Siderite after Calcite

specimen number:
2312227

location:
Turt Mine, Satu Mare Co., Romania

description:
Large rhombohedral crystals of what were once calcite to 6.0 cm in size across the top of matrix, these now completely replaced by crystallized siderite. The interior of the previous crystals is completely hollow. A neat and attractive replacement example that is in great condition.

Turt is a commune and mining locality situated in Satu Mare County, in northwestern Romania. Historically, underground mining operations in the area targeted rich polymetallic veins containing lead, zinc, copper, and antimony. While local mineral extraction dates back centuries in the broader Maramures metallogenic province, industrial-scale and organized mine exploitation peaked during the 19th and 20th centuries before operations were permanently shut down in 2005. Geologically, the region is characterized by Neogene volcanic rocks and hydrothermal vein systems that cut through sedimentary formations, creating world-renowned environments for diverse sulfide and sulfosalt mineral crystallization.

Pseudomorphs occur when one mineral replaces another while preserving the original crystal shape of the parent mineral. A classic example is siderite after calcite, where iron carbonate gradually substitutes for calcium carbonate. During this chemical replacement process, the external crystal form of the calcite remains completely intact, even though its internal composition and physical properties have completely changed to those of siderite.

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