Indium
In 1863, German chemists Ferdinand Reich and Hieronymus Theodor Richter examined ores from mines near Freiberg in Saxony. They dissolved minerals like pyrite and sphalerite in hydrochloric acid to distill raw zinc chloride. Reich worked as a color-blind scientist who needed an assistant to detect colored spectral lines. He hired Richter specifically for this task of identifying faint colors in the spectrum. While searching for the green emission lines of thallium, they instead found a bright blue line that did not match any known element. This distinct blue line led them to name the new element indium after the Latin word indicum meaning violet or indigo. The name reflected the indigo color seen in its spectrum rather than any connection to India itself. Richter later isolated the metal in 1864 and presented an ingot at the World Fair of 1867. The two scientists eventually fell out when Richter claimed sole credit for the discovery.
Indium is a silvery-white post-transition metal with a Mohs hardness of just 1.2. A knife can cut through it easily leaving a visible line on paper similar to graphite. When bent, the metal produces a high-pitched crackling sound caused by crystal twinning. Its melting point sits at 156.60 degrees Celsius which is higher than gallium but lower than thallium. Below 3.41 Kelvin the material becomes a superconductor allowing electricity to flow without resistance. Indium has greater solubility in liquid mercury than any other metal reaching over 50 mass percent at zero degrees Celsius. It crystallizes in a body-centered tetragonal system where each atom has four neighbors at 324 picometers distance. The element possesses 39 known isotopes ranging from mass number 97 to 135. Only two occur naturally as primordial nuclides: indium-113 and indium-115. Indium-115 makes up 95.7% of all natural indium and has a half-life of 4.41 years. This radioactive decay is spin-forbidden making its half-life four orders of magnitude greater than the age of the universe.
Indium holds 49 electrons with an electronic configuration of [Kr]4d5s5p. In compounds it most commonly donates three outermost electrons to become indium(III). Sometimes the pair of 5s-electrons remain undonated resulting in indium(I) oxidation states. The stabilization of this monovalent state comes from relativistic effects lowering the energy of the 5s-orbital. Indium(III) oxide forms when metal burns in air or when hydroxide heats up. Chlorination produces colorless InCl3 while iodination yields yellow InI3. These halides act as Lewis acids similar to aluminum trihalides. Hydrides like InH3 exist only transitorily in ethereal solutions at low temperatures. Organic derivatives such as trimethylindium serve as precursors for semiconductor materials. Indium does not form borides silicides or carbides but creates well-developed chalcogenides with oxygen sulfur selenium and tellurium. Many of these derivatives undergo rapid hydrolysis when exposed to water. Black powder indium(I) oxide appears when heating indium(III) oxide to 700 degrees Celsius.
Stars between 0.6 and 10 solar masses create indium through a slow neutron capture process called s-process. A silver-109 atom captures a neutron to become silver-110 which beta decays into cadmium-110. Further neutron captures produce cadmium-115 that decays via another beta decay into indium-115. This stellar mechanism explains why the radioactive isotope remains more abundant than the stable one. The element ranks 68th most abundant in Earth's crust at approximately 50 parts per billion. It rarely forms its own minerals or occurs in elemental form naturally. Fewer than 10 known minerals contain indium including roquesite CuInS2. None exist at concentrations high enough for direct economic extraction. Instead it appears as a trace constituent within common ore minerals like sphalerite and chalcopyrite. Enrichment levels in these deposits remain insufficient to support independent mining operations at current prices. The availability depends entirely on how much zinc and copper ore gets extracted annually rather than total crustal abundance.
