Unusual morphologies and the occurrence of pseudomorphs after ikaite (CaCO3·6H2O) in fast growing, hyperalkaline speleothems

Unusual speleothem, associated with hyperalkaline (pH>12) groundwaters have formed within a shallow, abandoned railway tunnel at Peak Dale, Derbyshire, UK. The hyperalkaline groundwaters are produced by the leaching of a thin layer (<2 m) of old lime kilnwaste above the soil-bedrock surface above the tunnel by rainwater. This results in a different reaction and chemical process to that more commonly associated with the formation of calcium carbonate speleothemsfrom Ca-HCO3-type groundwaters and degassing of CO2. Stalagmiteswithin the Peak Dale tunnel have grown rapidly (averaging 33 mm y-1), following the closure of the tunnel 70 years ago. They have an unusual morphology comprising a central sub-horizontally-laminated column of micro- to nano-crystalline calcium carbonateencompassed by an outer sub-vertical assymetric ripple laminated layer. The stalagmitesare largely composed of secondary calcite forming pseudomorphs(<1 mm) which we believe to be predominantly after the ‘cold climate’ calcium carbonatepolymorph, ikaite( calcium carbonatehexahydrate: CaCO3•6H2O), with minor volumes of small (<5 μm) pseudomorphsafter vaterite. The tunnel has a near constant temperature of 8-9°C which is slightly above the previously published crystallisation temperatures for ikaite(<6°C). Analysis of a stalagmiteactively growing at the time of sampling, and preserved immediately within a dry nitrogen cryogenic vessel, indicates that following crystallisation of ikaite, decomposition to calcite occurs rapidly, if not instantaneously. We believe this is the first occurrence of this calcium carbonatepolymorph observed within speleothem.