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hexagonal crystal
citrine
amethyst
2 tonne quartz crystal
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SCIENTIFIC DISCOVERIES

The Chemical Composition of Opal

In this section we will look at the following areas:

Introduction
Water in Opal
 
Dehydration of Opal
How is the Water Held?
Constituents other than Silica & Water
 
Major Constituents of the Residue
Minor Constituents of the Residue
Organic Constituents

Introduction

Most collectors, lapidarists and others interested in minerals and gemstones are aware of the fact that opal is a poorly crystalline form of silica containing some water. Silica itself is an oxide of silicon (silicon dioxide) with the chemical formula SiO2. At least one other oxide of silicon exists, silicon monoxide (SiO), but can only be formed in the laboratory; it does not occur in nature.

Silicon dioxide exists in many forms. The commonest is the mineral quartz, which occurs in a variety of physical forms. It contains no water, and is represented by the chemical formula SiO2. It is best known in the form of hexagonal crystals generally colourless, but may be almost black, brown, yellow, purple, pink, or more rarely, greenish or bluish in colour.

The coloured varieties are well known gemstones: cairngorm or morion (brown), the traditional stone of the Scots; citrine (yellow); amethyst (purple); rose quartz (pink). The colourless material is termed rock crystal.

The crystals may be of great size; a crystal exhibited in the Heimat Museum in Idar-Oberstein in Germany is nearly two metres high and weighs two tonnes.

However, Bauer and Bouska (R1599) in their book on precious stones state:

"Giant quartz crystals are known from Madagascar where crystals with circumferences of several metres around the prismatic faces have been found in local pegmatites."

One such crystal had a circumference of 7.5 metres and weighed some 45 metric tons. In September 1958 a crystal was found in Kazakhstan in the USSR that was as high as a two-storeyed house. Its estimated weight is 70 metric tons.

Flawless quartz is an important industrial material in the electronics field, where it is used as an oscillator, although nowadays most of the quartz for this purpose is synthetic. In fact quartz crystals of any colour can be grown, and the corresponding natural gemstones can thus be easily synthesised.

Quartz is also found abundantly as a constituent of many rocks. It is a necessary constituent of granite, and is the main constituent of sedimentary rocks such as sandstone, and metamorphic rocks such as quartzites.

It occurs in appreciable amounts in most sedimentary rocks except the carbonates (limestones and dolomites, for example), and in most metamorphic rocks, again except for the carbonates.

The weathering of these rocks (quartz being the most resistant to weathering), frequently forms accumulations of grains to form the sands of our beaches and creeks. Fine grained quartz often accumulates with clays.

Very finely crystalline aggregates of minute quartz crystallites are collectively called chalcedony (pj81). It is a hard, tough material with which, in the form of flint, man established his first manufacturing industry. It can be chipped and flaked to form useful tools and weapons, often with very keen edges.

Flints are abundant in the chalk deposits of southern and eastern England, as well as in Europe, and the remains of large prehistoric industries of this type are to be found in pits in the chalk deposits. In the middle ages, and even up to more recent times in these areas, flints were used as a building stone. They were of a convenient size to handle, so did not need physical quarrying or shaping, and are extremely resistant to weathering.

Many other forms of chalcedony occur; chert is an abundant type which commonly forms grey bands in bedded limestones. There are various forms of coloured chalcedonies, which, because of their beauty and durability, have been used as ornamental stones for thousands of years.

These include such stones as onyx, agate, chrysoprase,, prase, bloodstone, carnelian and others. These materials were commonly used for cylinders and other types of seals by people of ancient civilisations. They were often intricately carved and engraved, and were very durable.

Some forms of chalcedony are so fine grained that they are sometimes difficult to distinguish from common opal and in some cases may have been formed by the transformation of the less stable opal to the more stable chalcedony.

Chalcedony also differs from other types of quartz in that it usually contains a small amount, about 1%, of water, probably held in minute pores between the tiny quartz crystallites. The colours of the various types are caused by 'impurity' elements absorbed during formation of the material.

