Gem opal

Since the beginning of the 2000's, our team regularly contributes to the knowledge on gem opals. Numerous DUG studies  (Connoué, Rondeau, Letuve, Lanz, Mandaba, Villie, Courbes-Huillery...) and 4 PhDs (Aguilar-Reyes, Rondeau, Gaillou, Chauviré) have been devoted to this topic, giving rise to more than 50 scientific contributions (articles and presentations in conferences).

Opal: definition

Opal is an amorphous to poorly crystallized silica. It contains some water, and its chemical formulae is SiO2, nH2O. Water content may strongly vary from a sample to another, generally between 4 to 10%, with extreme values from 0.9% to 21% generally documented in common opals. Opal's specific gravity is about 2, but may vary from 0.8 (for very porous, hydrophane opals, that in this case float on water!) to 2.3. Index of refraction also varies, from 1.37 to 1.47 (representative value: 1.44). Hardness of opal may vary between 5.5 to 6.5.

Micro- to nanostructure

The archetypal image of opal structure is a regular stacking of silica spheres, 150 to 300nm in diameter, not cemented. This network is responsible for visible light diffraction, giving rise to play-of-color typical of precious opal.

MEB opaleA
Opal A from Slovakia observed by SEM. This picture is the front cover of the SILICA book. Shperes are clearly visible, because not cemented. This is actually very rare in natural opal.

MEB opaleA ciment
Most commonly, spheres in opal-A are strongly cemented.

This structure is encountered only in plany-of-color opal-A,where spheres are usually well cemented. Common opal-A, by contrast, are also made of spheres, but do not diffract light because either:

1- their diameter is not homogeneous,
2- their shape is not perfectly spheric,
3- they are either too large or too small to diffract visible light,
4- they are not regluarly stacked into a network.

MEB opaleA concentrique
Common opal-A from Slovakia observed by SEM after soft chemical attack by HF. Shperes show various diameters, and are made of concentric layers of nanograins about 25 nm.

MEB opaleA allongees
Common opal-A from Australia observed by SEM. Spheres are here not perfectly spherical, often elongated.

MEB opaleA petites spheres
Common opal-A from Slovakia observed by SEM showing too small spheres (about 80 nm in diameter) and randomly stacked.

In a general manner, opals are made of spheres built of nanograins about 25 nm.These nanograins form concentric layers to form a sphere. They more rarely form radial arrangements. The cement between spheres is also made of nanograins, but not regularly arranged. Structure of gem opal-CT is also composed of nanograins, but in this case they never arrange into true spheres. However, opal-CT display a great variety of structures, much greater that within the world of opal-A. Most opal-CT, and in particular fire opal (that show a orange-to-red bodycolor), are made of randomly packed nanograins, with no other structure.

AFM opale
AFM (atomic force microscopy) image of a common, fire opal-CT from Ethiopia. Structure is composed of non-arranged nanograins.

Pink opals (from Mexico, Peru) often contain palygorskite inclusion. These fibers act as a template (or a "guide") for the deposition of nanograins.

fibre opale rose
Pink, common opal-CT from Mexico observed by SEM. Nanograins follow the orientation of fibrous palygorskite inclusions.

Nanograins can also arrange into platelets, most often forming piles of 4 to 6 platelets. These platelets can in turn arrange themselves to form lepispheres, otherwise well-known in biogenic opals. However, lepispheres in common gem opal are most often cemented by nanograins (as in the case of opal-A), which is not the case in biogenic opals. On a fresh break, structure is then not visible. Opals must be submitted to soft chemical attack to reveal their internal structure. In the case of lepispheres, these dissolve more rapidly than the cement.

Lepispheres can arrange into a regular, diffracting network, as do true spheres. These are play-of-color opal-CT, which nano-structure is different from that of play-of-color opal-A. 

lepispheres 1
 White, common opal-CT from Mexico observed by SEM. Nanograins arrange to form platelets.

lepispheres 2

White, play-of-color opal from Mexico observed by SEM. Nanograins are arranged to form lepispheres, themselves regularly packed into a diffracting network.

lepispheres 3
Common fire opal from Mexico observed by SEM. After chemical attack, lepispheres are dissolved and nanograins building the cement are more prominent. 

Mode de formation

A l'aide d'un ICPMS en mode dilution, nous avons mesuré la composition chimique d'opales A et CT provenant de 11 pays afin de déterminer quelles impuretés sont présentes dans les opales et en quelle concentration. Les principales impuretés sont, dans l’ordre décroissant en concentration, Al, Ca, Fe, K, Na, et Mg (> 500 ppm). D’autres éléments ayant des teneurs plus faibles sont Ba, puis Zr, Sr, Rb, U, et Pb. La géochimie de l’opale s’est avérée être dépendante principalement de la chimie de la roche hôte, modifiée par des processus d’altération. Nous avons déterminé qu’il était possible de différencier les opales selon leur origine géologique : la concentration en Ba ainsi que les spectres de terres rares permettent de séparer les opales sédimentaires (Ba >110 ppm, anomalies en Eu et Ce) des opales volcaniques (Ba < 110 ppm, pas d’anomalie en Eu ou Ce). Il est également possible de distinguer quelques origines géographiques: les opales de feu d'Ethiopie ont une teneur en calcium bien supérieure (Ca > 1000 ppm) que leurs homologues mexicaines (moins de 500 ppm)

geochimie opales
Diagramme montrant la concentration en Ba en fonction de la concentration en Ca. Les opales ayant une teneur supérieure à 110 ppm de Ba se sont formées dans un environnement sédimentaire, tandis que celles qui ont une teneur inferieure à 110 ppm se sont formées dans un environnement de type volcanique. La concentration en Ca permet de séparer certains gisements, particulièrement les gisements du Mexique et d’Ethiopie où les opales ont pourtant une allure similaire (opales de feu).

En ce qui concerne l’origine de la couleur, nous avons par exemple déterminé que des grandes concentrations en fer induisent les couleurs les plus foncées (de jaune à chocolat). Cet élément induit également une inhibition de la luminescence pour des concentrations supérieures à 1000 ppm.

Perfect diffraction in opal

This video shows a 4.43 ct oval cabochon of white and play-of-color opal from the Wollo Province, Ethiopia. The play-of-color does not form large patches of spectral colors, but a network of sharp points of spectral colors moving together as the white light source (an optical fiber) moves around the sample. Note that the color of each spot slightly changes with the position of the white ligth source. All this indicates that this sample is made of a single optical crystal, i.e. a single network of silica spheres running from side to side of the sample, over one centimeter. This also explains that the diffraction produces sharp points and not large areas. This is absolutely exceptionnal.
From a crystallographical standpoint, this feature is like a Lauegram, first described by Max von Laue for the diffraction of X-rays by a (true) crystal. As opal diffracts visible light, such sample allows the visualization of the reciprocal lattice responsible for the diffraction of light. The reciprocal lattice is the Fourier transform of the "crystal lattice", i.e. the direct lattice of the silica spheres network. If you wish to learn more on the crystallography of opals, follow the DUG program!

This extraordinary phenomenon indicates a perfectly continuous network of silica spheres over more than one centimeter in all directions. Hence, the conditions of formation of such an opal must have been remarkably quiet, with no perturbation during the "crystallization" of this opal..

Additional information