Nucleation of aragonite upon carbonation of calcium oxide and calcium hydroxide at ambient temperatures and pressures: a new indicator of fire-related human activities

International audienceWood ash found at archaeological sites is the most direct evidence of the presence of combustion features.The preservation of wood ash within archaeological deposits is often poor due to diageneticprocesses and therefore its identification requires the use of spectroscopic methods. FTIR spectrometry isable to detect the main calcite component of wood ash, unless this is mixed with other calcitic phases ofgeologic origin. Using FTIR spectrometry we identified non-biogenic aragonite in archaeological heatalteredsediments and lime plasters. The presence of aragonite in such conditions is unusual, as it isunlikely to crystallize at ambient temperatures and pressures or in absence of Mg. The experimentalconditions which favor the nucleation of this aragonite phase were investigated with FTIR spectrometry,XRPD and SEM imaging of modern lime plaster and quicklime samples prepared in the laboratory usingdifferent calcium carbonate starting materials. We show that aragonite forms at ambient temperaturesand pressures upon carbonation of calcium oxide and calcium hydroxide together with calcite and itsnucleation and growth are influenced by environmental parameters, such as carbon dioxide partialpressure, relative humidity and temperature. This pyrogenic aragonite is a reliable indicator for calcareousmaterials exposed to temperatures above 600 C, and therefore it can be used to determine thepresence of heat-altered sediments and ash in the absence of structured combustion features. Thismineral phase could have applications in radiocarbon dating as well