Thaumasite with Prehnite

specimen number:
2282036

location:
Fairfax Quarry, Centreville, Virginia, USA

description:
White fibrous crystalline thaumasite very rich throughout a prehnite matrix with minor quartz.

The Fairfax quarry, located near Centreville in Fairfax County, Virginia, is a prominent aggregate operation famous among collectors for its diverse suite of secondary minerals. Geologically, the quarry cuts into a massive Triassic diabase sill, part of the Culpeper Basin, which intruded existing sedimentary strata of the Newark Supergroup. As the hot basaltic magma cooled, it formed a dense, crystalline igneous rock, while subsequent low-temperature hydrothermal fluids circulating through fractures, fault zones, and solution cavities deposited an array of minerals including prehnite, apophyllite, zeolites, and sulfides. Commercial quarrying at the site began in the mid-twentieth century around the 1950s to extract crushed stone for regional road construction and infrastructure development. The quarry operated continuously for several decades under major aggregate companies until corporate restructuring and local development led to the cessation of active quarrying operations in the late twentieth century, with the site later undergoing reclamation.

Thaumasite is an unusual calcium silicate mineral containing carbonate, sulfate, and hexacoordinated hydroxide groups, crystallizing in the hexagonal system. Geologically, it forms as a late-stage alteration product in low-temperature hydrothermal environments, typically occurring within cavities, veins, and fractures of altered basaltic or diabase igneous rocks, as well as in skarns and contact metamorphic limestone deposits. The mineral generally occurs as fibrous aggregates, radiating acicular crystal groups, or dense masses that are colorless, white, or pale yellow, displaying a vitreous to silky luster.

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