Glendonite is an unusual and distinctive form of calcite (CaCO3). Glendonites started out life as crystalline masses 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. It 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, glendonite is not a mineral. Rather, it is a calcite pseudomorph (“false-form”) after ikaite.
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. The sample shown below is from the Permian of Australia. During the Late Paleozoic, the supercontinent Gondwana (of which Australia was a part) experienced several ice ages (from the Mississippian to the Permian). The waxing & waning of the Late Paleozoic Gondwanan ice sheets is recorded in North America in the form of cyclothems (cyclic packages of sediments indicating numerous short-term transgressive-regressive events).
Glendonite occurs in three main morphologies: stellate glendonite, rosette glendonite, and bladed glendonite. The specimen shown below from the Sydney Basin of Australia is a fairly large stellate glendonite. The bladed glendonite type can be over twice the size of this sample. The largest reported bladed glendonite is 30 cm long. A very large stellate glendonite was recently illustrated by Selleck et al. (2007) in Journal of Sedimentary Research 77(11-12): 986.
Stratigraphy: glendonite from very dark gray siltstone matrix, Conjola Formation, Shoalhaven Group, Lower Permian.
Locality: coastal exposure at Dolphin Beach, southern side of mouth of river draining Burril Lake, just SW of Ulladulla, coastal southeastern New South Wales, Illawarra Coast, far-southeastern Australia.
Glendonite (stellate glendonite morphology) (12.5 cm across) from the Permian of New South Wales, Australia.
Collected & generously donated by Molly Tannian.
Some info. from Stephanie Thomas.