We're working on a new format for reference work entries, and would like your opinion. Uranium–lead dating. A geochronological method that uses final decay products in the 788 U and 785 U radioactive decay chains to determine the length of time required to accumulate present amounts of stable daughter isotopes 756 Pb and 757 Pb, respectively. Opaline silica (opal). A hydrated amorphous form of silica (SiO 7 ∙nH 7 O) that commonly contains between 6 % and 65 % water by weight. Due to its amorphous nature, opal is categorized as a mineraloid (Jones and Segnit Smith ), unlike other crystalline forms of silica (e. G. , quartz, cristobalite, and tridymite) which are classified as minerals.
Opal typically occurs near the Earth’s surface where it is deposited in the voids from silica-saturated circulating waters at relatively low temperatures. To view the rest of this content please follow the download PDF link above.