Cumengeite

specimen number:
9273154

location:
Amelia Mine, Boleo, Baja California Sur, Mexico

description:
A very good thumbnail specimen for the species, this with lustrous blue commensurate crystals in a cluster without matrix. In good condition.

The Amelia mine, located within the historic Boleo copper district near Santa Rosalia in Baja California Sur, Mexico, was a notable underground operation renowned for its complex geology and rare secondary copper minerals. Geologically, the deposit is hosted within late Miocene sedimentary layers of the Boleo Formation, where copper-rich hydrothermal solutions infiltrated volcanic tuffs and clay-rich sandstones, creating unique stratabound layers called mantos. Extensive reaction with ancient seawater and oxidizing meteoric fluids led to the precipitation of an exotic suite of rare, lead-copper chloride-hydroxide minerals within localized clay beds. Formal mining at the Amelia mine began in the late 1880s under the French-owned Compagnie du Boleo, which managed a vast network of tunnels to extract high-grade copper ore until economic exhaustion and high operational costs forced the cessation of primary commercial mining at the site in the late 1930s.

Cumengeite is an exceptional, rare lead-copper chloride-hydroxide mineral that was first discovered and described from the Boleo district, with the Amelia mine serving as its type locality. Geologically, it forms under highly specific, low-temperature alkaline conditions where copper-bearing and lead-bearing solutions react with chlorine in an oxidizing environment. The mineral belongs to the tetragonal crystal system and typically occurs as deep blue, lustrous, six-pointed star-shaped epitaxial growths perched symmetrically on the faces of cube-like pseudoboleite or boleite crystals. These distinct, intricate twinned groupings are highly sought after by mineral collectors, and because the original underground workings have been abandoned for nearly a century, specimens are only recovered through sporadic, hazardous hand-digging by independent collectors re-entering the old, unstable tunnels.

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