Other types of silica are rare or non-existent in nature. The best known are cristobalite and tridymite. These are occasionally encountered in small cavities in volcanic rocks; they are formed at high temperatures.

Theoretically, on heating, quartz transforms to tridymite at 873°C, and tridymite transforms to cristobalite at 1470°C. However, the changes occur only slowly, and are complex, as each of these minerals themselves have two or more forms (polymorphs).

Quartz has two forms, tridymite three, and cristobalite two. These latter forms convert readily into one another within their groups. This is an important phenomenon industrially, as all of these forms of silica are frequently encountered, and are often essential components of refractories, slags, and other ceramics.

Even rarer are those forms of silica which are formed only at very high pressures. Such forms are coesite, stishovite and keatite. They are formed in the laboratory under very high pressures, and the first two have been found in the environments of craters formed by the impact of large meteorites with the earth. Enormous pressures are generated on impact, and quartz in the impacted rocks has in some cases been converted to one or another of these forms. One of the best known localities for coesite, for example, is the great Meteorite Crater in Arizona. Keatite has not been found in nature.

The melting point of pure silica is 1723°C. It melts to a very viscous liquid, and when cooled does not tend to crystallise, but forms a glass. Occasionally natural silica glass is found. Such high temperatures are rare in nature, but can occur in, for example, a lightning strike. Silica glass has sometimes been found in sandy areas where lightning has struck; it has been called lechatelierite. Silica glass is readily made industrially, and is a valuable material for a variety of purposes in industry and in the laboratory.

'Haystack glass', formed by the combustion of large quantities of hay or similar materials, is an impure form of silica glass. Its melting has been assisted by the absorption of impurities such as potassium, sodium, calcium and aluminium in small amounts.

Other forms of silica containing water are known in nature. The rare form silhydrite has the chemical formula 3SiO2.H2O. It is known mainly from a spring deposit in California, and has been formed by the natural leaching of sodium from the silica-rich hydrated mineral magardiite. The other form of hydrated silica common in nature is opal.

The Water in Opal

Water is regarded as an essential constituent of opal and opaline silica. Its content is variable, ranging from about 3% in true hyalites to up to 20% and more in some earthy types such as diatomite. The chemical formula of opal is therefore usually represented as SiO2.nH2O.

A large number of determinations of water in opal have been made over the last 150 years or more, and it is of interest to plot the percentage of water found (at, say 0.5% intervals) against the number of analyses for each 0.5% range. An illustration has been compiled from some 170 analyses.

In some cases the analyses are weighted towards a specific occurrence, but the general trend is clear. Two main maxima occur, one around 3-3.5% water, and the other in the range 4.5-9.5% of water. Many of the opals associated with the lower maximum are hyalites, the glass-clear mineral found mostly in association with volcanic rocks.

The main maximum covers most other opals, including common and gem varieties. The samples showing around 1% water are most likely to be chalcedonies wrongly reported as opals, although it is possible that some volcanic opals have been dehydrated. Those with very high water contents are most commonly earthy types such as diatomites.

Dehydration of Opal

Most of the water in opals is not tightly held. In fact careful weighing between summer and winter can sometimes detect a weight change, the higher temperatures of summer driving off a small amount of water. Further heating results in a greater loss of water.

The loss of water by heating is most clearly shown by differential thermal analysis (DTA), a technique in which the powdered sample is heated at a uniform rate, and the temperature of the sample monitored against a neutral material. In general, loss of water from the sample causes a change in its temperature; it becomes slightly cooler than the furnace temperature.

This is recorded on a graph; if the change in temperature is sudden, a peak appears on the graph. If the water loss is slow and gradual, no peak is seen. Parallel with the DTA determination it is usual to monitor the weight change of the sample. This technique is called thermogravimetric analysis (TGA) and records continuously any weight change, usually a loss in weight.

In the case of opals, variable results are obtained. A graph shows a selection of DTA curves for different specimens of opal. They range from a typical sedimentary gem opal, to common opals. Gem and related opals (potch) seldom show a peak (termed an endotherm) associated with water loss, indicating that the water is slowly driven off as the temperature rises (Curve A).

