Hematite after Magnetite

specimen number:
2282007

location:
Cerro de Mercado, Durango, Mexico

description:
Sharp octahedral crystals of what were once magnetite measuring to 1.5 cm are in size, these now replaced by hematite, across the front of matrix. Classic material for this locality. In great condition.

The Cerro de Mercado mine, located on the northern outskirts of Victoria de Durango in Durango, Mexico, is a world-famous iron ore deposit dominated by a massive prominent hill of high-grade iron oxides. Geologically, it is classified as an iron oxide-apatite (IOA) or Kiruna-type deposit, formed during the Oligocene epoch in connection with extensive ignimbrite volcanism of the Sierra Madre Occidental. The ore body is hosted within rhyolitic tuffs and flows, consisting heavily of massive magnetite, hematite, and martite, accompanied by notable yellow-green fluorapatite crystals. Large-scale exploitation began around 1828 and intensified significantly with the opening of local ironworks in the late nineteenth century, eventually supplying major industrial steel mills across Mexico. Open-pit quarrying and extraction operations have continued systematically through the twentieth century and remain active today, making it one of the longest-operating and most productive iron mines in North America.

A pseudomorph occurs when a mineral chemically replaces or alters another mineral while completely retaining the original mineral's external crystal form or shape. In the case of hematite after magnetite, often referred to as martite, the process is driven by supergene or hydrothermal oxidation where the iron atoms within the isometric magnetite crystal matrix are oxidized. This chemical alteration transforms the structural arrangement from a spinel structure into the hexagonal structure of hematite without disturbing the outer sharp octahedral geometry of the original magnetite crystal. The resulting specimen looks exactly like an octahedral magnetite crystal, but it possesses the physical properties, dark red streak, and chemical composition of pure hematite.

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