Weathering of hydroxyapatite, Ca5(PO4)3(OH) in bone probably is initiated by organic and carbonic acids formed by the microbial decomposition of collagen. This weathering, independent of soil properties, is caused by protons replacing Ca from hydroxyapatite. As collagen is depleted, proton production decreases and weathering may either continue if protons are available from the soil or be arrested if Ca from the soil displaces the protons previously added to the hydroxyapatite. The theoretical Ca/P weight ratio of unweathered bones is 2.15. Weathered bones that have been stabilized by Ca may have this ratio or a higher one if extra Ca has been added. A group of weathered bones from one site with a slightly acid soil had an average ratio of 1.67, which probably promotes further weathering, while bone at the same site with an average ratio of 4.09 was less weathered and apparently stabilized.
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