blue

Photo

Locality: Herzog Julius smelter slag locality, Astfeld, Goslar, Harz, Lower Saxony, Germany Picture width is 1,4mm Copyright © Elmar Lackner
Locality: Herzog Julius smelter slag locality, Astfeld, Goslar, Harz, Lower Saxony, Germany Picture width 1 mm Copyright © Stephan Wolfsried
Locality: Herzog Julius smelter slag locality, Astfeld, Goslar, Harz, Lower Saxony, Germany FOV 1.5 mm Copyright © Vincent Bourgoin
Locality: Herzog Julius smelter slag locality, Astfeld, Goslar, Harz, Lower Saxony, Germany FOV: 0.3 mm Copyright © Robert O. Meyer
Locality: Amelia Mine, Santa Rosalía (El Boleo), Boleo District, Mun. de Mulegé, Baja California Sur (BC Sur), Mexico Largest Crystal Size: 5.5 mm Copyright © John Rakovan
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