Rutile paramorph after Brookite

specimen number:
2262014

location:
Magnet Cove, Arkansas, USA

description:
Choice classic from Arkansas, a pair of large crystals of what were once brookite measuring to just under 3.5 cm in size in a cluster and now replaced by rutile. This is in very good condition.

Magnet Cove, located in Hot Spring County, Arkansas, is a world-renowned mineralogical site famous for its complex alkaline igneous geology. The area is defined by a ring-dike complex that formed approximately 100 million years ago during the Cretaceous period when a series of alkaline magmas intruded into Paleozoic sedimentary rocks. This unique geochemistry created a concentrated area of rare minerals, including varieties of garnet, perovskite, and various titanium-bearing species. Historically, the area was never a site for a single large-scale industrial mine, but rather a collection of scattered pits and quarries exploited for different materials. Magnetite was collected for iron as early as the mid-1800s, and later operations in the 20th century, specifically between the 1930s and the 1970s, focused on the extraction of barite and titanium ores like rutile and brookite. Today, Magnet Cove is considered a premier research locality, with over 100 different mineral species identified within its relatively small geographic footprint.

A paramorph is a type of pseudomorph where a mineral undergoes a change in its internal atomic structure while maintaining the external crystal shape of the original mineral species. Unlike other pseudomorphs where one mineral is replaced by a chemically different substance, a paramorph involves a polymorphic transition where the chemical composition remains identical. In the case of rutile after brookite, both minerals share the same chemical formula as titanium dioxide, but they belong to different crystal systems. At Magnet Cove, large and distinct crystals originally formed as brookite under specific temperature and pressure conditions. As those conditions changed, the internal structure rearranged into the more stable rutile form. The resulting specimen looks like a classic dipyramidal brookite crystal from the outside, but it is composed entirely of a microscopic or fibrous aggregate of rutile on the inside.

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