Thursday, June 25, 2015
Calcite after Ikaite (CaCO3·6H2O) var. Glendonite concretion.The rock was found at Kola Peninsula, Russia. Ikaite is a rare mineral that occurs in nature at temperatures up to 7°C in alkaline, phosphate-rich marine and continental waters. Although ikaite has generally been replaced by calcite (this replacement is called glendonite), it has preserved its original crystal shape. Because of the rather restricted conditions under which ikaite/glendonite forms, it serves as a marker for near-freezing water temperatures. Hydrohalite, a form of salt, precipitates at low temperatures from highly saturated brines. It is not well-preserved in sediments because of its high solubility, but its former existence can be inferred from the presence of halite or other minerals that preserve the shape of the original crystals. Minerals are just one of many proxies used by geologists to reconstruct past climates. Some minerals like ikaite define a very narrow set of conditions, while others, like the clays, appear in a number of geologic and climatic settings. Minerals along with other indicators — fossils, tree rings, pollen, ice cores, geochemistry or isotopes — provide tantalizing clues to ancient climates.