Silver is widely distributed in nature, occurring in the crust of the earth at concentrations of about 80 parts per billion...

In the beginning...

Silver has been known by humans since prehistoric times, and its discovery is estimated to have happened shortly after that of copper and gold. One of the earliest reference to the element appears in the book of Genesis (13, 2). The Egyptians considered gold to be a perfect metal, and gave it the symbol of a circle. Since silver was the closest to gold in perfection, it was depicted as a semi-circle. Later this semi-circle led to a growing moon symbol, presumably due to the likeness between the shining metal and the moon glow. The Romans called silver argentum, and we keep this alive by retaining Ag as the chemical symbol for this element.

Silver, like gold, was considered by the Ancients as an almost sacred metal and consequently, it had extremely restricted use. Because of its malleability and ductility it was easily worked and this, coupled with the lustre, made it ideal for ornamental purposes. It was also used for paying debts, for decoration in religious places, as utensils in the wealthiest houses and, of course, for personal adornment. Silver ornaments and decorations have been found in royal tombs throughout the world dating back as far as 4000 BCE.

Some mineral slags from old mines in the near-East and from some islands of the Aegean Sea reveal that, by 5000 BCE, a method was already known to extract silver from lead ore. The silver mines worked by the Carthaginians in Spain were well known and Roman envy of this wealth helped to bring about the Punic Wars.

Silver connections

With such a long history, few people are unaware of silver. Most peopleís knowledge stretches to knowing that it is a precious metal and it is mined. But where is it found, and how does it occur? Names such as Silver City in New Mexico (where Billy the Kid spent his early years and where Apollo 17 astronaut and geologist Harrison 'Jack' Schmitt grew up), Silverton, NSW, Australia (where some Castlemain xxxx beer advertisments were made and where Mad Max strutted his stuff) and our own (Scottish) Silver Glen near Alva, openly testify to the occurrence of nearby silver. Other localities are less obvious but are of no lesser importance and there are few countries throughout the world where silver is not found. We are familiar with the appearance of silver in jewellery but how does it look in its unprocessed state?

Silver minerals

There are 248 mineral species that contain silver as an essential component. Very very few names give an indication that the mineral contains silver. Minerals such as argentite (Ag₂S) and the more obscure argentocuproaurite ((Cu,Ag)₃Au) obviously indicate a silver content but what of eskimoite (Ag₇Pb₁₀Bi₁₅S₃₆) and vikingite (Pb₅Ag₂Bi₆S₁₅) and the almost unpronounceable uchucchacuaite (oo-chew-cha-co-a-ite for those willing to try) (AgMnPb₃Sb₅S₁₂). These last three belong to the sulphosalt category.
 

Silver occurs as a wide range of minerals and also in its elemental form as native silver. Silver-bearing minerals are widely distributed but only a small number can be considered common.

Silver halides are found in the near surface oxidised portions of ore-bearing lodes, especially in arid climates. Small amounts of silver form in the oxidation zone as the more complex compounds erode and weather (sulphur depletion). At deeper levels silver occurs as sulphides, arsenides and antimonides (compounds of silver with sulphur, arsenic and antimony respectively). In these deposits, formation is the result of deposition from primary hydrothermal solutions. Argentite (Ag₂S), silver sulphide, occurs in low temperature hydrothermal veins in association with other silver minerals, or sometimes in the cementation zone of lead and zinc deposits. Most silver is recovered from argentiferous galena and tetrahedrite or produced as a by-product in porphyry copper deposits, where it is present as a minor constituent in chalcopyrite. The contrast in silver mineralogy among deposits reflects differences in the availability of sulphur, semi-metals and base-metals.

Native silver is rare and often consists of wires that are curved and intertwined together, making an inspiring mineralogical curiosity. Such specimens are highly prized by collectors.

Silver was mined at Kongsberg, in south-eastern Norway, from the early 17th century until the mid-20th Century. The mines are one of the most famous silver localities in the world and produced some of the best silver specimen ever seen. The finest preserved specimens are today located in the Kongsberg Mining Museum and in the Geological Museum in Copenhagen. Natural wire silver has become so desirable that some dishonest people have resurrected an old recipe to manufacture wire silver by sublimation and are passing on the synthesised wire silver as natural.

