Siderite after Calcite

specimen number:
2312209

location:
Turt Mine, Satu Mare Co., Romania

description:

Rhombohedral crystals of what were once calcite measuring to 4.7 cm in size across the front of matrix, these now completely replaced by crystallized siderite. In terrific condition. This is classic material for this locality.

The Turt Mine, located in the Oas Mountains of Satu Mare County in northwestern Romania, is part of a historic metallogenic region with long-standing underground mining operations targeting base and precious metals. Geologically, the deposit is situated within the Neogene volcanic belt of the Inner Carpathians, where subvolcanic structures and complex hydrothermal vein systems cut through older formations. These fluid systems deposited concentrated pockets of lead, zinc, and copper sulfides. The mine has gained recognition among mineral collectors for producing fine specimens of galena, quartz, and distinctive pseudomorphs or crystal casts involving siderite replacing earlier carbonate minerals.

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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