Quartz after-on Fluorite

specimen number:
2292103

location:
Weardale, Durham, England, UK

description:
Cubic crystals of what were once fluorite measuring to 4.0 cm on edge, the prominent crystal exhibiting sharp penetration twinning, coated and replaced by colorless crystallized quartz. In good condition. Neat specimen.

Weardale, located in the North Pennines of County Durham, England, is a globally famous historical mining district situated within the northern half of the Alston Block. Geologically, the region comprises a thick, cyclically bedded Carboniferous sedimentary sequence of limestones, sandstones, and shales that was intruded from below by the Devonian-aged Weardale Granite, which provided a structural and thermal high. The extensive mineral deposits are Mississippi Valley-Type hydrothermal vein systems hosted along steeply dipping faults, where low-temperature, mineral-rich fluids deposited massive quantities of fluorite, galena, iron carbonates, and quartz, often forming vast open flats and cavities. Early lead extraction by the Romans and medieval monks gave way to intense industrial operations in the eighteenth and nineteenth centuries under the Blackett-Beaumont family and the London Lead Company, followed by a major transition to industrial fluorspar mining in the twentieth century. Large-scale underground commercial operations steadily declined due to changing market conditions, with the last major working fluorspar operation, the Groverake mine, permanently closing its shafts in the summer of 1999.

An epimorph, in this specific context involving quartz after fluorite, is a type of pseudomorph where a younger mineral forms an external crust or shell over an earlier, pre-existing mineral crystal. Geologically, this development occurs during late-stage hydrothermal events when silica-rich fluids enter an existing mineral pocket and deposit a continuous layer or coating of chalcedony or microcrystalline quartz over the sharp faces of well-formed fluorite cubes. Subsequently, a shift in the chemical composition, temperature, or acidity of the circulating groundwater causes the original, internal fluorite crystal to completely dissolve away, leaving behind a hollow, empty cast that perfectly preserves the external cubic geometry and surface textures of the departed host. These distinct hollow molds provide mineralogists with critical evidence regarding the complex, multi-stage paragenetic sequences and changing fluid chemistries within historical vein systems.

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