The ‘fire’ of opals

Abstract

T he mineral opal, chemically a form of hydrated silica, is found on practically all continents, but mostly as an “opalescent”, milky–white, soft rock. However, in some parts of Australia small pieces of beautifully coloured gemstones, “precious opal”, are to be found embedded in a matrix of ordinary opal. What makes these quintessentially Australian gemstones sparkle with flecks of pure spectral colour (Fig.1)? Oddly enough, the answer to this question was a mystery to mineralogists for a long time until noted CSIRO electron microscopist John Sanders discovered the surprising answer as recently as the 1970s [1],[2]. Because of the spectral colours exhibited, the phenomenon of diffraction from periodic features was suspected to be the cause, but nobody could guess at the nature of such periodic structures until they were revealed by electron microscopy. It was surmised that the optical properties of precious opal, as distinct from the milky-white appearance of common opal that shows no ‘fire’, depends on the existence of orderly, regular arrays of optical discontinuities, spaced at repeat distances of the order of 150 to 350 nanometers, i.e., distances that correspond to half the wavelength of visible light. Chemically, opals are made of pure, transparent, hydrated silica, i.e., hydrated Silicon Dioxide. But what the electron microscope revealed was that the silica is in the form of tiny spheres, of the appropriate range of sizes, stacked in close-packed regular arrays, as may be seen in Fig. 2, just like atoms or molecules in crystalline substances. How are these little spherical objects formed is answered by noting that the solubility of silica in water increases markedly with temperature so that, upon cooling, silica is usually deposited as quartz crystals that are said to grow in what is called a hydrothermal process. Alternatively, in the presence of centres of nucleation, the silica can precipitate from saturated solutions in the form of amorphous clusters. These continue to grow as concentric spheres, which then fall through the solution and end up in interstitial cavities. In most cases, a poly-disperse range of sizes results, which when dried out results in a milky-white solid of ordinary, or so-called ‘potch’ opal. However, in rare cases, where the little spheres have a greater distance through which to fall, a gravitational separation can take place, where the larger spheres fall more quickly than the smaller ones and arrange themselves in layers upon layers of uniformly sized regions of hexagonal close-packed groups, like oranges in a crate. Hence the quasi-crystalline arrangement of precious opal, usually in small pieces consisting of separate small regions, is analogous to crystal grains. The opal ‘grains’ can vary in size, from a few millimeters, known as pin-fire opals, up to quite large ones in what is known as boulder opals. Because the individual silica spheres are completely transparent, pieces of opal show practically no colour when viewed in transmission. However, when viewed in reflection, strong diffraction colours are seen. These diffraction phenomena are not like those from a two-dimensional grating, such as is seen from the surface of a CD or DVD, and erroneously shown in illustrations in some popular articles. On the contrary, 3-dimensional diffraction