Dolomite after Calcite with Quartz

specimen number:
2312607

location:
Boldut Mine, Cavnic, Maramures Co., Romania

description:
Classic specimen for this locality with crystals of what were once calcite measuring to 5.5 cm in size, these now replaced by glistening colorless crystallized dolomite. Minor quartz is present. In great condition.

The Boldut Mine, located within the Cavnic mining district of Maramures County, Romania, is part of a historic mining region with roots dating back to medieval times and earlier, while intensive industrial extraction of its polymetallic ores operated heavily through the 20th century until final closure around 2005. Geologically, the deposit sits within the Neogene volcanic belt of the Inner Carpathians, where hydrothermal activity driven by calc-alkaline magmatism emplaced rich veins of lead, zinc, copper, gold, and silver into volcanic and sedimentary host rocks. The underground workings became internationally renowned among mineral collectors for producing diverse sulfide and carbonate assemblages, including quartz, dolomite, rhodochrosite, calcite, sphalerite, and galena.

Pseudomorphs of dolomite after calcite occur when a precursor crystal of calcite is chemically altered or replaced by dolomite while preserving the original crystal's external geometric form. This geological phenomenon typically happens when hydrothermal solutions or groundwater rich in magnesium ions interact with preexisting calcium carbonate crystals under specific temperature and chemical conditions. Mineralogically, the process involves the introduction of magnesium to replace a portion of the calcium ions in the crystal lattice, transforming the mineral structure from calcite to dolomite without altering the distinctive scalenohedral, rhombohedral, or prismatic shapes dictated by the original calcite template.

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