Dolomite after Aragonite

specimen number:
2282063

location:
Yeso, De Baca Co., New Mexico, USA

description:
A loose cluster of pseudo-hexagonal crystals of what were once aragonite, these now replaced by granular crystalline dolomite. In great condition all around.

A pseudomorph of dolomite after aragonite forms through a specialized chemical replacement process where the original mineral structure is altered while retaining its external shape. Geologically, this transformation typically occurs in sedimentary, evaporitic, or low-temperature hydrothermal environments where early-formed crystals of aragonite, an unstable polymorph of calcium carbonate, come into contact with magnesium-rich fluids or groundwaters. Over time, the magnesium ions systematically substitute into the carbonate lattice via a dissolution and reprecipitation mechanism, converting the mineral to the more chemically stable calcium-magnesium carbonate, dolomite. The resulting specimens are highly notable because they perfectly preserve the distinct, sharp, often pseudo-hexagonal twinned crystal habits or cyclic trillings characteristic of aragonite, despite now possessing the chemical composition, physical hardness, and internal crystal structures of dolomite.

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