Hematite after Magnetite

specimen number:
2072087

location:
Cerro de Mercado Mine, Durango, Mexico

description:
Large crystals of what were once octahedral magnetite measuring to 3.5 cm in size on matrix and replaced by hematite. Classic pseudomorph specimen for this famous Mexican locality. In good condition. Previously from the collection of Steve Pullman, and before that Jeanne Mager.

The Cerro de Mercado Mine is located on the northern outskirts of Durango City, Mexico, and is one of the most famous iron ore deposits in the world. Geologically, it is a massive Kiruna-type iron oxide-apatite (IOA) deposit that formed approximately 30 million years ago during the Oligocene Epoch. The deposit is situated within a thick sequence of rhyolitic volcanic rocks and is believed to have originated from iron-rich magmatic fluids or melts that erupted or were emplaced near the surface. Historically, the "Iron Mountain" was noted by Spanish explorers as early as 1552, who initially hoped it was a mountain of silver. Large-scale industrial mining began in the late 19th century and has continued for over 100 years, making it a cornerstone of the Mexican steel industry. For mineral collectors, the site is legendary for producing world-class specimens of yellow-green fluorapatite crystals and exceptional "martite" specimens found in the oxidized zones of the open pit.

A pseudomorph is a mineral specimen where one mineral has replaced another while retaining the original external shape or crystal habit of the first mineral. In the case of the Cerro de Mercado specimens, this process involves the oxidation of magnetite into hematite, a specific type of pseudomorph known as "martite." Magnetite naturally crystallizes in the isometric system, typically forming sharp octahedral (eight-sided) crystals. When these crystals are exposed to oxidizing conditions, the iron atoms within the structure rearrange to form hematite, which belongs to the hexagonal (trigonal) system. Despite this internal chemical and structural change, the resulting hematite perfectly preserves the outward appearance of the original magnetite octahedron. These specimens are highly prized by collectors for their sharp geometric form and metallic luster, serving as a perfect physical record of the changing chemical environment within the ore body.

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