Other silver minerals are also highly desirable, notably the so-called ëruby silversí proustite (Ag₃AsS₃) (see cover illustration) and pyrargyrite (Ag₃SbS₃). These are isostructural, proustite being the rarer of the two. Proustite and pyrargyrite occur as a late-stage mineral in hydrothermal veins of low temperature origin with other silver sulphosalts. These two minerals have a striking scarlet red colour and adamantine lustre. Because of their magnificent colour and brilliance, some crystals have been cut into gemstones but, as they have low hardness (2-2.5), they are far too soft to wear. Proustite is the more transparent of the two minerals and yields the best gems. Fortunately most collectors value the uncut crystals more than the gems and few specimens are faceted, making these gems, proustite in particular, exceedingly rare and amongst the most sought after of the 'collectors' gems. This is probably just as well as the material darkens on exposure to light (a photochromic effect due to the silver present).
 

The finest proustites known are in the collection of the Natural History Museum, London. The locality that produced the finest specimens is Chanarcillo, Chile, with superb crystals up to 7.5 cm long and 2.5cm thick. The best pyrargyrite specimens come from the Harz Mountains of Germany.

Proustite and pyrargyrite were first described in 1832 and 1831 respectively. The most recently described silver mineral is sicherite (TlAg₂(As,Sb)₃S₆) (2001) from the famous Lengenbach quarry, Binntal, Switzerland. It is named in honour of Valentin Sicher, active member of Interessengemeinschaft Lengenbach, IGL, an organisation with the aim of mining minerals at Lengenbach for research and collectors. Lengenbach is noted for its extensive range of rare sulphosalt minerals.

Silver in Scotland

The Leadhills orefield produced the greatest amount of silver. Between 1845 and 1919 twenty three tonnes were recovered from argentiferous galena, the richest lead ore producing 312 g/tonne. Significant amounts of native silver were found at Alva, near Stirling and Hilderston, near Bathgate.

Alva
Stephen Moreton presents the history and mineralogy of the silver deposit at Alva in an excellent article in the Mineralogical Record. In addition to describing the geology and mineralogy, he paints a lurid tale of Jacobite rebellion, fortunes made and lost, buried treasure, betrayal, deals with the government and a diversity of charming and not-so-charming characters.
The deposit, epigenetic hydrothermal veins in andesitic lavas, was worked intermittently from 1715 until 1768 producing an estimated five to six tons of silver. Latterly the deposit was worked, albeit for a very short period, for cobalt. Virtually the only silver mineral present is native silver, though EPMA detected minute grains of an unnamed silver-bismuth-selenide. Argentite has been reported but no specimens exist to confirm this. The native silver occurs as beautiful crystal dendrites up to 3Ýcm in length.

Hilderston
The most comprehensive account of mining is given by Meikle in the Journal of the Russell Society. The deposit was discovered in 1606 and production ceased in 1616 although in the 1750s, nickeline (NiAs), previously discarded by the silver miners, was recovered from the surface dumps and sold. The deposit is found alongside a quartz dolerite dyke intruded into Lower Limestone Group Carboniferous sedimentary and volcanic strata. Two distinct metallogenic assemblages have been identified, only one of which contains silver. Few specimens from the deposit exist today. From the literature (Aitkinson, 1619) it appears that the native silver occurred as wire silver:

the manner how it grew was like unto the haire of a manís head and the grasse in the fielde.

Uses

The naturally occurring silver halide minerals, chlorargyrite (AgCl), bromargyrite (AgBr) and iodargyrite (AgI) (no problem about working out how these minerals got their names) exhibit light sensitive properties. The reaction of silver halides to light spawned the greatest use of silver today, such that 40% of the silver mined is used in the photographic industry. A common use in the past for silver was to make coins, but today, only six percent of the worldís silver is used for coinage. Another use for silver is the silvering of mirrors. Silver is used to coat smooth glass surfaces by vaporisation of the metal or by precipitation from a solution, though today, aluminium has largely replaced silver in this process. Silver is also used in the manufacture of switches, printed circuits, long-lasting batteries and bearing alloys for aeroplanes, diesel engines and some motor cars. When alloyed with copper, silver is used for welding. In its colloidal form, silver serves as a catalyst in the manufacturing of certain alcohols. When silver is alloyed with cesium, it is used in photocells and, in the form of silver iodide, it is used to seed clouds for weather modification purposes. So at least some clouds do have a silver lining.

Further reading

Aitkinson, Stephen, 1619. The discoverie and historie of the gold mynes in Scotland written by Stephen Aitkinson in the year MDCXIX pp.47-50, James Ballantyne & Co., Edinburgh 1825.

Meikle, 1994. Native silver from Hilderston mine, West Lothian, Scotland, Journal of the Russell Society , vol. 5, part 2, pp. 83-90.

Moreton, Stephen, 1996. The Alva silver mine, Mineralogical Record, vol. 27, pp. 405-414.