Indium: The Rare Metal Behind Flat Screens and Touchscreens

Indium is a rare metal that makes modern screens possible. As part of its research on nonfuel mineral resources, the U.S. Geological Survey studies how and where indium deposits form, how indium affects human and ecosystem health, trends in its supply and demand, and where future resources might be found.

A rare, soft metal

• The continental crust averages about 50 parts per billion of indium and the oceanic crust about 72, similar to meteorites and comparable to silver. Only 12 indium minerals are known.
• Pure indium is a soft, lustrous, silvery-white metal with a low melting point. It stays ductile and malleable even near absolute zero, which suits it to cryogenic uses.
• Two German chemists discovered it in the mid-1800s in zinc ores from Freiberg, Saxony, and named it for the indigo-blue colour in its emission spectrum. Long a curiosity, it found early uses in light-emitting diodes and in coatings for aircraft engine bearings; indium-bearing nuclear control rods spread in the 1970s.
• Much less is known about indium's behaviour and toxicity in the environment than about metals such as lead and zinc, though many indium compounds are toxic to animals.

How we use it

• Indium-tin oxide (ITO), the leading use: a thin-film coating in virtually every flat-panel display and touchscreen that turns incoming electrical data into an optical image, with few chemical substitutes. Most is made in Japan, with significant amounts in China, the Republic of Korea and Taiwan.
• Solders and alloys, the second use: indium solders resist cracking and heat fatigue better than tin-lead ones. Copper-indium-gallium-diselenide (CIGS) thin films are increasingly favoured for their low material and manufacturing costs, and CIGS solar cells, in early commercialisation, may one day compete with other forms of energy production and help cut its carbon footprint. Indium alloys also bond non-metallic materials, as in dental alloys, have replaced mercury in some uses, and are still used in nuclear control rods.
• Semiconductors in LEDs and laser diodes: indium LEDs mainly transmit data optically and also light outdoor signs, billboards and transit displays; indium laser diodes carry fibre-optic communications.

Where it comes from

Indium occurs in base-metal hydrothermal ore deposits of all ages that formed at active plate margins and mountain belts with abundant volcanic activity, from active seafloor volcanoes to billion-year-old volcanic layers in greenstone belts. It is never concentrated enough to be mined for itself: it is recovered as a by-product of refining zinc, mostly from the mineral sphalerite. Indium-rich ores typically hold 10 to 22% zinc and 2% or more copper; refinery residues ("anode slimes") carrying 20 to 25% indium are concentrated and refined to over 99.99% purity.

More than half the world's by-product indium comes from southern China, from seafloor volcanogenic massive sulfide (VMS) and sedimentary exhalative (SEDEX) deposits; much of the rest comes from zinc concentrates from Mississippi Valley-type (MVT) deposits. Many other deposit types also hold significant indium.

Supply and demand

• Reserves are hard to estimate for a by-product found in many kinds of zinc deposit; more than 95% of production comes from electrolytic zinc refining, and indium levels in sphalerite are rarely reported. U.S. zinc reserves are about 5% of the global total of 250,000,000 tonnes.
• In 2011 world primary production was about 662 tonnes, more than half from China; with the Republic of Korea, Japan, Canada and Belgium, five countries made almost 95%. Many producers refine residues bought from elsewhere and have no large deposits of their own. The number of producers and specialised refineries has grown greatly since 2000, and some refineries outside China have moved from low-grade to higher-purity indium.
• In 2011 the United States produced no indium as a by-product at any smelter or refinery; it upgraded imported metal and powder. Imports reached 150 tonnes, up 43% from 2009; U.S. refinery capacity is about 40 tonnes a year, mostly at two companies.

Securing future supply

Recovery from secondary sources, refinery residues and end-of-life scrap, already exceeds primary production by about 250%, and recycling is important in Japan, the Republic of Korea, the United Kingdom, Canada, the Philippines, Taiwan and Germany. Meeting future demand will need better recovery from ore concentrates, more careful recycling of electronics and production waste, and more exploration.

Exploration picked up with growing markets for displays and solar cells and after the 2006 closure of the world's largest producer, Japan's Toyoha deposit. High-grade deposits are reported from the Mount Pleasant porphyry tin-tungsten deposit in New Brunswick, Canada, and from Argentina, Australia, Brazil, Bolivia, Finland, Portugal and the United States. In Utah, the Crypto Zinc-Copper-Indium Project in Juab County is a new domestic resource: its skarn deposits average about 31 parts per million indium, with inferred resources of about 475 tonnes.

More: indium statistics and information · recent U.S. uses · historical statistics. The USGS Mineral Resources Program is the principal federal source of research on nonfuel minerals (minerals.usgs.gov).

Sources

• C. N. Mercer, Indium—Bringing liquid-crystal displays into focus, U.S. Geological Survey Fact Sheet 2015–3012. https://pubs.usgs.gov/publication/fs20153012
• The original's photographs (stock images, a CC-licensed mineral specimen and a mining company's mine site) are not reproduced here.