Siderite on Marcasite after Pyrrhotite with Sphalerite

specimen number:
2312061

location:
Herja mine, Maramures Co., Romania

description:
Neat combination specimen showing large flattened platy crystals of what were once pyrrhotite, these now replaced by crystallized marcasite and coated with crystallized siderite. Calcite is scattered throughout. This is all set up on a crystallized sphalerite matrix. In great condition.

The Herja Mine, located in Maramures County, Romania, within the Baia Mare metallogenic district, is a prominent polymetallic hydrothermal vein deposit. Its geology is closely tied to Neogene post-collisional volcanism and subvolcanic intrusions, which created an extensive network of over 260 complex veins hosted within sedimentary and volcanic formations. Mining activity in the surrounding area dates back to the Middle Ages, with industrial extraction focusing heavily on lead, zinc, copper, and silver ores. The mine gained international recognition among mineral collectors for its diverse assemblage of hydrothermal minerals, including distinct stibnite clusters, sulfosalts, and rare bicolored calcite spheres. Commercial operations at the site permanently ceased in 2005, closing the underground infrastructure.

Pseudomorphs occur when one mineral chemically replaces another while preserving the original crystal shape and external morphology. A fascinating example of this process is marcasite after pyrrhotite. Pyrrhotite, an iron sulfide with a hexagonal crystal system, typically forms under high-temperature magmatic or hydrothermal conditions. As geochemical environments shift and low-temperature, acidic fluids circulate through the deposit, the original pyrrhotite crystals become unstable and undergo dissolution-reprecipitation. Through this replacement mechanism, iron disulfide precipitates as marcasite, faithfully replicating the distinct tabular or prismatic forms of the precursor pyrrhotite even though the internal crystal structure changes from hexagonal to orthorhombic.

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