Ikaiet, het oorspronkelijke mineraal van de pseudomorfosen van het jarrowiet-type

The pseudorphs known as jarrowite which were found during the middle part of the last century in muds at Jarrow Lake 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 Celcius, but breaks down at normal temperatures to form calcium carbonate and water. Jarrowites are pseudomorphed in calcite which appeared to have been generated by redistribution of the calcium 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 occurences of pseudomorphs of jarrowite-type (pseudogayllusite, thinolite, White Sea hornlets, glendonite, gersternkorner, gennoishi, fundylite and others), the discovery of ikaite and the ways in which the pseudomorphs atter ikaite may have a role as palaeometers.