Fluorite with Chalcopyrite

specimen number:
2092194

location:
Annabel Lee Mine, Hardin Co., Illinois, USA

description:
A grouping of sharp gemmy semi lustrous twinned fluorite crystals coming off matrix with minor chalcopyrite. These penetration twins were never very common in the mining district. This is a good one. This glows Amber in color when backlit as seen at the end of the video.

The Annabel Lee Mine, located in Hardin County, Illinois, was one of the last major underground operations in the historic Illinois-Kentucky Fluorspar District. Geologically, it exploited a classic vein-type deposit hosted within the Mississippian-aged St. Louis and Ste. Genevieve Limestones, where mineralizing hydrothermal fluids filled structural faults. The mine is legendary for its world-class fluorite specimens, often occurring as sharp, transparent cubes in striking colors such as deep purple, amber-yellow, and pale blue, frequently found in association with lustrous sphalerite, galena, and barite. Historically, the mine was developed by the Ozark-Mahoning Company, with shaft sinking beginning in 1982 and full-scale production starting in 1984. It remained a primary producer of acid-grade fluorspar until the decline of domestic mining led to its closure and flooding in 1995. Today, the Annabel Lee is remembered as a premier locality that produced some of the most aesthetic and geometrically perfect fluorite specimens in the world during the final decade of the district's industrial activity.

Penetration twinning in fluorite is a common and highly sought after crystallographic phenomenon where two individual cubic crystals appear to grow through one another, sharing a common symmetry axis. Geologically, this occurs during the initial stages of crystal nucleation when two separate embryos intergrow at a specific geometric angle, typically following the spinel twin law with a 180 degree rotation about the (111) axis. The resulting specimen displays two cubes that are interpenetrated, often with the corners of one cube protruding from the faces of the other, creating a complex and visually striking geometric form. This type of twinning is particularly iconic in specimens from classic localities like the North Pennines in England or the various mines of the Illinois-Kentucky Fluorspar District. Because fluorite belongs to the isometric crystal system, these twins maintain a high degree of mathematical precision and symmetry, making them a favorite subject for study in morphological crystallography.

Questions about this specimen?
Send an email or call us: 314-341-1811.