A historical discussion of Opal Mineralogy

introduction to an historical view of opal mineralogy

Existing early literature on silica, chalcedony an common opal (names found in older literature) is extensive, and by comparison there is only a small amount of this literature that refers to precious “sphere type” opal that shows a Play-of-colour. Perhaps a number of factors are at work here:

The scientific and mineralogical examination performed on precious play-of-colour opal is more likely to be restricted to atomic structure, or the place of individual atoms of silicon (Si) and Oxygen(O) within that structure, and the overall categorisation of the mineral interactions especially with reference to geology, and rock formation, perhaps just where “opal” as a general name fits in.
Or it is a possibility that the expense involved in obtaining suitable samples of precious opal for destructive testing can be challenging. Good precious opal material may cost as much as $25,000 AUD per gram, other gem quality material is valued even higher. The consequence of this is that most materials used or examined experimentally is of a lower quality. (Editors note, at some point in time higher quality precious opal is going to be needed to be examined).
Much opaline silica in inhomogeneous and may be a mixture of silica opal polymorphs.
Having discussed these idiosyncrasies of mineralogy, it is interesting to note that more often when opal occurrences are discussed these occurrences are directed to a discussion of where precious opal occurs (or commercially used opal) rather than where common opal occurs.

A distinct division in scientific literature

The historical text presented here and published in 1962 presents an overall and correct view of what scientists knew about opal up until that time. Things changed when Australian scientists investigated precious opal in particular in 1964.

Before the pioneering electron microscopy work performed at the Australian CSIRO by Ralph Segnit, John Sanders, Jones and Arthur Gaskin, and Peter Darragh and recorded in the literature listed below, the basic understanding and knowledge of opal formation and mineralogy had remained much the same for over a century. However even though the structure of precious opal was not completely understood, the general descriptive work outlining the optical, physical and chemical properties were essentially correct. The following discussion is taken mostly from the well known mineralogical text of “Dana’s system of Mineralogy” as edited and republished by Clifford Frondel in 1962. You can read an extract of the full text here:

Mode of formation and occurrence- The view in 1962

Editors Note: one part of this discussion remains interesting and may also be pertinent to this discussion, The opal samples given in Table 37 of literature are for 42 samples, none of which are determined as precious opal, and none of these samples are from Australian opal fields, the same can be said for Table 38 which discusses Specific Gravity (SG or density), and refractive index (RI) of a further 31 specimens. Only Table 39 a very brief acknowledgement of precious opal in White Cliffs NSW. It would seem that “Precious Opal was not the discussion of the mineralogy at all, “Precious Opal” from Slovakia (Hungary), Mexico, and Australia appear not to have been examined in the literature reviewed at the time.

Opaline silica was considered as a micro-crystalline aggregate of crystallites of cristobalite, formed inorganically in a variety of ways, which included chemical weathering processes, direct deposition from hot springs, deposition from low temperature hydrothermal solutions at shallow depths and deposition from meteoric circulation. Opal can occur as crusts with botryoidal, globular, reniform or ropy surface as small stalactitic to colloidal masses, and as concretionary masses with a tuberose or irregular shape. Opal is commonly found as a cavity filling and as veinlets or as siliceous sinters or geyserites when deposited from hot springs. Opal can be deposited from the chemical action of circulating ground waters and is deposited in veins and cavities as gelatinous silica. Opal often replaces wood, shells and other organic remains and can be found as the cementing agent in detrital sediments and as detrital fragments in other sediments.

Physical Properties

Opal in its purest form is white to colourless and ranges in diaphaneity from transparent to opaque. Through the admixture of included material the colour of opal can vary from yellow through brown, reddish brown to orange, green and blue, although usually in paler shades. Australian opals are often observed in all shades of grey, dark grey or black. Three different mechanisms causing colour are believed to occur in opal:

Pigmentation by finely divided foreign material.
Interference of light causing the play of colour seen in precious opal.
Scattering of light.

The third mechanism is dominant in the “milky” white types of opal. All three mechanisms can occur in a single opal specimen. Opal streak is white, the fracture is conchoidal but of varying degree. Opal is moderately brittle and may spontaneously crack by a sudden change of temperature or partial dehydration. The hardness is 5.5 – 6.5 on Moh’s scale and the specific gravity is variable from 1.99 – 2.25 depending on both the porosity and water content.

Optical properties

Opal is amorphous and thus isotropic under crossed polarised light, although some specimens show a weak anomalous birefringence because of strain, which is uniaxial negative. The index of refraction of different specimens varies due to water content and decreases as water content increases. However it is usually in the range of 1.435 – 1.455. Inclusions of minute fibre bundles of chalcedony, cristobalite or lussatite (another silica polymorph) are sometimes seen. The colours are seen in reflected light and are of considerable spectral purity. Although the colours are caused by the interference of light, the nature of the structure that results in the play of colour is problematic. Some of the postulations for the play of colour are:

A regularly spaced grid-work of crystallites of cristobalite.
Thin lamellae with a difference of index of refraction next to adjoining material.
Systems of cracks and fissures that are perhaps filled with opal of a slightly different index of refraction, or strains set up during the drying of the initial gel during opal formation.

Chemical composition.

Opal is hydrous silica with a chemical formula, SiO2.nH2O. The water content is quite variable ranging as high as 20 per cent or more. Commonly the range is from 4 – 9 per cent. Some of this water may be held structurally in interstitial positions between the cristobalite crystallites. Most of the water, however is adsorbed. Some of the other principal constituents identified in opal are: Al2O3, Fe2O3, CaO, MgO and alkalies.