Pyrite casts after Anhydrite with Galena

specimen number:
2013553

location:
Sweetwater Mine, Viburnum Trend, Reynolds Co., Missouri, USA

description:
Sharp tabular crystals of what were once anhydrite measuring to 2.5 cm in size, these crystallized pyrite that has formed cast after the now gone anhydrite. Bright lustrous crystals of galena are in association along with white crystallized dolomite. This was collected in the very early 1990s. In excellent condition all around.

The Sweetwater Mine, located in Reynolds County within the Viburnum Trend of southeast Missouri, is a significant lead and zinc producer. Geologically, it is a Mississippi Valley-Type (MVT) deposit hosted within the Cambrian Bonneterre Formation, where mineralization occurs as replacements and vug fillings in dolomitized limestone and ancient reef structures. The ore body is dominated by galena, sphalerite, and chalcopyrite, but the mine is particularly famous among collectors for producing exceptional specimens of golden chalcopyrite, iridescent marcasite, and aesthetic calcite. Historically, the mine began operations in 1974 under the Ozark Lead Company, a subsidiary of Kennecott. It was later acquired by ASARCO and eventually became part of the Doe Run Company operations. The mine has remained an active industrial producer for over fifty years, contributing to Missouri's status as a global leader in lead production while consistently providing high-quality mineral specimens for research and curation.

Mineral casts, such as the pyrite after anhydrite found at the Sweetwater Mine, represent a specific type of pseudomorph where a secondary mineral encrusts or replaces a primary mineral that later dissolves. In this process, the primary mineral, anhydrite, originally formed as distinct, elongated or bladed crystals. Later, iron-rich hydrothermal fluids deposited a layer of pyrite over these crystals. When the original anhydrite was eventually dissolved by changing groundwater chemistry, it left behind a hollow shell or a solid replacement that perfectly preserves the external shape of the vanished crystal. These specimens are prized by mineralogists because they act as physical records of minerals that are no longer present, revealing the complex sequence of mineral deposition and dissolution within the ore body.

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