The structure of opal-A was not fully understood due to its poorly crystalline nature. To better understand its structural characteristics, we have analyzed opal-AN (amorphous-network) and opal-AG (amorphous-gel) using synchrotron X-ray diffraction (XRD), pair-distribution function (PDF) analysis, and transmission electron microscopy (TEM). Opal-AN mainly exists as an aggregation of different sizes of nanospheres (<5 nm) generating banded features, whereas opal-AG displays close-packed silica nanospheres with a diameter of ~400 nm. TEM energy-dispersive X-ray spectroscopy (EDS) indicates that Na, Al, K, and Ca are present as trace elements in opal-AN and opal-AG. XRD patterns of both samples show one prominent peak at ~4.0 Å, together with broad peaks at ~2.0, ~1.45, and ~1.2 Å. Previous studies only identified the ~4.0 Å diffraction peak for the definition of opal-A. Hence, opal-A needs to be redefined by taking into account the newly observed three broad peaks. PDF patterns of opal-AN and opal-AG reveal short-range atomic pairs (<15 Å) with almost identical profiles. Both phases exhibit Si-O correlation at 1.61 Å and O-O correlation at 2.64 Å in their [SiO4] tetrahedra. The currently accepted opal structure is disordered intergrowths of cristobalite- and tridymite-like domains consisting of six-membered rings of [SiO4] tetrahedra. Our PDF analyses have identified additional, coesite-like nanodomains comprising four-membered [SiO4] rings. Moreover, we have identified eight-membered rings that can be generated by twinning and stacking faults from six-membered rings. The coesite nanodomains in opal-A may be a precursor for coesite micro-crystals formed by the impact of supersonic micro-projectiles at low pressures. More broadly, our study has also demonstrated that the combined approach of synchrotron XRD/PDF with TEM is a powerful approach to determine the structures of poorly crystallized minerals.

中文翻译：

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opal-A结构的复查：同步辐射X射线衍射与对分布函数分析的联合研究

opal-A 的结构由于其结晶性差而未被完全了解。为了更好地了解其结构特征，我们使用同步加速器 X 射线衍射 (XRD)、对分布函数 (PDF) 分析和透射电子显微镜分析了蛋白石-AN（无定形网络）和蛋白石-AG（无定形凝胶） （透射电镜）。Opal-AN 主要以不同尺寸的纳米球 (<5 nm) 的聚集体形式存在，产生带状特征，而 opal-AG 显示出直径约为 400 nm 的紧密堆积的二氧化硅纳米球。TEM 能量色散 X 射线光谱 (EDS) 表明 Na、Al、K 和 Ca 作为微量元素存在于蛋白石-AN 和蛋白石-AG 中。两种样品的 XRD 图谱均在 ~4.0 Å 处显示一个显着峰，在 ~2.0、~1.45 和 ~1.2 Å 处出现宽峰。以前的研究只确定了~4。0 Å 衍射峰用于蛋白石-A 的定义。因此，蛋白石-A 需要通过考虑新观察到的三个宽峰来重新定义。opal-AN 和 opal-AG 的 PDF 模式显示具有几乎相同轮廓的短程原子对 (<15 Å)。两相在它们的 [SiO4] 四面体中都表现出 1.61 Å 的 Si-O 相关性和 2.64 Å 的 OO 相关性。目前公认的蛋白石结构是由 [SiO4] 四面体的六元环组成的方石英和鳞石英状域的无序共生。我们的 PDF 分析已经确定了包含四元 [SiO4] 环的额外的柯石英状纳米域。此外，我们已经确定了可以通过六元环的孪晶和堆垛层错产生的八元环。蛋白石-A中的柯石英纳米域可能是由超音速微弹在低压下撞击形成的柯石英微晶的前体。更广泛地说，我们的研究还表明，同步加速器 XRD/PDF 与 TEM 的组合方法是确定结晶不良矿物结构的有效方法。