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= Iridium =
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Introduction
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Iridium is a chemical element; it has the symbol Ir and atomic number
77. This very hard, brittle, silvery-white transition metal of the
platinum group, is considered the second-densest naturally occurring
metal (after osmium) with a density of as defined by experimental
X-ray crystallography. 191Ir and 193Ir are the only two naturally
occurring isotopes of iridium, as well as the only stable isotopes;
the latter is the more abundant. It is one of the most
corrosion-resistant metals, even at temperatures as high as .
Iridium was discovered in 1803 in the acid-insoluble residues of
platinum ores by the English chemist Smithson Tennant. The name
'iridium', derived from the Greek word 'iris' (rainbow), refers to the
various colors of its compounds. Iridium is one of the rarest elements
in Earth's crust, with an estimated annual production of only 15,000
lb in 2023.
The dominant uses of iridium are the metal itself and its alloys, as
in high-performance spark plugs, crucibles for recrystallization of
semiconductors at high temperatures, and electrodes for the production
of chlorine in the chloralkali process. Important compounds of iridium
are chlorides and iodides in industrial catalysis. Iridium is a
component of some OLEDs.
Iridium is found in meteorites in much higher abundance than in the
Earth's crust. For this reason, the unusually high abundance of
iridium in the clay layer at the Cretaceous-Paleogene boundary gave
rise to the Alvarez hypothesis that the impact of a massive
extraterrestrial object caused the extinction of non-avian dinosaurs
and many other species 66 million years ago, now known to be produced
by the impact that formed the Chicxulub crater. Similarly, an iridium
anomaly in core samples from the Pacific Ocean suggested the Eltanin
impact of about 2.5 million years ago.
Physical properties
=====================
A member of the platinum group metals, iridium is white, resembling
platinum, but with a slight yellowish cast. Because of its hardness,
brittleness, and very high melting point, solid iridium is difficult
to machine, form, or work; thus powder metallurgy is commonly employed
instead. It is the only metal to maintain good mechanical properties
in air at temperatures above 1600 C. It has the 10th highest boiling
point among all elements and becomes a superconductor at temperatures
below 0.14 K.
Iridium's modulus of elasticity is the second-highest among the
metals, being surpassed only by osmium. This, together with a high
shear modulus and a very low figure for Poisson's ratio (the
relationship of longitudinal to lateral strain), indicate the high
degree of stiffness and resistance to deformation that have rendered
its fabrication into useful components a matter of great difficulty.
Despite these limitations and iridium's high cost, a number of
applications have developed where mechanical strength is an essential
factor in some of the extremely severe conditions encountered in
modern technology.
The measured density of iridium is only slightly lower (by about
0.12%) than that of osmium, the densest metal known. Some ambiguity
occurred regarding which of the two elements was denser, due to the
small size of the difference in density and difficulties in measuring
it accurately, but, with increased accuracy in factors used for
calculating density, X-ray crystallographic data yielded densities of
for iridium and for osmium.
Iridium is extremely brittle, to the point of being hard to weld
because the heat-affected zone cracks, but it can be made more ductile
by addition of small quantities of titanium and zirconium (0.2% of
each apparently works well).
The Vickers hardness of pure platinum is 56 HV, whereas platinum with
50% of iridium can reach over 500 HV.
Chemical properties
=====================
Iridium is the most corrosion-resistant metal known. It is not
attacked by acids, including aqua regia, but it can be dissolved in
concentrated hydrochloric acid in the presence of sodium perchlorate.
In the presence of oxygen, it reacts with cyanide salts. Traditional
oxidants also react, including the halogens and oxygen at higher
temperatures. Iridium also reacts directly with sulfur at atmospheric
pressure to yield iridium disulfide.
