Ikaite is the mineral name for the hexahydrate of calcium carbonate, CaCO3·6H2O. Ikaite is colorless when pure. It is usually considered a rare mineral, but this is likely due to difficulty in preserving samples. Ikaite is a metastable and has only been observed in nature at temperatures <7 °C.
It was first discovered in nature by the Danish mineralogist Pauly in the Ikka (then spelt Ika) fjord in SW Greenland, close to Ivigtut, the locality of the famous cryolite deposit.
Ikaite Readily dehydrates to calcite above 8°C. The resulting pseudomorph of calcite after ikaite is also called "glendonite."
| Ikaite And Glendonite. Precious opal after glendonite |
from White Cliffs Opal Field, New South Wales, Australia.
Natural ikaite growth sites vary widely in their chemical conditions, including saline lakes and fjords (where it forms tufa towers; cold saline spring waters, Antarctic sea-ice, and marine sediments (up to 10 m below the sediment–water interface). These sites are all characterised by having low temperatures (≤7 °C) and unusual chemical conditions.
This "melting mineral" is more commonly known through its pseudomorphs. Ikaite or its pseudomorphs have been reported as occurring in marine, freshwater, and estuarine environments.
The aqueous chemical conditions vary, but all are believed to feature high alkalinity in combination with chemical inhibitors of the thermodynamically more stable anhydrous carbonate polymorphs, calcite, aragonite, and/or vaterite.
Furthermore, although in nature, ikaite has only been found in environments with temperatures ranging from −2 to + 7 °C. In nature, ikaite is most commonly found growing below the sediment–water interface in continental shelf settings.
Color: Chalky white.
The presence of ikaite may be recorded through geological time through the presence of pseudomorphs of other calcium carbonate phases after it. Although it can be hard to uniquely define the original mineral for every specimen, there appears to be good evidence for ikaite as the precursor for the majority of the following locality names of pseudomorphs:
Glendonite, Thinolite, Jarrowite, Fundylite, Gersternkorner, Gennoishi, Molekryds, and Pseudogaylussite.
|Calcite after ikaite var. glendonite concretion|
Photo: Brocken Inaglory
The term “glendonite” does not refer to a mineral, but to a class of pseudomorphs. A pseudomorph is a mineral that has taken the characteristic crystal shape of another mineral by processes such as replacement or recrystallization. Glendonite named after type locality, Glendon, New South Wales, Australia.
Glendonites started out life as crystalline masses of ikaite, Ikaite only forms in near-freezing water 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.
Marine sedimentary ikaite is the parent mineral to glendonite,
stellate pseudomorphs found throughout the geological record which are
most usually composed of calcite. Ikaite is known to be metastable at
earth surface temperatures and pressures, readily breaking down to more
stable carbonate polymorphs when exposed to warm (ambient) conditions.
Yet the process of transformation of ikaite to calcite is not well
understood, and there is an ongoing debate as to the palaeoclimatic
significance of glendonites in the geological record.
|Orange-brown Crystals of Calcite (Var. Glendonite),|
from Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia
Photo: Rob Lavinsky
|Precious opal after glendonite |
(White Cliffs Opal Field, New South Wales, Australia)
Photo: James St. John
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