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XRD powder patterns of the tunnel structure phase, cryptomelane (K-cryptomelane) (A), and the birnessites (B), which are labeled as follows: A, sodium birnessite (Na-Birnessite); B, potassium birnessite (K-Birnessite); and C, calcium birnessite (Ca-Birnessite). The birnessite powder patterns are y-offset to improve readability. arb, arbitrary units.
