A relation between sphere size and chemical composition of Australian opal

Opal forms in dissolution-reprecipitation processes in an interplay of factors such as pH, solution chemistry, host rock composition, temperature, pressure and time. The key to their understanding is on the chemical and structural micro- and nanoscale. Opal-A has a very low degree of order on the atomic scale. In contrast, on microscale precious opal is characterized by highly ordered stacking of monodisperse spheres. Despite progress in the synthesis of opaline structures, there is no satisfactory answer to the ordering process leading to natural photonic crystals due to the multiple influences in geological systems. Also, the processes leading to the preservation of microscopic structures through opal pseudomorphs are widely unknown. We investigated silicified materials and their host rocks from the opal fields of Andamooka (South Australia) and Yowah (Queensland) by petrographic microscopy, XRD, SEM, Raman spectroscopy and EMPA. The opaline silica filled voids and replaced several substances such as fossil shells, trigonal crystals, clasts in a silicified breccia, wood, ooids and rhombic, elongated minerals of uncertain origin. SEM investigations reveal the abundant presence of opal-A, commonly consisting of spheres 150-260 nm in diameter. Monodisperse spheres with a relative standard deviation in sphere diameter of less than 9% are composed of up to three shells. XRD and Raman spectroscopic investigations show that opal-A initially possesses varying degrees of crystallinity. The main broad Si-O-Si band in Raman scattering spectra at 412±6 cm-1 and the main peak in X-ray diffractograms at 4,004pm0,011 A (FWHM of 7,1pm0,4 2theta) are negatively correlated. The mean water content of opal-A is in the range of 4 - 8 wt%. Impurities of all opal-A samples are Al, Fe, Ca, Na and K, all but Al appearing only in traces. Polydisperse spheres have more uniform element concentrations than monodisperse ones. Recrystallized fossil shells with perfectly replicated polysynthetic twinning and cleavage planes contain the highest concentration of impurities and consist of well ordered spheres with distinct monodispersity. Opal-A consisting of mono- or polydisperse spheres can be discriminated based on the proportions of monovalent cations. The average size of opal-A spherules correlates positively with the sodium content, implying an effect of sodium on the polymerization of silicic acid or the aggregation rate of primary silica particles eventually leading to the formation of larger particles. The most striking difference in the opal chemistry is theNaKratiorestricting the appearance of monodisperse spheres to values below 1.3 and highlighting the importance of colloid chemical aspects for growth and ordering processes. Accompanying minerals like alunite, k-feldspar and illite indicate large pH variations during the deep chemical weathering of the near-surface opal bearing profiles and may serve as a tracer for the solution composition. We conclude that the NaK ratio represents different formation conditions that vary in pH and anion composition and have strong influence on the stability of colloidal silica. It seems plausible, that sedimentation plays only a minor role in the formation of regular arrays and that other factors have to be considered to explain the self-assembly of spherical colloidal silica.