Common opals show a good deal of variability in the shape of their DTA curves. Curves B and C are the type usually obtained from common opal (opal-CT in the main). However the white and translucent opals illustrated here, which are identical in most respects, give different DTA curves. The white material shows a marked endotherm, while the translucent gives a result similar to that obtained for gem opal. In this case the difference was simply due to a system of microcracks in the white opal. A scanning electron micrograph clearly shows the cracks in this opal. Other samples showed minor variations in the shape of the endotherm, and one sample even showed two endotherms.

It can be seen from these curves that water is easily driven off from opal. Most of it is gone by 200°C, although small amounts cling on until much higher temperatures. In pure synthetic silica powder, traces of water are retained up to 1100°C.

How is the Water Held?

The different types of differential thermal analysis (DTA) curves obtained from opals and opaline silicas were the catalyst which resulted in a considerable amount of work being carried out in recent years to determine the nature of the bonding of the water in opaline silica.

Most of the earlier work, some of which has been outlined previously, dealt mainly with the actual amount of water in the opal, and its effect on physical properties. The DTA work of Jones et al (R0371) suggested that there were different ways in which water was held in opals from different sources. They noted three main types of DTA curves which they classified as being derived from, in general, three types of opal:
1. Most glassy opals, including precious opal. These produced a DTA trace showing few or no inflections, and certainly no peak corresponding with a loss of water.
2. Most opaque opals. These usually gave a definite rounded endothermic peak between 100°C and 200°C corresponding with a loss of water.
3. A small group of translucent red or brown opals, which gave a sharp water-loss peak at a little above 100°C.

This classification was somewhat simplistic, as later work was to show, although it was observed at the time that this limited evidence indicated that most of the water in opal was not chemically bonded.

Detailed work had been published on the bonding of water by synthetic amorphous hydrous silica, and this was relevant to the nature of the water in opal. Later work on opal took note of this, with investigations on the nature of the water in opal using infrared spectroscopy, nuclear magnetic resonance, and a more detailed examination by thermal techniques (see bibliography).

All of the techniques indicated that most of the water was physically absorbed in the internal pores of the opal. The rate of release of this water, either by dehydration at constant temperatures, or by continuous heating was variable, a phenomenon explained by the physical structure of the particular opal.

A good example of this is the Lake Eyre wood opal, described in the previous section. The clear variety lost water slowly, the white rapidly. Scanning electron micrographs showed that the latter, milky variety, contained a network of minute cracks which allowed water to escape rapidly, while, with the clear variety, it must necessarily diffuse mainly through the silica gel structure. Nevertheless, most of the water, in both cases, was lost by 200°C under isothermal conditions. This result was confirmed by other methods, indicating that most of the water was physically held.

 

Other opals gave similar results, but, in some cases, less water was evolved at the lower temperatures; this was particularly the case with X-ray amorphous opals, those with a physical structure of sub-microscopic silica spheres, either uniform in size and close packed (precious opal) or irregular in size (potch).

It was clear that the spaces between the spheres could hold free water, but, after this was evolved, there was found to be a considerable proportion of the water still in the structure. This was attributed to surface bonding of hydroxyl groups. The silica spheres themselves were composed of very small (10-20 nm) particles of silica which suggested a very high internal surface area.

A calculation indicated that the internal surface area of the opal based on these small particles was 200-100m2/gram. A complete coverage of these surfaces by hydroxyl groups requires a water content of 2.4-1.2% water.

Many of these phenomena may be explained by the widely accepted view that all silica surfaces tend to be covered by a layer of (OH) groups, either as single hydroxyl groups, or as pairs of hydroxyls.

The twin groups may lose water by condensation of neighbouring hydroxyls at as low a temperature as 200°C under vacuum, but the reaction is reversible. At 400°C, however, changes take place in the nature of the silica surface, and the twin groups cannot be regenerated. With increasing temperature, further condensation of hydroxyls to form water from the silanol groups occurs, with consequent gradual loss of water.


