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Mineral Notes: Silver

By Frederick H. Pough, Ph.D.

While 56 or 57 times cheaper than gold on the commodities market, silver is likely 57 or 56 times less common in mineral collectors’ showcases. The reasons may be several, but two are outstanding.

First and most important is the susceptibility of silver and several silver-containing species to darken by surface exposure to sulfur or light. Silver’s behavior, in a crystal or a spoon, limits its popularity among collectors and dinner-party throwers alike.

Second, in the last century, there haven’t been many good crystals of silver species found. Although mineral books, such as Dana’s 6th (or 7th?) and Bideaux’s Handbook of Mineralogy, list many localities for silver, and while in a 300-year perspective this may be true, these books make silver seem too common. There once were many mines producing silver specimens, but lists, like those in Dana and Bideaux, are pretty meaningless in terms of current specimen production, for practically none of the mines of past centuries are sources of specimens today: as mines deepen, the veins soon become less rich, open pockets diminish, and everything conspires to minimize specimen recovery.

Silver in most or normal veins is of little or no collector worth: the specimens from Cobalt, Ontario, for example, are just ugly black masses. In some cases, silver may assume the form of elongated, often skeletal or dendritic crystals, often penetrating calcite; collectors may enjoy these specimens sliced and polished. With the likelihood of tarnish soon taking over the silver surfaces, however, when sliced, these specimens had best be promptly lacquered.

Collectors do have some sources of interest. There are two major but completely unlike silver specimen sources with specimens of very different growth habits: the wires from Könsberg, Norway, and the crystals of Michigan’s Upper Peninsula.

Könsberg was the world’s greatest silver specimen mine, with its magnificent, twisting wires rising practically alone in the exhibit circuit. So abundant at one time was silver at this mine that Norway’s king established a mint in Köngsberg, where, still maintained, is a museum with fabulous examples.

It is an unusual silver vein, containing little ore other than silver and silver sulfide. Associated gangue minerals are few: barite, zeolites, and somewhat important, spectacular octahedral fluorite and abundant, large acanthite, née argentite. (For readers not familiar with silver sulfides, orthorhombic acanthite is a silver sulfide crystallizing from solutions cooler than 173°C solutions. Usually, the separating sulfide, growing at a higher temperature, is cubic and has been long known as argentite. Restacking its atoms, argentite’s paramorphism to acanthite occurs spontaneously when vein temperature falls below 173°C.)

Though hardly a non-Mexican collector would venture to put anything but a Köngsberg wire in a competitive case, there are, and were (Mexico, Peru, and, once, Germany) lesser sources of wire silvers. Pachuca and especially Batopilas (Mexico) were good sources. All the wire silvers find errant airborne sulfur most welcome, however, and Quick-Dip’s shine is regrettably shortlived.

The silver wires of Norway and other localities seem to rise from a floor. We have other examples of minerals growing up or out from various porous bases: gypsum “horns” on cavern walls; chalcanthite and H2O needles rise as slender rods in copper mines and over frosty mud flats. Hunting a solution of this observation, we note that the wires usually seem to spring from a gray metallic mineral surface, generally the octahedral face of a one-time argentite crystal. There seems to be a catalytic extraction of silver from ore-forming solutions taking place at that interface as the wires twist and climb, each doubtless a single crystal, one or dozens fused in a parallel bundle. Perhaps this happens when the sulfur emissions diminish. In any case, what is suggested is that an argentite-acanthite presence is essential to the development of wires.

The Michigan occurrence is quite different: there seems to be little or no silver sulfide, so there are no wires. Instead, there are well-formed crystals, which terminate grouped copper crystals. Growing as crystals normally do, the Michigan silvers swell as they rest on a matrix, forming shiny white peaks, crowning the crystallized copper masses. Remarkably, unlike the wires from elsewhere, these silver crowns atop copper seem eminently displayable, for they tend to remain untarnished and silvery, at least as far as your author has noted. Perhaps all were lacquered as they came from the mine, treated at the source like tanzanite! Perhaps one of the five or six individual mineralogical ignoramusses now claiming authorship of some trivial academic paper could take it on as a one-person job, not too difficult for a single author to grasp.

In Canada, the Czech Republic, Germany, Peru, Chile, and Mexico, other less pure silver veins contain mixtures of minerals; some have the beautiful (but darkening with light and time) “ruby silver” antimonides and arsenides, pyrargyrite and proustite, that you won’t see on display. Gangues in these veins include many non-Ag ores, cobalt minerals, and uraninite, with calcite, barite, and often, zeolites, as well as recrystallized secondary silver species like polybasite, pearceite, and stephanite.

All of these suites are largely epithermal, that is, low-temperature, near-surface deposits. Born under less pressure, epithermal deposits often have open pockets, which permit the unhampered growth of free crystals: hence the existence of the beautiful crystals of ruby silver for which Chanarcillo, Chile, is so famous. The other side of the coin for us, however, is that being near-surface deposits, most were discovered, and worked out, centuries ago. Being near the surface, the veins are also susceptible to alteration by weathering.

Silver is often, even to 50 percent, also a contaminant in gold (which it pales), and both silver and silver minerals commonly host mercury, copper, iron, and antimony. Often, mining and refining of common metals like lead, zinc, copper, even uranium, provide a small gold and/or silver bonus; crinkly, cleaving galena is commonly regarded as having a relatively high silver content, though this may be only an old kobold’s tale, a kobold being a sort of gnome or mining spirit. Another story, believable if not true, has it that EK Co. bought some silver from Great Bear Lake, a Canadian uranium deposit, but the film they made eventually had a tendency to self-expose, a consequence of years of keeping such bad company.

From Lapidary Journal: Volume 54, No. 8 — November 2000