File:Glendonite (Neogene; Kola Peninsula, Russia) 7.jpg

English: Glendonite is an unusual and distinctive form of calcite (CaCO3). This material started out as a crystalline mass of ikaite, a hydrous calcium carbonate mineral (CaCO3·6H2O). Ikaite only forms in near-freezing water (~0º to 7º C) of high alkalinity, in organic-rich sediments at the sediment-water interface. At warmer temperatures, ikaite is not stable, and the mineral loses its water content and converts to calcite (anhydrous calcium carbonate). During the ikaite-calcite conversion, the original crystal structure of the ikaite may be retained. Calcite masses that retain ikaite crystal shapes are called glendonites. So, the term "glendonite" does not refer to a mineral; rather, it refers to the pseudomorph of calcite-after-ikaite. Pseudomorphs ("false-forms") are minerals that have replaced a previous mineral, but have retained the original crystal form.

The presence of glendonite in a succession of rocks is diagnostic evidence for the presence of glaciers in the geologic past, because ikaite forms at cold temperatures. Many ancient successions known to be deposited in glacial settings have glendonite.

Glendonite occurs in three main morphologies: stellate glendonite, rosette glendonite, and bladed glendonite. This example is a stellate glendonite.

Stratigraphy: supposedly an estuarine facies, upper Neogene

Locality: unrecorded/undisclosed site in the Olenitsa River area, White Sea coast of the Kola Peninsula, far-northwestern Russia