Quartz after Calcite

specimen number:
2312075

location:
Droujba mine, Laki, Plovdiv, Bulgaria

description:

A large crystal grouping of what would have been calcite, these now completely replaced by crystallized white quartz. Neat pseudomorph specimen complete all around. In good condition with minimal micro-damage present as seen.

The Droujba Mine, located near Laki in the Rhodope Mountains of southern Bulgaria, is part of a major polymetallic ore district rich in lead, zinc, and base metal deposits. The regional geology is defined by complex metamorphic basement rocks of the Rhodope Massif, which were intruded by Tertiary magmatic activity that drove extensive hydrothermal ore deposition. Commercial mining operations developed significantly during the twentieth century to extract industrial metals from these hydrothermal vein and replacement structures. In addition to its industrial history, the underground workings have become celebrated among mineral collectors worldwide for producing exceptional specimen combinations, including quartz, chalcedony, sphalerite, galena, and well-formed calcite crystals.

Pseudomorphs also provide fascinating insights into changing geochemical environments, such as when quartz replaces calcite. This alteration process occurs when shifts in hydrothermal or groundwater chemistry cause acidic, silica-rich fluids to circulate through a rock formation containing pre-existing calcite crystals. Because calcite is highly soluble under these conditions, it gradually dissolves away molecule by molecule. Simultaneously, dissolved silica precipitates out of solution and fills the microscopic void left behind, or coats the crystal faces in a process called epimorphosis. Through this pseudomorphic or epimorphic replacement, the incoming quartz faithfully preserves the sharp rhombohedral or scalenohedral external geometry of the original calcite crystal, even though the chemical composition changes entirely from calcium carbonate to silicon dioxide.

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