Solubilities of bone mineral from archaeological sites: The recrystallization window

Bone mineral solubility is an important parameter for understanding the preservation of bones in the archaeological and palaeontological records. In this study we have measured the solubility of the carbonated hydroxyl apatite of sub-recent and fossil bones, as well as synthetic hydroxyl apatite in deionized water and in pH-buffered solutions. The solutions were open to the atmosphere and the pH values were around neutral; measurement conditions that are relevant to bone mineral preservation in nature, but that were absent from most previous studies. We obtained internally consistent results from both the water and the buffered experiments supporting the notion that we are measuring an inherent property of the mineral phase. We found that bone mineral is much more soluble than synthetic hydroxyl apatite. We measured the ionic activity products at "steady state" conditions and we identify a recrystallization window between pH 7.6 and 8.1, which defines the conditions under which bone crystals dissolve and reprecipitate as a more insoluble form of carbonated hydroxyl apatite. As these conditions are common in nature, most fossil bones will not maintain their original crystals with time. We also found that bones that contained small amounts of calcite did not dissolve at all during our experiments. These results provide a basis for better understanding the conditions in sediments under which bones are preserved and the relative states of preservation of bone. They also have important implications for the selection of the most appropriate bone samples for paleoenvironmental and paleodiet analyses and dating.