Ikaite, the parent mineral of jarrowite-type pseudomorphs

Abstract

The pseudomorphs known as jarrowite which were found during the middle part of the last century in muds at Jarrow Slake on the River Tyne are now identified as being pseudomorphs after crystals of the mineral ikaite. Ikaite, CaCO3.6H2O, is a metastable mineral in sedimentary rocks. It crystallizes at near to zero Celsius, but breaks down at normal temperatures to form calcium carbonate and water. Jarrowites are pseudomorphed in calcite which appears to have been generated by redistribution of the calcium carbonate formerly present in the parent ikaite. The high porosity of the jarrowites is the result of the decrease in volume which accompanied the change. The paper reviews the known occurrences of pseudomorphs of jarrowite-type (pseudogaylussite, thinolite, White-Sea hornlets, glendonite, gersternkorner, gennoishi, fundylite and others), the discovery of ikaite and the ways in which the pseudomorphs alter ikaite may have a role as palaeothermometers.