Indium production occurs exclusively as a by-product during processing of other metal ores. Sulfidic zinc ores serve as the main source material where indium hosts primarily within sphalerite. Minor amounts also extract from sulfidic copper ores during smelting processes. During roast-leach-electrowinning procedures indium accumulates in iron-rich residues before final purification. Electrolysis completes further cleaning steps varying with each smelter operation mode. Recent estimates place supply potential at minimum 1,300 tonnes yearly from zinc ores plus 20 tonnes from copper ores. These figures exceed current annual production which reached 655 tonnes in 2016. China led global output with 290 tonnes while South Korea produced 195 tonnes that same year. Japan contributed 70 tonnes and Canada added 65 tonnes to world totals. The Teck Resources refinery in Trail British Columbia served as a large single-source producer generating 41.8 tonnes in 2004. Average pricing dropped significantly from 705 dollars per kilogram in 2014 down to 240 dollars per kilogram in 2016. Major future increases remain possible without significant cost hikes or price spikes.
Flat-panel displays consume half of all indium worldwide today through transparent conductive coatings called indium tin oxide. This material applies directly onto glass substrates for electroluminescent panels used in LCD screens. Demand rose rapidly from the late 1990s until 2010 driven by computer monitors and television sets. Thin-film applications became the largest end use by 1992 after decades of development. Indium phosphide semiconductors and indium tin oxide films aroused interest during the middle and late 1980s. Some compounds like indium antimonide function as semiconductors useful for low-temperature transistors. High-temperature transistors utilize indium phosphide while light-emitting diodes employ InGaN and InGaP compounds. Photovoltaic cells known as CIGS solar cells combine copper indium gallium selenide for second-generation thin-film energy capture. Vacuum seals made from ductile indium wire deform to fill gaps in microwave circuits. Gallium-indium-tin alloy galinstan replaces mercury in thermometers due to its room temperature liquidity. Recycling rates remain below one percent according to UNEP data despite increased manufacturing efficiency.
Radioactive indium-111 appears in very small amounts within nuclear medicine tests as a radiotracer. It follows labeled proteins and white blood cells to diagnose various types of infection. Biological half-life spans about two weeks before excretion occurs through normal body processes. Compounds tend to store temporarily in muscles skin and bones prior to elimination. Growth hormone analogues tagged with indium help locate receptors in neuroendocrine tumors. No metabolic role exists for indium in any organism on Earth. Inhalation or injection causes toxicity though ingestion absorption remains poor. Industrial exposure carries health risks despite claims of no evidence regarding hazards from standard use. One overview states no clear health hazard emerges from routine industrial applications. The element serves critical diagnostic functions while maintaining strict safety protocols around handling procedures.
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Common questions
Who discovered indium and when was it first identified?
German chemists Ferdinand Reich and Hieronymus Theodor Richter discovered indium in 1863 while examining ores from mines near Freiberg in Saxony. They found the element by observing a bright blue spectral line during their analysis of minerals like pyrite and sphalerite.
What are the physical properties and melting point of indium metal?
Indium is a silvery-white post-transition metal with a Mohs hardness of just 1.2 that melts at 156.60 degrees Celsius. It produces a high-pitched crackling sound when bent due to crystal twinning and becomes a superconductor below 3.41 Kelvin.
How does indium form naturally within Earth's crust and which isotopes exist?
The element ranks 68th most abundant in Earth's crust at approximately 50 parts per billion and rarely forms its own minerals or occurs in elemental form naturally. Only two primordial nuclides occur as natural indium-113 and indium-115, with indium-115 making up 95.7% of all natural indium.
Where does global indium production come from and what were the output figures for 2016?
Indium production occurs exclusively as a by-product during processing of sulfidic zinc ores where it hosts primarily within sphalerite. China led global output with 290 tonnes while South Korea produced 195 tonnes and Japan contributed 70 tonnes to world totals in 2016.
What are the primary industrial uses of indium tin oxide and other compounds today?
Flat-panel displays consume half of all indium worldwide today through transparent conductive coatings called indium tin oxide applied onto glass substrates for LCD screens. Other applications include indium phosphide semiconductors for transistors and CIGS solar cells that combine copper indium gallium selenide for second-generation thin-film energy capture.