A more detailed analysis of the water in several types of opaline silica was carried out by Langer and Flörke (R1551). Using polished plates for infrared spectroscopy, they were able to differentiate more clearly between molecular water and -SiOH groups in the opals. They were able to evaluate quantitatively the contents of 'water' of different types in the samples. They gave the estimates for the three different types of opal they examined (four samples of opal-AN, five of opal-AG and three of opal-CT).

The figures for opal-AG and opal-CT agree fairly well with the figures suggested by Jones and the author, who did not examine opal-AN in detail.

 

Langer and Flörke also noted two types of molecular water:

"single, almost nonhydrogen bonded molecules in small cages (diameter ca. 0.35 nm) of the SiO2-matrix (type A); liquid-like, hydrogen bonded water as film on inner surfaces (type B)."

They found the highest fraction of type A water in opal-AN, and the lowest in opal-CT. They also found two types of -SiOH groups, A and B, the latter with stronger hydrogen bonding.

In the case of precious and related potch opals (opal-AG), with their silica sphere microstructure, the type A water is held within the silica network 'cages' inside the larger spheres. The type B water is held, largely by hydrogen bonding, in the pores caused by the close packing of the larger spheres.

Constituents other than Silica and Water

An indication of the amounts of the foreign constituents may be obtained by evaporation of the opal sample with hydrofluoric and sulphuric acids, followed by ignition at about 1000°C. The residue will consist largely of sulphates of alkali and alkaline earth elements, and oxides of the heavier elements.

Occasionally, opals are found which are remarkably low in any components other than silica and water. A hyalite from Victoria yielded a residue of 0.14 weight percent of the original sample, and an opal-CT from South Australia left a residue of only 0.08%. Most opals contain less than three percent by weight of foreign elements calculated as oxides. Of 75 opals analysed in this manner, 70% had a residue of less than 3%. Six opals had a residue of more than 10%; the maximum was 23.4%.

A triangular plot of silica/water/residue ratios is instructive; the concentration of compositions in the 5-10% water and 0-5% residue area is clearly shown.

Major Constituents of the Residue

The major constituents of the residue are usually iron, aluminium, calcium, magnesium, sodium and potassium, although in widely varying proportions. A survey of published and unpublished chemical analyses of opals of all kinds, as well as analyses of the residues from the hydrofluoric-sulphuric acid treatment, also shows that there are certain patterns in the chemistry of the 'impurity' elements.

A triangular plot shows the distribution of residue compositions on a three component diagram with iron oxide at one corner, calcium and magnesium oxides at another, and aluminium oxide at the third corner.

It can be seen that there is one concentration of analyses (group A) towards the iron oxide corner; that is, if the iron content is high, there are only small amounts of the other elements. Along the base of the diagram, there are scattered a group which tend to be high in alumina, often with some alkaline earths, but always low in iron.

Those samples labelled 'S' on the figure are siliceous sinters and geyserites; these clearly make up a characteristic group (group C). There is another group which plots towards the (Ca,Mg)O corner of the diagram (group B). These tend to be rich in magnesium; they are labelled 'M'. The latter are, in fact, compositions of the residues of an opaline material which was called 'menilite' in earlier days.

Finally, in the central part of the diagram there are numerous samples which form a scattered field. These largely comprise precious opals (opal-AG) and ordinary common opals, mainly opal-CT, which can have a wide range of ratios of the common elements of the residues.

 

Such concentrations of certain elements suggest that the pattern is related to the mode or environment of formation of the particular groups of opals. A selection of analyses of some iron-rich opals and the corresponding residues is given in a table. All of these opals were of the opal-CT variety. The optical microstructure usually showed an inhomogeneous mixture of iron oxide material in the opal.

The iron content was present in various forms in these samples:
19023: This opal, of a dark brown colour, occurs in pods in a basalt. The XRD pattern was of opal-CT; no form of iron was detected, so it was evidently present as an amorphous admixture.
19236: This sample was yellow brown in colour, with a shiny conchoidal fracture. It also occurred in a basaltic environment, but was collected from decomposed surface material. XRD indicated opal-CT, but no crystalline iron oxides.
21588: This was obtained from veins in a volcanic tuff; it was brown in colour. The XRD pattern showed highly disordered opal-CT; the iron was present as goethite, and there was a little quartz.
19002: Another opal-CT with no crystalline iron oxide. Its associations are not known.
19032: Also opal-CT with goethite and a little quartz; associations are not known.

