PROPERTIES

Properties

Silver is a naturally occurring element with an atomic number of 47 in the periodic table. It is a constituent of Group 11 (1B) and is classified as both a Transition Metal and a Precious Metal. Its neighbours in Group 11 are copper and gold. Other Transition Metals close to silver in the periodic table include platinum, palladium, nickel, zinc, cadmium and mercury. All original silver in the universe is a direct product of giant star explosions or supernovas.

Silver was first known to man around 4,000 BCE. It’s the third oldest of the ‘Seven Metals of Antiquity’. These seven metals, fundamental to the development of human civilization, are gold, copper, silver, lead, tin, iron and mercury.

Physical Properties

Pure silver is a soft white solid metal with a brilliant lustre. It can be polished to a mirror finish.

Silver has unique physical properties;

  1. High ductility and malleability
  2. The highest electrical and thermal conductivity of all metals
  3. The highest optical reflectivity of all metals

Silver’s ductility is second only to gold. One ounce (31.1 g) of silver can be drawn into 8,000 feet (2,438 m) of wire. A high malleability allows silver to be hammered into sheets more than fifty times thinner than a piece of paper.

Native ‘wire’ silver

The relatively low melting point of silver is 961.8 °C and the density is 10.49 g cm-3. By comparison, gold melts at 1,064.2 °C and has a density of 19.30 g cm-3. Silver and gold are both relatively soft metals that measure a low 2.5 on the Mohs qualitative hardness scale. This is about equivalent to the hardness of a human finger nail. Silver is physically resilient and virtually indestructible.

Chemical Properties

Silver is a corrosion resistant metal. It doesn’t react chemically in pure air or water. However, silver reacts readily with sulphur (S) and hydrogen sulphide (H2S) to produce silver sulphide (Ag2S). This reaction is most commonly recognized as tarnish on silver and silver coins. 

Black silver sulphide on a US silver dollar

Black silver oxide (Ag2O) forms in a reaction with high concentrations of ozone (O3). Silver does not react with oxygen in normal air conditions. The common view that silver ‘oxidises’ in the air is chemically incorrect. Tarnishing is due to the reaction of silver with sulphur and the oxidation of minor alloy metals like copper. Silver dissolves easily in nitric acid (HNO3) to produce silver nitrate (AgNO3), the precursor to inorganic compounds used in chemical photo processing.

Natural Occurrence

Native silver crystals

Silver ranks 65th in the estimated abundance of metals in the earth crust. Gold ranks 72nd. The average concentrations of silver and gold in the crust are in the order of only 0.08 ppm and 0.001 ppm respectively. By comparison, copper ranks 26th with a concentration of 68 ppm.

Native silver is rare. In this form it’s most frequently found in association with other more common minerals in a non-crystalline form or with the appearance of delicate, silver wires, grains and fine coatings. Electrum is a natural alloy of gold, silver and trace copper, platinum and other metals. The natural silver content of electrum ranges from 20-80%. Electrum coins were made in ancient Lydia (western Anatolia) in the late seventh century BC.

Native electrum

The most common silver mineral compound is silver sulfide (Ag2S) which occurs in two forms. The only stable form at a temperature below 173 °C is the black-grey mineral Acanthite which crystallises in the monoclinic mineral system. The mineral name derives from the Greek word ‘akantha’ meaning thorn or arrow. Argentite is a cubic form of Ag2S that is only stable above 173 °C.

Partially crystalline Acanthite

Acanthite ore from the Namtu-Bawdwin mine. Shan State, Myanmar

‘Horn silver’, or chlorargyrite (AgCl), is a secondary silver chloride mineral formed due to the oxidation or weathering of silver deposits. Scientists have identified over 60 silver minerals in total, although most species are obscure and rarely found or used as a source of silver metal.

Silver also occurs in a group of complex sulfide minerals known as sulfosalts which typically contain silver, copper or lead in association with arsenic (As), antimony (Sb) or bismuth (Bi). Pyrargyrite (Ag3SbS3) and proustite (Ag3AsS3) are the two most commonly found silver sulfosalts. Pyrargyrite is an important silver ore and is locally known as dark red silver ore or ruby silver due to the colour of its small prismatic crystals. Proustite forms long dog-toothed prismatic crystals with a scarlet-vermillion colour. Accordingly, the mineral is also known as light red silver ore.

Pyrargyrite crystals

Silver is an important by-product of mining lead-zinc ores. Argentiferous galena (PbS) can contain up to 2% silver by mass. The silver occurs as silver-bearing minerals or in solid solution within galena’s crystal structure.

Galena crystals

Silver is also recovered from cerussite (PbCO3), a common lead oxide mineral. In 2013, 38% of world silver production was a by-product of the mining and processing of lead and zinc ore. A further 20% was a by-product of copper production. Primary silver mines produced only 29% of the total world silver output.