Calcite after Aragonite

specimen number:
2062976

location:
Puerta de Luna, Guadalupe Co., New Mexico, USA

description:
An impressive large crystal of what was once a pseudo-hexagonal twinned aragonite, this now replaced by crystalline calcite. Collected on May 7, 1977, the identification has been confirmed. This is in super condition all around. Very choice large example for this material. The specimen comes with a previous label from the collection of Ray DeMark.

Puerta de Luna, located in Guadalupe County along the Pecos River in New Mexico, features prominent Triassic red-bed sedimentary formations like the Santa Rosa Formation. While not a commercial mining district, this area became famous among rockhounds and mineral collectors for discovering unique examples of pseudomorphs after aragonite weathered out from the local strata.

A pseudomorph forms when one mineral is replaced by another while retaining the original external crystal shape. In the case of calcite after aragonite, changing environmental conditions, temperature, or groundwater chemistry cause the unstable aragonite structure to dissolve or transform, allowing calcite to precipitate in its place and preserve the distinct morphology of the original crystal.

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