Hematite after Magnetite

specimen number:
2072251

location:
Payun Volcano, Mendoza, Argentina

description:
A grouping of sharp magnetite crystals, these now replaced by bright lustrous metallic hematite. In great condition all around with no points of attachment.

The Payun Matru volcanic field, located in the Malargue Department of Mendoza Province, Argentina, is a premier world locality for the variety of hematite known as martite. Geologically, the deposit is associated with Quaternary basaltic volcanism where iron-rich fluids underwent rapid oxidation within the volcanic vents and surrounding scoria. The site is famous for producing sharp, lustrous black octahedral crystals of hematite that have replaced original magnetite crystals, often found as loose individuals or clusters within the volcanic ash and debris. Historically, the area has not been the site of large-scale industrial ore quarrying, but rather has been worked by specimen collectors and local miners since the late 20th century, particularly gaining international prominence in the 1990s and 2000s. Because the locality is situated within the Payunia Provincial Park, collecting is strictly regulated, yet the specimens recovered during its peak periods of activity remain the global standard for this specific mineral replacement.

A pseudomorph is a mineralogical phenomenon where one mineral replaces another in such a way that the external shape of the original mineral is preserved, effectively creating a "false form." In the case of hematite after magnetite, the process is specifically called martitization, where the iron in the magnetite (Fe3O4) is oxidized to form hematite (Fe2O3). While the internal chemical structure and symmetry change from the cubic system of magnetite to the hexagonal system of hematite, the outward appearance remains that of a magnetite octahedron. This allows the new mineral to inhabit a crystal habit that is not naturally its own, providing a permanent record of the changing chemical conditions, such as increased oxygen fugacity, within the geological environment.

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