Reexamination of the structure of opal-A: A combined study of synchrotron X-ray diffraction and pair distribution function analysis.

It assigns the interstratified phase (to model stacking disorder or interstratification between cristobalite and tridymite) to cristobalite (0.31) and tridymite (0.23) portions, and the scale factors for cristobalite, tridymite, coesite, and the twin boundary are 0.65, 0.46, 0.21, and 0.09, respectively. Keywords: Synchrotron X-ray diffraction; pair distribution function analysis; transmission electron microscopy; local structure; opal-A EN Synchrotron X-ray diffraction pair distribution function analysis transmission electron microscopy local structure opal-A 1353 1360 8 07/05/22 20220701 NES 220701 Introduction Opal is a mineral term for amorphous and paracrystalline hydrated silica species that occur commonly in vesicles, veins, and fissures of young rocks (e.g., sandstone, chalk deposits, limonite, geodes, and rhyolite) with environments ranging from the deep ocean to the terrestrial ([9]; [33]; [49]; [20]). The PDF refinement used low tridymite, low cristobalite, coesite nanodomain, and twin boundary model, suggesting the occurrence of clusters of six-membered rings of cristobalite and tridymite, four-membered rings, and eight-membered rings in the opal-A structure (Fig. The interstratified phase can account for stacking disorder or interstratification between cristobalite and tridymite in the 4:3 ratio, and the final refined scale factors of cristobalite, tridymite, coesite, and the twin boundary are 0.62, 0.46, 0.22, and 0.10, respectively. [Extracted from the article]

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