U radioactive decay chains to measure the length of time required to reach the current state of radioactive disequilibrium from an initial condition.Opal. Amorphous or only partially crystalline hydrated silicon dioxide (SiO 2 •nH 2 O) precipitated from aqueous solutions in near-Earth-surface environments.
Mineralogy and Geochemistry of OpalOpal is a common mineraloid found near the surface of the Earth that consists of silica containing nonstructural water (SiO 2 •nH 2 O). Opal typically forms by precipitation of colloidal silica from silica-saturated aqueous solutions in response to decreases in temperature, neutralization of alkaline solutions, increases in salinity, or evaporative concentration. Sedimentation of colloidal microspheres (<500 nm) results in deposits of amorphous silica gel. Subsequent dewatering and diagenesis causes gels to solidify and gradually increase their structural ordering. Water contents typically range between 0.5 % and 10 % and tend to decrease with increasing crystallinity.Opals have been distinguished on the basis of their degree of crystallinity determined by x-ray diffraction (Jones and Segnit 1971). These include amorphous opal (opal-A), disordered a-cristobalite with a-tridymite stacking (opal-CT), and more ordered a-cristobalite with minor tridymite stacking (opal-C). Progressive increases in the degree of ordering from opal-A through chalcedony (Fig. 1) have been observed in response to increasing temperature as well as increasing age (Kano 1983;Herdianita et al. 2000;Neymark et al. 2003).Opal is most common in volcanic or sedimentary settings and is associated with alteration or weathering of silicate rocks. Materials display a wide range of colors (known as play of color), textures, opalescence, and luminescent properties. Opal is also classified as gem opal (also precious or noble opal) or common opal (also, potch opal) based on its ability to diffract visible light into a spectral array of colors (Smith 1998). Play of color is the property observed when well-sorted (~150-250 mm) opal microspheres are clustered into ordered arrays (Jones et al. 1964;Darraugh et al. 1965). Common opal forms when microspheres are less well sorted and cluster without orderly packing.