Acanthite

Long before it had a mineralogical name, acanthite was the dark, leaden stuff that silver miners pulled out of veins across central Europe. They called it silver glance in English, Silberglanz in German — names that described the metallic sheen of a fresh-broken crystal rather than what the mineral was.

The formal name came in 1855, when the mineralogist Gustav Adolf Kenngott described specimens with sharp, pointed crystals and named the mineral from the Greek akantha — meaning thorn — in reference to that crystal shape. The type locality was the silver district around Jáchymov — Joachimsthal in German — in the Ore Mountains of Bohemia.

For a time, two minerals seemed to coexist. Kenngott's pointed acanthite was monoclinic, with crystals tapering like spines. But silver miners had long known a second form with cubic, blocky crystals: that one had been called argentite, from the Latin argentum for silver. With the arrival of X-ray and other instrumental analysis, the two names resolved into one substance. The cubic argentite of the old labels is the high-temperature form of silver sulfide, stable only above 173 °C. The room-temperature form is monoclinic acanthite. As argentite crystals cool below that temperature, they distort internally into acanthite while keeping their outward cubic shape — a paramorph, meaning a crystal whose external form was inherited from a higher-temperature mineral but whose internal lattice has been rebuilt. Every old "argentite" label in a museum drawer is, by atomic structure, an acanthite.

Acanthite in the great silver districts

The history of acanthite is, in practice, the history of silver mining. The mineral itself was the ore — under whatever name miners gave it locally.

In 1546, Spanish prospectors struck the great silver mountain of Potosí in what is now Bolivia. The Mexican district of Zacatecas was discovered the same year. The two became the engines of the Spanish silver economy and triggered what historians call the Spanish Price Revolution across Europe. Acanthite, together with native silver and the silver halides, was a principal ore in both fields.

Three centuries later, the centre of gravity shifted north. The Comstock Lode at Virginia City, Nevada, opened in 1859 as the first major silver ore deposit in the United States.

Acanthite is the most important ore of silver. Where the mineral occurs in workable concentrations — together with native silver, the silver halides like chlorargyrite, and the silver sulfosalts — it is mined for the metal it carries.

The bulk of the world's silver, however, no longer comes from ores worked for silver alone. Most of it is recovered as a by-product of copper, gold, lead, and zinc refining. Acanthite is one of the minerals that accompanies these base-metal sulfides, and the silver it carries enters the same refinery streams.

When acanthite is processed for silver, the route runs through froth flotation, which separates the fine-ground sulfide grains from the surrounding rock. A cyanide leach then dissolves the silver out of the concentrate — the same chemistry used for gold. When silver instead comes out of a copper smelter, it is recovered during the electrolytic refining of the copper, the step that uses an electric current to deposit pure metal on a cathode. Peru and Mexico remain among the principal silver-producing countries, and roughly a fifth of world supply comes from recycling rather than fresh mining.

What the silver becomes

Silver has the highest electrical and thermal conductivity of any metal, and the highest optical reflectivity. That set of properties drives most of its modern industrial use.

The largest current demand sits in electronics. Silver is used in electrical contacts and conductors, and at radio frequencies in the very-high-frequency band and above, where its conductivity advantage matters most. Silver is also used in solar panels.

A second cluster of uses is medical and antibacterial. Dilute solutions of silver nitrate and other silver compounds serve as disinfectants and microbiocides; they are added to bandages, wound dressings, catheters, and other medical instruments.

The remaining demand is spread across jewellery, high-value tableware, brazing alloys, specialised mirrors and window coatings, catalysis of chemical reactions, photographic and X-ray film, and as a colorant in stained glass.