The preparation of blue pigment CoKPO
4 was studied by grinding the mixture of reddish violet colored CoNH
4PO
4.H
2O and K
2CO
3 (mixing molar ratio 1.0 : O. 5) in ball mill. The color of the ground materials was gradually tinged with blue with increasing grinding time, and after 16 days grinding changed to indistinct blue. After more days of grinding the vividness increased gradually without changing its hue. After 30 days the ground material showed most vivid blue. This vivid blue colored material was identified as CoKPO
4 by X-ray diffraction measurement, etc. Grinding CoNH
4PO
4.H
2O and K
2CO
3 independently, the crystallinity of the former disappeared somewhat, and the latter changed into K
2CO
3.1. 5 1120.
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