Calcite after Ikaite

specimen number:
2133308

location:
Lake Winnemucca, Nevada, USA

description:
A loose grouping of prismatic double terminated crystals of what were once ikaite now replaced by calcite. These are often called glendonites. this one is in terrific condition all around.

Glendonites are distinctive mineral pseudomorphs consisting of calcite that has completely replaced the highly unstable precursor mineral ikaite, which is a calcium carbonate hexahydrate compound. The parent mineral, ikaite, is metastable and typically precipitates within cold, organic-rich marine or lacustrine sediments at temperatures near freezing, requiring unique geochemical conditions such as high alkalinity and the presence of phosphate or magnesium ions to inhibit normal calcite growth. When environmental temperatures rise above approximately 4 to 7 degrees Celsius, or when pore-water chemistry shifts, ikaite rapidly loses its six structural water molecules, triggering a dramatic internal volume loss of 70 to 80 percent. This dehydration process results in a porous, multi-generational aggregate of secondary calcite crystals that fills the void while faithfully preserving the original monoclinic, often blade-like, stellate, or rosette macroscopic morphology of the parent ikaite. Because of this strict temperature dependency in nature, geologists and mineralogists widely use the presence of glendonites in the ancient rock record as a valuable paleoclimatic and paleoenvironmental indicator to infer cold-water depositional settings and ancient glacial episodes.

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