Opal... rainbow of the desert

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geyserite

opal spheres

FORMATION OF OPAL

Opal from Hot Spring Waters

Water from thermal sources, whether originating from deep wells or springs in sedimentary rocks, or from volcanic activity, commonly contains silica in solution, and, as has been mentioned elsewhere, may deposit silica as siliceous sinter or geyserite. An interesting early observation on this type of deposit is that by the famous 19th century German geologist, Professor C. Rammelsberg (1859, R1602), entitled 'Ueber den Bianchetto der Solfatara von Pozzuoli'. A translation of this paper reads:

"On the white deposits of the Solfatara of Pozzuoli

The white earthy mass, which covered the ground and the lower slopes of the Solfatara, is a decomposition product of the trachyte caused by fumarolic activity. There, sulphurous acids and hydrogen sulphide accompany the hot steam of these fumaroles, so that sulphuric acid has decomposed the rock, formed soluble sulphates which are found in part as crystalline salts, and silicic acid is deposited, which one can easily mistake for clay or gypsum, of which it actually contains only traces.

During a visit to the Solfatara in August 1858, I collected some of this material, and soon investigated, and proved that it was composed mainly of silicic acid which is permeated with water, free sulphuric acid and small amounts of sulphate salts.

If one treats the white mass with water, one gets a strong, sour solution which contains free sulphuric acid. The same is the case if it is digested with absolute alcohol. It contains 21% water. On standing in a desiccator for a few days, more than two thirds is lost. The dehydrated residue amounted to 69.15% and consisted almost entirely of silicic acid, which showed no crystallinity under the microscope, and of which nine tenths dissolved in hot potash solution within half an hour. The analysis of the white mass gave:

Sulphuric acid 7.81 |

Alumina 0.38 | extracted by

Lime 0.18 )

Potash 1.34 ) water

Silica 0.10 )

Silica 66.84

Alumina 1.40

Magnesia 0.91

Water 21.0 "

(End of translation)

More recently, Watson (R1635) records analyses of water from seven hot springs in the eastern United States. While their salinity varied from 82 to 735 parts per million (ppm), there was much less variation in the actua silica content.

Five of the spring waters contained between 21.5 and 28.5 ppm silica; the other two contained 12.0 and 53.9 ppm silica. It is probable that the water from hot springs deposits silica because of lower pressures and temperatures as it rises to the surface. The remaining silica is ultimately deposited around the spring vents as a form of opal.

A comprehensive study of silica in hot spring waters was made by White, Brannock and Murata (R1445). They stress that most of the silica in these waters is, in fact, in true solution. The hot spring waters they studied contained about 315 ppm silica at 90°C, and 110 ppm at 25°C, in good agreement with the data of Krauskopf. They do, however, make the point that:

"Monomeric silica polymerises so slowly to colloidal silica that many waters are supersaturated with respect to amorphous silica. The rate of polymerisation is influenced by pH, temperature, degree of supersaturation, presence of previously formed colloidal and gelatinous silica, and contact with opal and other substances. Precipitation of colloidal silica is favoured by high temperature and contact with opal."

Amongst their conclusions relating to formation they make the point that opal is formed when silica deposition of rapid, and the temperatures are low. It may occur at temperatures as high as 140°C, but is unstable and is likely to be converted to chalcedony or quartz.

They also observed the formation of gelatinous silica in quiet pools, but this did not seem to form opal. On drying it formed a powdery dust; the identity of this material is not specifically stated, but can probably be assumed to be opaline silica of the type opal-AN. They also state that:

"Masses of gelatinous silica that have remained for a year or more at 80°C to 90°C show no obvious evidence of conversion to opal. Other samples have been stored in water at room temperature for as much as ten years without apparent formation of opal."

It was thought that repeated wetting and drying with further deposition of silica over a number of years probably contributed to the hardening process to form the siliceous sinter. They also make the point that opal of some hot spring areas is an end-product of near surface leaching of silicate minerals by sulphuric acid formed by the oxidation of hydrogen sulphide above the water table; this is consistent with the observations made by Rammelsberg some 100 years earlier.

A curious form of spherulitic opaline silica from hot springs occurs in Japan. Opaque spheres of the order of five mm or more in diameter may be formed, or small, transparent spheres of the order of a millimetre or less in diameter.