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V.V. Serdobintseva, D.V. Kalinin, S.V. Vosel
The structural unit of the gel crystals of precious opal is monodisperse silica spheres (MSS) 150–350 nm in size consisting of smaller (about 10 nm) closely packed spherical microparticles. The porous system of regular structure formed by MSS is filled with a solidified interstitial gel with particles less than 5 nm in size. The conditions of MSS growth, their stable existence in suspension and formation of gel crystals (the first generation of silica) differ from the conditions of formation of the interstitial gel and its subsequent silicification (the second generation of silica). The silicification mechanism depends on the types of initial crystalline gel structures.