Vivianite and Its Oxidation Products in Mammoth Ivory and Their Implications to the Burial Process

The phase composition and distribution characteristics have been obtained from two mammoth ivory samples with typical blue and yellowish-brown outer layers. The results reveal that hydroxyapatite, newberyite, organic matter, and quartz exist in all structures of mammoth ivory. Vivianite and santabarbaraite mainly contribute to the blue and yellowish-brown oxide layers of mammoth ivory, respectively. Meanwhile, metavivianite also occurs and partly influences the appearance of oxide layers. Vivianite is a common and complex product that can be formed by the interaction of gradually infiltrated Fe²⁺ and the original PO₄ ^3- in mammoth ivory. At the later stage, vivianite can be oxidized into metavivianite and santabarbaraite. As a result, mammoth tusks present dark bluish-green and yellowish-brown appearances. The multi-colored oxide layers are formed by different contents of vivianite and its oxidation products, which also provides valuable information on the relative burial intensity and time in different structures. It is inferred that the burial intensity increases in the sequence of yellowish-white dentin → blue outer layer → yellowish-brown outer layer. These observations are hopeful to be widely used in evaluating the changeable burial environment and exploring historical events that occurred on mammoth ivory.