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569
Rubric: Faculty
Section: Earth Sciences
The "Living" Silver of Olonho

The "Living" Silver of Olonho

Silver is one of the most ancient metals extracted from the earth interior. It does not oxidize in the air, has a record electric and thermal conductivity, and easily interfuses with other metals. These properties account for its many applications in industrial technologies, medicine, and jewelry. Silver consumption has been continuously increasing, and demand for it is higher than supply since most rich deposits have been depleted. A rare exception is silver deposits of Yakutia, which can boast of remarkable native silver bearing small resemblance to ordinary minerals.

Geologists have always been searching for native silver - such finds are considered a great piece of luck. The matter is that silver nuggets are testimony to specific conditions of ore formation giving rise the richest areas of silver deposits - ore shoots.

Therefore, samples from the lodes of the Mangaseya deposit were primarily examined for native silver. As a rule, it was not detected; sometimes, carbonate and quartz were accompanied with what looked liked bunches of rusty wires or tufts of dry moss. The geologists ignored these findings until, from a depth of 180 meters, a sample was lifted that contained the same "moss". The treasure had been there all along: these wire-shaped nuggets were made of pure silver!

Silver, as becomes a ptecious metal, is by no means a widespread chemical element: its content in the earth crust is up to a hundred thousandth of one percent. Silver predominantly occurs in nature in the form of minerals: compounds of sulfur, antimony, etc, and in polymetallic ores.

Native silver is quite rare. The main industrial deposits are in Germany (Erzgebirge), North America (Cordilleras), Europe (Carpathians), Japan, and New Zealand. Finding large silver nuggets is an extraordinary event. This has mainly happened in Western Europe: for instance, at the Schneeberg deposit of Erzgebirge a nugget weighing 20 tons was discovered! A lot of silver nuggets were found in Russia (in the Altai and on Medvezhiy Island) in the 18th century.

Today, most rich silver deposits have been depleted; at many of them even ores poor in silver are mined. An area with totally virgin silver deposits is the West-Verkhoyan geological and economic region of eastern Yakutia, whose resources potential is up to 60,000 tons of silver. These promising deposits have been studied by the scientists of the Institute of Diamond and Precious Metal Geology, Yakutsk Branch, Russian Academy of Sciences.

Being quite a rare mineral, silver may have been studied in more detail and more carefully than many other minerals, which has resulted in some unexpected discoveries. This is what happened to Yakut scientists at the Mangaseya deposit in the West-Verkhoyan region.

Silver “moss”

Geologists have always been searching for native silver – such finds are considered a great piece of luck. The matter is that silver nuggets are testimony to specific conditions of ore formation giving rise the richest areas of silver deposits – ore shoots.

Therefore, samples from the lodes of the Mangaseya deposit were primarily examined for native silver. As a rule, it was not detected; sometimes, carbonate and quartz were accompanied with what looked liked bunches of rusty wires or tufts of dry moss.

The geologists ignored these findings until, from a depth of 180 meters, a sample was lifted that contained the same “moss”. The treasure had been there all along: these wire-shaped nuggets were made of pure silver!

Native silver is a mineral belonging to the class native elements; its chemical composition varies from virtually pure silver (less than 1.5 % of adulterants) to natural solid solutions containing gold, antimony, and mercury. Native silver usually occurs in the form of thin irregular plates and leaves, wire-shaped or dendrite-like bunches, grains of irregular shape and larger entire clusters (nuggets), and occasionally in the form of cubes and octahedrons. Native silver is white in color at the fresh break; the surface often has a black coating. Polished sections exhibit a very strong reflective power

Some of them were coated with iron hydroxides and looked like rusty wire as a result; others were sparkling white as though they had come right from a jewelry counter.

Some time later such native silver was found in great amounts in the lodes at the deposit surface. The shape of the nuggets varied: wires, dendrites, thin plates and even foil that has grown between the faces of carbonate crystals.

Surprising as it was that silver came in great quantities and various shapes, even more surprising was that geologists finally learned to “see” it. They certainly became interested in the origin, composition, and structure of this mineralogical phenomenon.

Coming from freibergite?

Wire-shaped native silver mainly forms in the cavities of druses composed of quartz, carbonate, and sometimes galena (lead sulfide) crystals. Alongside silver, the samples almost always contain the silver-bearing mineral freibergite ((Ag,Cu,Fe)12(Sb,As)4S13). The latter is regarded to be among the most important silver bearers occurring in ores.

As a rule, to study mineral composition of ores, special polished samples are prepared; they are examined and analyzed in the reflected light of an ore microscope of different power. It has turned out that polished freibergite crystals can generate nugget silver in response to light – this was confirmed experimentally. This fact suggested that some of the silver “wires” might have grown from freibergite. However, there is no sunlight inside the earth so what could have triggered the process? We believe that it could have been high pressure created by the sedimentary rock mass or tectonic activity.

How and where?

Studies using electronic microscopy have shown that at the early growth stage the silver wire is hollow. Silver dendrites start growing from the wire ends and, gradually, the entire cavity gets filled. The wire can grow several centimeters long; its root is thicker than the top, which makes it look like a hair.

This shape of silver is attributed to crystal whiskers growing not by “head” but by foot, squeezing out of a porous substrate like toothpaste from a tube. The thickness and shape of a whisker matches the pore diameter; a tuft of many whiskers can be up to two millimeters thick. As the whisker “head” runs into an obstacle, the metal continues to grow at the foot, and, as silver is a quite plastic mineral, it bends and grows on. This must be the explanation for tousled silver wires in druse cavities, where the room is limited.

Native silver frequently forms in the oxidation zones of deposits. The necessary condition for this is that silver ores must occur at flat mountain tops. Oxidation zones can normally be traced by rushes of “rusty” rocks, remains of a carbonate lode having silver minerals. If such a lode crops out to the surface at the mountain slope, it crumbles and gets washed out by rain and melt water, and no silver can be found there.

A most illustrative and educational oxidation zone of the Mangaseya deposit is Sterzhnevaya, the ore zone discovered in 1991 by the geologist V. V. Shoshin from oxidized ore debris containing numerous formations of native silver and malachite. Joint occurrence of these minerals proves that the primary ores were composed predominantly of freibergite which contains silver and copper.

As heavy concentrate from oxidized silver ores is being washed in the cradle, glistening amid dark-brown carbonate is a run of silver dust and silver nuggets. In this way, oxidation and destruction of primary silver ores brings about their secondary enrichment: readily soluble components are washed off, and the low-activity mineral – silver – remains. The preserved samples help to estimate how rich the primary ore was and to make a production forecast. Among these samples is the amazing wire-shaped silver, the solved puzzle of the Mangaseya silver deposit.

The photos are courtesy of the author

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