China
Africa
Explore chapters and articles related to this topic
Crystalline Colloidal Array Photonic Crystal Optical Switching
Published in Anwar Sohail, Raja M Yasin Anwar Akhtar, Raja Qazi Salahuddin, Ilyas Mohammad, Nanotechnology for Telecommunications, 2017
Opal is a naturally occurring colloidal crystal, composed of a three-dimensional array of 150–400 nm silica spheres that form and bind together naturally in sediment. Visible light diffracts from the lattice planes, resulting in an iridescence that earns opal a place among the world’smost-prizedgem-stones. Synthetic opals can be made by sedimenting or centrifuging synthesized monodisperse silica spheres in the laboratory for several months to a year or more, after which they are dried in an autoclave and sintered in a furnace at temperatures of up to 1000°C (Filin et al. 2002, van der Beek et al. 2007).
The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia
Published in Australian Journal of Earth Sciences, 2019
J. R. Herrmann, R. Maas, P. F. Rey, S. P. Best
Opal-AG is a low-temperature (<45°C), amorphous and vitreous form of hydrous silica (SiO2·nH2O) consisting of small (100–400 nm) silica spheres and is commonly associated with sedimentary opal deposits (e.g. Darragh, Gaskin, & Sanders, 1976; Sanders, 1964; Vysotskiy, Ignatiev, Khlestunova, Velivetskaya, & Okrugin, 2013). The characteristic play-of-colour in ‘precious’ opal-AG is produced by visible light diffraction within a regular cubic or hexagonal, close-packed array of monodisperse 150–350 nm silica spheres (Jones, Sanders, & Segnit, 1964). Black opal is ‘non-precious’ opal-AG with irregular sphere sizes and a dark coloration ranging from dark grey to black (e.g. Ball, 1985; Banerjee & Wenzel, 1999). It is found in many fields in Australia (e.g. White Cliffs, Coober Pedy, Lightning Ridge, Tintenbar, Mintabie, Welbourn Hill, Andamooka, Coolgardie, and throughout Queensland) and elsewhere, notably the Czech Republic (the so-called ‘Hungarian opal’), Mexico, Virgin Valley (Nevada), Ethiopia (Kieffert, Hardy, Sintayehu, Abate, & Woldetinsae, 2014), Indonesia (Tay, Atichat, Fritsch, Kang, & Wijaya, 2009) and Honduras, but the opal fields of the Lightning Ridge region, NSW, remain the most important source. Black opal from Lightning Ridge is mainly used as a natural or manufactured backing to play-of-colour opal; black opal backing enhances the play-of-colour and contributes to the commercial value of the precious opal.
Which water formed Australian sediment-hosted precious and potch opal?
Published in Australian Journal of Earth Sciences, 2019
Despite over 100 years of study, the means by which these opals were formed has remained an enigma. Many different models have been proposed with a wide range of different processes considered. Major models are reviewed by Rey (2013) but there are many additional models such as one involving alkaline lakes (Kalinin & Serdobintseva, 2003), which are not commonly reviewed. In this paper, the formation of opal is addressed starting from the basis that opal is hydrated silica and hence a product of aqueous chemistry. Thus, it can be described in terms of a conventional mineral deposit formation model represented by the expression where S is the source rock, Fl is the flow of a water that dissolves and transports the components of the mineral from the rocks hosting the flow, Fo is a mechanism that focuses the flow into an area of deposition, and D is a physical (e.g. evaporation) and/or chemical (e.g. oxidation, polymerisation) processes that lead to deposition of the mineral. Here, the {}J expression represents the ‘just right’ environment for a near-surface mineralisation, which results in the mineral formation at a particular place and time and within a particular climate.