Rutile after Brookite

specimen number:
2262357

location:
Magnet Cove, Hot Spring Co., Arkansas, USA

description:

Classic specimen from Arkansas, this previously a large crystal of brookite, now replaced by rutile. In great condition.

Magnet Cove, located in Hot Spring County, Arkansas, is a unique geological structure and a world-famous locality for diverse mineral species. Geologically, it is an alkaline igneous intrusive complex formed during the Cretaceous period, roughly 100 million years ago, consisting of a roughly circular basin defined by a ring of hills made of igneous rocks like syenite and carbonatite. Historically, the area attracted mineralogists and prospectors in the nineteenth and twentieth centuries due to the rich concentration of uncommon titanium, niobium, and zirconium minerals found within its weathered soils and rock outcrops.

Paramorphs occur when a mineral changes its internal crystal structure without altering its chemical composition or external crystal form. A notable example of this process is rutile after brookite, which is frequently found at localities like Magnet Cove. Brookite, an orthorhombic titanium dioxide polymorph, is often unstable under changing geological conditions such as rising temperatures or hydrothermal activity. Through a solid-state transformation, the brookite lattice rearranges into the more stable tetragonal crystal structure of rutile, resulting in a paramorph where the external shape of the original brookite crystal is preserved while the internal atomic arrangement becomes rutile.

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