On some coastal rock platforms of the South Coast of NSW one can occasionally find unusual symmetrically shaped objects, known as "glendonites". These are not really fossils since they are not a trace of something that was once alive (like a shell, or a fish) but, rather, are geological, mineral specimens. Examples are also found where the mineral specimen is no longer present and all that remains is an external cast or imprint. An example of this is shown in the accompanying image.
But glendonites are even more interesting. Even where an actual mineral specimen is present it is a pseudomorph. In other words the original mineral has been substituted by another. In scientific terminology "a glendonite is a pseudomorph after ikaite"; where ikaite is the original mineral and is a crystalline version of calcium carbonate (CaCO3.6H2O for those who are interested). These pseudomorphs have been called glendonites after a location called Glendon in the Hunter Valley (west of Singleton) where they were first noted in 1840 by visiting geologist, J. D. Dana, who was part of an American scientific exploration expedition (United States Exploring Expedition) to the Pacific in the period 1838-1842.
The mineral, ikaite, is formed in seawater near freezing temperatures (less than 5oC) and is only formed in a particular chemical and physical environment, e.g. it forms in organic rich mud in water with higher than normal alkalinity. The crystals were first identified growing in the organic rich mud at the bottom of the Ika Fjiord in southern Greenland in 1962 but since
Locally glendonites are found in several horizons of the Lower to Middle Permian strata including the Wandrawandian Siltstone which forms the rock platforms around Ulladulla and Huskisson. In age the crystals range from ~285 MY to ~270 My BP indicating that they grew on the seafloor in very cold Antarctic waters because at that time this region was part of Gondwana and was located at about 80o S, well inside the Antarctic Circle .
Once the temperature rises above 5oC the ikaite crystals dissolve leaving an external mould or imprint which can be filled by another mineral dissolved in the seawater. Because of the nature of the original ikaite crystals, glendonites come in several different shapes, e.g. star-like as in the image here, and blade-like. The replacement mineral is generally a matrix consisting of small calcite crystals held in place by a cement.
Glendonites as Palaeoclimatic and Palaeoceanographic Indicators: A Case Study from the Glacially Influenced Permian System of Eastern Australia, T.D. Frank, S.G. Thomas, and C.R. Fielding, 2008. (adapted from an oral presentation at the AAPG Convention, San Antonio, TX, USA, April 2008)
A Review and Synthesis of Glendonites (Pseudomorphs after Ikaite) with New Data, B.W. Selleck, P.F. Carr, and B.G. Jones, J of Sedimentary Research, 2007, v 77, pp 980-991.
Some input from Phil Smart is much appreciated.