A red opal (residue content 14.6%) from Cooma, New South Wales was highly disordered opal-CT; at least some of the iron was present as hematite. Most of these opals appear to have been formed at a late stage in basaltic rocks, which have presumably been the source of iron. One would also expect other ions to be present in this type of environment. However, iron hydroxide can precipitate at a much lower pH than most other common elements; it can begin to precipitate at pH3. Therefore, if silica precipitates in a slightly acid environment, and iron is present, the two materials can co-precipitate, with only minor contamination by other common ions.

 

Bayliss and Males (R1504) analysed precious and potch opals from three of the major opal fields of Australia. The major constituents of the residues, which were low in iron, were alumina and lime. These analyses have been recalculated and presented in a similar form to those of the iron-rich-opals for more direct comparison. The total residue content is considerably less than in the iron-rich opals.

There were no very significant differences between the compositions of potch and precious opals, and this is not to be expected in the present knowledge of the relationships between the formation of these two types.

In general, where a high aluminium content is found in opals of whatever type, there is usually a low iron content, and vice versa. There are, of course, some exceptions to this; a precious opal from Nevada, for example, is recorded as containing 3.22% Al2O3 and 1.85% Fe2O3. Precipitation at a somewhat higher pH will engender the co-precipitation of these two elements.

The major 'impurity' elements in the opals from the Australian opal fields are dominated by the major elements in the weathering environment, in this case, aluminium and calcium. The Cretaceous beds associated with opal deposition are largely pale sandy clays, and the movement of aluminium in the environment is shown by the presence of alunite, which is abundant in some areas. Calcium is also plentiful, especially in the opal horizons, in the form of gypsum.

Another group of opaline silicas which are low in iron but high in aluminium are the geyserites and siliceous sinters. These are formed around volcanic vents and hot springs where they have been brought to the surface and deposited by steam and hot water. The compositions of the residues from this group form a narrow zone across the low iron area illustrated in a triangular plot.

In the case of common opals (opal-CT mainly) the residue compositions vary widely, and are clearly to be related to the environment in which the opal formed. This is also illustrated and shows the distribution of the main constituents of the residues of a wide variety of opals and opaline silicas, including precious opals.

In general, therefore, it can be stated that the main 'impurity' elements which are found in opals and opaline silicas are iron, aluminium, calcium and magnesium, and smaller amounts of sodium and potassium. The relative amounts are determined by the chemical environment in which the opal is formed; in some cases, especially with those with a high iron content, pH probably plays an important part.

Minor Constituents of the Residue

In addition to the major constituents of the residue from the evaporation of opaline silicas with hydrofluoric and sulphuric acids, a wide range of minor constituents have been found. Apart from alkalies, which are usually present to some extent, there occur a variety of elements in trace amounts. Opaline silicas, have, by the nature of their physical structure, a large internal surface area exposing a large silica/hydroxyl surface to the chemical environment.

This surface is capable of adsorbing significant amounts of the trace elements occurring in the environment of formation of any particular opal. In other cases, where we note a high phosphorus content, for example, it is likely that another mineral has co-precipitated with the opal. This could be the case, for example, with sample 17048, where high rare earth contents are associated with a high phosphorous content. In the case of C26, it is probable that barite has been coprecipitated with the opal.

As an example of the range of elements, the diagram shows the results of semi-quantitative spectrographic analysis of four opals from Australia and New Zealand, together with the average content of these elements in the earth's crust for comparison. These analyses, as well as the germanium analyses referred to below, were carried out on behalf of J.B. Jones and the author by the Australian Mineral Development Laboratories, South Australia.

The high content of gallium in the siliceous sinter probably reflects the association with aluminium, which appears to be a common element in this type of opal. Similarly, the high lithium and rubidium contents reflect the volcanic (hydrothermal) origin of the opal.

The heavy metal content of some opals is surprisingly high; these metals are probably absorbed on the silica surface. They must reflect an abnormally high content of these metals in the local environment, suggesting the possibility that opaline silicas may be a useful geochemical exploration tool.

