Abstract

Glendonites are carbonate pseudomorphs after monoclinic crystals of ikaite (calcium carbonate hexahydrate: CaCO3.6H2O). Ikaite forms at present within organic-rich marine or brackish sediments that are at near freezing temperatures and decomposes to a mush of calcium carbonate and water when the temperature rises above 5°C. These unusual pseudomorphs were first recognised in southern Africa in 1972 as a rare component in marine shales just above the top of the Dwyka Group in the Great Karoo basin in South Africa. The crystal casts are rhomb-shaped in cross-section, show well-developed pyramid faces and are up to 19cm long. The glendonites occur in two forms: 1. as hollow or ochre-filled casts of ikaite crystals in concretions in fossiliferous marine Dwyka shales at Blaauwkrantz near Kimberley and also in the Ganigobis Shale at Zwartbas on the Orange River. 2. as carbonate pseudomorphs after ikaite, in concretions within shaly siltstone in the upper part of the marine glacial boulder mudstone at De Kalk (adjacent to Blaauwkrantz). The concretions are unweathered and the crystal casts are filled with a hard dark brown granular carbonate. Each granule encloses one or more stellate aggregates of zoned calcite crystals with an outer layer of brown carbonate. Electron microprobe analysis shows that the stellate crystals are pure calcite CaO: 55% (by wt.), Mg: 0.19% and the enclosing brown carbonate layer is Fe/Mg/Mn-enriched calcite (FeO+MgO+MnO: 6.18%, CaO: 48.5%).

The presence of glendonite demonstrates that the fossiliferous host sediments accummulated under near freezing, highly alkaline, orthophosphate-rich bottom water conditions. These stressful conditions help to explain the observed severely limited species diversity of the associated fossil assemblage.

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