Smithsonite after Calcite

specimen number:
2282122

location:
San Antonio Mine, Santa Eulalia, Chihuahua, Mexico

description:
A grouping of what were once scalenohedral crystals of calcite, these now replaced by smithsonite. In good condition.

The San Antonio mine, situated within the East Camp of the Santa Eulalia mining district in Chihuahua, Mexico, is a world-class polymetallic carbonate replacement deposit. Geologically, the ore bodies consist of massive hydrothermal replacement chimneys and mantos hosted within heavily folded Cretaceous limestones, particularly the Aurora Formation. Mineralization was driven by deep-seated Tertiary felsic intrusions that released high-temperature, tin-bearing and lead-zinc-silver-rich hydrothermal fluids, which interacted with the host carbonates to deposit immense sulfide zones that later underwent extensive oxidation. Discovered in the early eighteenth century, the mine saw centuries of intermittent, small-scale extraction before modern industrial development accelerated in the early twentieth century under major corporate operators, including the American Smelting and Refining Company (ASARCO). Large-scale underground operations continued systematically until the late twentieth century, and while primary commercial ore extraction has declined, the mine remains an iconic locality for mineral collectors due to its spectacular, highly prized crystal pockets.

A pseudomorph occurs when a mineral chemically replaces or encrusts a pre-existing crystal structure, completely altering the original mineral composition while perfectly preserving the external crystal habit and geometry of the initial species. In the case of smithsonite after calcite, the process represents a classic example of substitution and alteration within the deeply weathered, oxidized zones of zinc-bearing carbonate deposits. Zinc-rich meteoric waters slowly infiltrate the host rock, dissolving the calcium carbonate of the original calcite crystals and simultaneously precipitating zinc carbonate (smithsonite) in its place, atom-by-atom. The resulting specimen displays the distinct rhombohedral, scalenohedral, or dogtooth crystal form characteristic of calcite, yet the actual mineral substance has been entirely transformed into dense, lustrous, or botryoidal smithsonite, offering a fascinating physical record of localized geochemical shifts over geological time.

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