Nucleation of aragonite upon carbonation of calcium oxide and calcium hydroxide at ambient temperatures and pressures: a new indicator of fire-related human activities
Résumé
Wood 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 diagenetic
processes and therefore its identification requires the use of spectroscopic methods. FTIR spectrometry is
able to detect the main calcite component of wood ash, unless this is mixed with other calcitic phases of
geologic origin. Using FTIR spectrometry we identified non-biogenic aragonite in archaeological heataltered
sediments and lime plasters. The presence of aragonite in such conditions is unusual, as it is
unlikely to crystallize at ambient temperatures and pressures or in absence of Mg. The experimental
conditions 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 using
different calcium carbonate starting materials. We show that aragonite forms at ambient temperatures
and pressures upon carbonation of calcium oxide and calcium hydroxide together with calcite and its
nucleation and growth are influenced by environmental parameters, such as carbon dioxide partial
pressure, relative humidity and temperature. This pyrogenic aragonite is a reliable indicator for calcareous
materials exposed to temperatures above 600 C, and therefore it can be used to determine the
presence of heat-altered sediments and ash in the absence of structured combustion features. This
mineral phase could have applications in radiocarbon dating as well.