Fluorite

specimen number:
2013535

location:
Auglaize Quarry, Junction, Ohio, USA

description:
Gorgeous lustrous iridescent cubic crystals of fluorite lining an open pocket in matrix. In very good condition. Excellent for this classic locality. From the 1970s.

The Auglaize Quarry, located near Junction in Paulding County, Ohio, is a classic Midwestern mineral locality situated within the Findlay Arch, a broad structural uplift. Geologically, the quarry exposes a sequence of Middle Devonian carbonate rocks, primarily the Dundee Limestone and the overlying Traverse Group. These sedimentary layers were deposited approximately 380 to 390 million years ago in a shallow, subtropical marine environment. The mineralization at Auglaize is considered a Mississippi Valley-Type (MVT) occurrence, where hydrothermal fluids migrated through the porous limestone and dolomite, depositing minerals in open vugs and solution cavities. Historically, the quarry has been an active source of crushed stone for aggregate and industrial lime since the mid-20th century. While it remains an industrial site, it earned a prestigious reputation among mineral collectors for producing some of the most unique and colorful fluorite specimens in the United States.

For mineral collectors, the Auglaize Quarry is world-famous for its iridescent fluorite, which exhibits a spectacular "rainbow" or "oil-slick" play of colors on the crystal surfaces. The fluorite typically occurs as small, sharp cubes, ranging in body color from a pale amber or honey yellow to a deep, dark purple that can appear nearly black. The iridescence is caused by a microscopic thin-film interference effect, occurring when a very thin layer of secondary minerals -- most commonly hydrocarbons (bitumen) or a fine coating of iron-rich minerals like marcasite or pyrite -- forms on the surface of the fluorite cubes. When light hits this micro-layer, it reflects off both the top and bottom surfaces, creating the shimmering metallic blues, purples, and golds.

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