Siderite after Calcite with Pyrite

specimen number:
2312031

location:
Turt Mine, Satu Mare Co., Romania

description:
Rhombohedral crystals of what were once calcite to 1.0 cm in size, these now replaced by siderite, across the top of a pyrite crystal matrix. In good condition.

The Turt Mine, located in the Oas Mountains of Satu Mare County in northwestern Romania, is situated within a prolific metallogenic region associated with Neogene volcanism along the Inner Carpathians. The local geology is characterized by volcanic and subvolcanic structures where hydrothermal activity deposited complex polymetallic ore veins containing lead, zinc, and copper sulfides. Mining operations in the area date back generations to extract these base metals. The underground workings became well known among mineral collectors for producing fine specimens of galena, often associated with siderite, pyrite, and other sulfide minerals, before industrial extraction eventually ceased.

Pseudomorphs occur when one mineral replaces another while retaining the original external crystal shape. A notable example is siderite after calcite, a process where iron-bearing hydrothermal fluids or groundwater react with pre-existing calcite crystals. As the chemical composition changes from calcium carbonate to iron carbonate, the original crystal structure is preserved. This results in a mineral specimen that maintains the distinct rhombohedral or scalenohedral form of the original calcite while possessing the brown color and chemical makeup of siderite.

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