Weddellite

Formula: Ca(C₂O₄).2H₂O
Organic compound, oxalate
Specific gravity: 1.94
Hardness: 4
Streak: White
Colour: Colourless, white, yellowish brown to brown, colourless in transmitted light
Solubility: Insoluble in water
Environments

Sedimentary environments
Hydrothermal environments

Whewellite and weddellite are hydrated oxalates of calcium, found naturally in plant tissues, in sediments as a mineral of organic origin, and in urinary stones. About 70 percent of human urinary stones contain whewellite and/or weddellite (AM 65.327-334).
Weddellite is authigenic (formed in place) in bottom muds and in peat sediments, in calcareous lake-bottom sediments and it may be formed by reaction of calcite with oxalic acid produced by lichens. Associated minerals include whewellite, urea, phosphammite and aphthitalite from bat guano (HOM). It is also formed by lichens living on basalt, serpentinites, copper-bearing rocks, gabbros, dolerites, andesites and volcanic clasts, and it may occur in coral reefs. It dehydrates to whewellite upon exposure to air (Mindat).

Localities

At the type locality, the Weddell Sea, Western Antarctica, Antarctica, weddellite occurs as tiny isolated crystals in bottom muds of the arctic sea (Mindat).

At the Moore Lake occurrence, Lutterworth Township, Haliburton County, Ontario, Canada, weddellite has been found as snow-white spherical aggregates up to 0.5 mm in diameter with altered amphibole and mica on marble. Oxalic acid produced by lichens presumably reacted with calcite to form the weddellite (CM 21.509-511).

At Milltown Quarry, Milltown, Ashover, North East Derbyshire District, Derbyshire, England, UK, a weddellite crystal 0.3 mm across has been found perched on the edge of a fluorite crystal (RES p104-105).

At Biggs, Sherman county, Oregon, USA, weddellite occurs in nodules of chalcedony variety jasper found in lake-bottom sediments associated with flows of the Columbia River basalt plateau. Fish scales and bones were found in several of the jasper nodules. These and other organic material in the sediments probably provided the calcium and the oxalic acid necessary for the formation of the weddellite. Whewellite pseudomorphs after weddellite have been found here. A thin section of the jasper showed that this host material for the weddellite, whewellite and quartz is made up of very small grains of quartz (CM 21.503-508).

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