There are many references in the literature, right back to the 19th century, of opals containing surprisingly high contents of unexpected elements or compounds. For example, Royer (R1552) described an opal rich in arsenic from St. Nectaire (France). Ahlfeld (R0389) examined opal from an antimony deposit in Argentina, and found up to 17.9% antimony, probably present as the oxide. Gerasimovski (R1023) found, on blocks of sodalite syenite from the Kola Peninsula (Russia), incrustations of opal containing nearly 8% sodium oxide, as well as fluorine.

Payne and Mau found (R1346), in an altered basalt from Kilauea (Hawaii), an opal containing 10.9% titanium oxide. High contents of sulphur, usually as sulphate, have been encountered in opal from hot springs. Rammelsberg (R1602) found 7.8% sulphur (19.5% sulphur trioxide, SO3) in a geyserite from Pozzuoli (Italy), while Payne and Mau reported 2.7% SO3 in the Kilauea opal.

Some of the older references also reported carbonate in opal. Kokta (R1582) quotes Schaffgotsch as finding 9.1% calcium carbonate in a 'Schwimkiesel' (geyserite) from St Ouen (France), and Damour as finding 3.2% carbon dioxide (= 6.2% calcium carbonate) in an opal from Buttes-Chaumont (France).

The low germanium figures are also of interest. Of 48 opals and opaline silicas, both opal-A and opal-CT, analysed for this constituent by emission spectrography, 42 yielded less than one part per million (ppm) germanium; the maximum content observed was 2 ppm. For comparison, a chalcedonic (mineralogically quartz) petrified wood contained 50 ppm. It is clear that the conditions under which opal forms separates germanium from silicon.

Organic Constituents

From time to time, organic constituents have been reported in opaline materials. One area of interest in this field is in the nature of the famous black opals from the Lightning Ridge field in northern New South Wales. This variety of opal, which, when it was first displayed on the world markets was considered to have been treated in some manner, has a dark background against which the diffracted colours look their best.

The origin of the black colour of the background has long remained a mystery, but recent work suggests that it is caused by the presence of small amounts of dark organic material or finely dispersed carbon. An analysis of black potch from Lightning Ridge, using the technique for determination of carbon in steel, showed the presence of 0.16% carbon. If very finely dispersed, this is probably enough to cause a dark body colour. It may be mentioned that elements such as iron, sulphur and manganese were not detected in the sample.

Organic matter in opaline materials has been reported from other sources. A black opal was reported from a pegmatite at Volyn, (former) USSR (Gigashvili, R0255). It appears to have been an opal-CT, and firing under vacuum at 600°C caused blackening of the sample, with condensation of an oily liquid on the cold parts of the sealed tube. The organic material had a carbon/hydrogen ratio of 84.7/15.3, which was close to the ratio for natural petroleum.

Theories of origin were:
1. Migration of bituminous materials from country rock and concentration with the primary minerals at a late stage of mineralisation.
2. Formation of organic materials from aqueous colloids and co-deposition with colloidal silica.
3. By reaction between carbon monoxide and hydrogen in the presence of bivalent iron (from pyrite) as a catalyst.

A third reported occurrence was in an opal claystone from Kulkurna Station, west of Wentworth, south western New South Wales. The earthy material was white in colour, but DTA recorded a sharp exotherm at 228°C. Chloroform extracted footnote organic compounds which made up some 0.1% of the opal claystone. Much of this organic matter comprised a series of alkanes and branched and cyclic hydrocarbons.

The conclusions were:

"The exact origin of the organic matter in a rock of this type is uncertain but adsorption of carbonaceous material on colloidal or hydrated silica may have occurred during sediment deposition. While some organic molecules may have been oxidised or solubilised during diagenesis, hydrocarbons present as films on particles are likely to have been retained and could even have controlled the formation of the structural lattice. However, the high percentage of hydrocarbons in the total organic extract (35%) and the smooth distribution of the normal alkanes having a maximum at C25 could also be explained by recent contamination of the sample with a refined petroleum product."