Pyrite cast after Calcite

specimen number:
2013559

location:
Rensselaer Quarry, Pleasant Ridge, Jasper Co., Indiana, USA

description:
What was once a scalenohedral crystal of calcite, this now completely gone and replaced by octahedral semi lustrous crystals of pyrite. Neat specimen that is in super condition all around. Collected in the 1970s.

Casting in minerals, such as the pyrite after calcite specimens found in various hydrothermal deposits, is a form of pseudomorphism where a later mineral forms a crust or replacement over a pre-existing crystal. In this process, the primary mineral, calcite, initially crystallizes into its characteristic rhombohedral or scalenohedral forms. Subsequently, changing chemical conditions in the surrounding fluids trigger the precipitation of pyrite, which grows as a thin layer or "epimorph" over the calcite surfaces. If the original calcite is later dissolved by acidic or under-saturated groundwater, it leaves behind a hollow shell of pyrite that perfectly maintains the external geometric shape of the departed calcite crystal. These casts are highly valued by mineralogists because they preserve the architectural history of the mineral vein, showing the exact sequence of growth and the environmental shifts that caused the original mineral to disappear while the secondary mineral remained.

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