New Mexico Mineral Symposium — Abstracts

Silver and silver-bearing minerals hold a particular fascination for the mineral collector. Superb specimens are known from hundreds of localities worldwide, and unlike gold, silver is quite a reactive element, forming more than a hundred different silver-bearing minerals. Further, silver mining has played a very important role in development of the New World, so mineral specimens are often associated with a rich historical lore. Today the annual worldwide production of silver is approximately 14,000 metric tons, more than at any time in history. Most of this silver is recovered as a byproduct of low-grade copper and lead mining. This fact, coupled with the historically low price of silver (— $5 US/troy ounce) and the continued mechanization of even high-grade mines, has greatly reduced the availability of "new" silver mineral specimens on the market. Only Mexico, Peru, and, to a lesser extent, Kazakhstan have produced significant volumes of specimen material in the last decade. Nevertheless, silver is an extremely important industrial metal, and the future is bright for more spectacular specimens.

The name silver is an old English word related to the German Silber. This name apparently has no descriptive value, but the Latin name argentum means "white and shining". Silver is widespread in the Earth's crust, with literally thousands of ore deposits exploited in the last few hundred years. Ordinary rocks contain roughly 100 parts per billion silver, whereas typical ore deposits contain 10-1,000 grams of silver per ton. There are a variety of ways that silver enrichment can take place, but most commonly it is by hydrothermal activity. It is possible to divide silver mineral localities into two broad categories: deposits where silver is the dominant metal and deposits where silver is produced as a byproduct. Fine specimens are found in both types of deposits, but the best are usually from silver-rich deposits. Silver-dominant ore deposits generally form veins that contain the most varied and spectacular specimens.

Silver has an atomic number of 47 and has two stable isotopes (Agl' and Agin that occur in nature in approximately equal amounts. Another 25 radioactive isotopes have been created in the laboratory. When not in its native form, silver is generally monovalent and is present as a cation in approximately 130 different minerals. The relatively large number of known silver minerals can be attributed to several factors, including crystal radius. Ordinarily, when cations and anions bind together, each gathers as many ions of opposite sign as size permits, the number corresponding to the coordination number. However, silver has an unusual electronic structure that leads to a preference for low coordination numbers (typically 2, 3, and 4). There are four basic groups of silver minerals that share general properties. These are the silver sulfides, silver sulfosalts, silver halides, and amalgams. There are a few unusual silver minerals that include an oxide (stetefeldtite) and oxysalts (bellite, an apatite group mineral, and argentojarosite, a silver-iron sulfate). Most silver minerals are stable only at low temperatures (<250°C), and at higher temperatures they convert to solid solutions, commonly with large amounts of base metals. The elemental composition of deposits is important for determining which silver minerals will be present.