Uranyl binding mechanism in microcrystalline silicas: A potential missing link for uranium mineralization by direct uranyl co-precipitation and environmental implications

•Microcrystalline silicas (agates and opal-CT) contain 27–137 ppm U.•Uptake and retention of uranyl silicate complexes in microcrystalline silicas.•Uranium mineralization by direct uranyl co-precipitation without reduction.•Microcrystalline silicas are effective for U attenuation at elevated temperatures.