Polybasite after Acanthite (G-039)

specimen number:
9273148

location:
Fresnillo, Zacatecas, Mexico

description:

A shapely cluster of what was once acanthite crystals, this now replaced by metallic crystallized polybasite. There are no points of attachment, this is crystallized all around and in excellent condition.. Choice pseudomorph specimen for this famous locality.

The Fresnillo mining district, located in the central portion of the State of Zacatecas, Mexico, is an immensely productive, world-class underground operation primarily extracted for silver alongside significant lead, zinc, and gold. Geologically, the deposit is a major epithermal vein and stockwork system hosted within a complex sequence of Cretaceous sedimentary and volcanic rocks, including the Proano Group limestones and greywackes, which were heavily fractured and intruded by Tertiary felsic dikes. Hydrothermal fluids migrating through these fault networks deposited rich bands of base and precious metal sulfides, notably argentite, acanthite, galena, and sphalerite, making it one of the largest continuous silver accumulations on Earth. Discovered by Spanish explorers in 1554, small-scale silver extraction began almost immediately, but modern corporate industrialization took off in 1831 with the founding of the Zacateco-Mexicana company. The district experienced various operational phases, including a major revival by the Mexican Corporation in 1919 and subsequent long-term development by the Fresnillo Company, which transitioned the site into a massive, highly mechanized underground complex that continues continuous commercial production through recent years under Fresnillo plc.

Pseudomorphs represent a fascinating mineralogical phenomenon where a mineral chemically replaces or alters an existing crystal while perfectly preserving the external shape and habit of the original species. In the specific case of polybasite after acanthite, the process begins with a primary crystal of acanthite, a silver sulfide that typically forms robust cubic, octahedral, or arborescent crystal habits. Over time, changes in the temperature, pressure, or chemical composition of the surrounding hydrothermal fluids introduce copper and antimony into the system. These elements react with the existing silver sulfide, gradually substituting into the crystal lattice to form polybasite, a complex silver copper antimony sulfosalt, without disturbing the original outer crystal boundaries. The resulting specimen displays the classic, blocky isometric form of an acanthite crystal, but it is composed entirely of the platy, hexagonal-structured polybasite.

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