Isotopes
==========
Iridium has two naturally occurring stable isotopes, 191Ir and 193Ir,
with natural abundances of 37.3% and 62.7%, respectively. At least 37
radioisotopes have also been synthesized, ranging in mass number from
164 to 202. 192Ir, which falls between the two stable isotopes, is the
most stable radioisotope, with a half-life of 73.827 days, and finds
application in brachytherapy and in industrial radiography,
particularly for nondestructive testing of welds in steel in the oil
and gas industries; iridium-192 sources have been involved in a number
of radiological accidents. Three other isotopes have half-lives of at
least a day--188Ir, 189Ir, and 190Ir. Isotopes with masses below 191
decay by some combination of β+ decay, α decay, and (rare) proton
emission, with the exception of 189Ir, which decays by electron
capture. Synthetic isotopes heavier than 191 decay by β− decay,
although 192Ir also has a minor electron capture decay path. All known
isotopes of iridium were discovered between 1934 and 2008, with the
most recent discoveries being 200-202Ir.
At least 32 metastable isomers have been characterized, ranging in
mass number from 164 to 197. The most stable of these is 192m2Ir,
which decays by isomeric transition with a half-life of 241 years,
making it more stable than any of iridium's synthetic isotopes in
their ground states. The least stable isomer is 190m3Ir with a
half-life of only 2 μs. The isotope 191Ir was the first one of any
element to be shown to present a Mössbauer effect. This renders it
useful for Mössbauer spectroscopy for research in physics, chemistry,
biochemistry, metallurgy, and mineralogy.
Chemistry
======================================================================
colspan=2| Oxidation states
−3
−1
0
| **+1**||
| **+2**||
| **+3**||
| **+4**||
+5
+6
+7
+8
+9
Oxidation states
==================
Iridium forms compounds in oxidation states between −3 and +9, but the
most common oxidation states are +1, +2, +3, and +4.
Well-characterized compounds containing iridium in the +6 oxidation
state include Iridium(VI) fluoride and the oxides and . iridium(VIII)
oxide () was generated under matrix isolation conditions at 6 K in
argon. The highest oxidation state (+9), which is also the highest
recorded for 'any' element, is found in gaseous .
Binary compounds
==================
Iridium does not form binary hydrides. Only one binary oxide is
well-characterized: iridium dioxide, . It is a blue black solid that
adopts the fluorite structure. A sesquioxide, , has been described as
a blue-black powder, which is oxidized to by . The corresponding
disulfides, diselenides, sesquisulfides, and sesquiselenides are
known, as well as .
Binary trihalides, , are known for all of the halogens. For oxidation
states +4 and above, only the tetrafluoride, pentafluoride and
hexafluoride are known. Iridium hexafluoride, , is a volatile yellow
solid, composed of octahedral molecules. It decomposes in water and is
reduced to . Iridium pentafluoride is also a strong oxidant, but it is
a tetramer, , formed by four corner-sharing octahedra.
Complexes
===========
Iridium has extensive coordination chemistry.
Iridium in its complexes is always low-spin. Ir(III) and Ir(IV)
generally form octahedral complexes. Polyhydride complexes are known
for the +5 and +3 oxidation states. One example is (iPr = isopropyl).
The ternary hydride is believed to contain both the and the
18-electron anion.
Iridium also forms oxyanions with oxidation states +4 and +5. and
can be prepared from the reaction of potassium oxide or potassium
superoxide with iridium at high temperatures. Such solids are not
soluble in conventional solvents.
Just like many elements, iridium forms important chloride complexes.
Hexachloroiridic (IV) acid, , and its ammonium salt are common iridium
compounds from both industrial and preparative perspectives. They are
intermediates in the purification of iridium and used as precursors
for most other iridium compounds, as well as in the preparation of
anode coatings. The ion has an intense dark brown color, and can be
readily reduced to the lighter-colored and vice versa. Iridium
trichloride, , which can be obtained in anhydrous form from direct
oxidation of iridium powder by chlorine at 650 °C, or in hydrated form
by dissolving in hydrochloric acid, is often used as a starting
material for the synthesis of other Ir(III) compounds. Another
compound used as a starting material is potassium
hexachloroiridate(III), .
Organoiridium chemistry
=========================
Organoiridium compounds contain iridium-carbon bonds. Early studies
identified the very stable tetrairidium dodecacarbonyl, . In this
compound, each of the iridium atoms is bonded to the other three,
forming a tetrahedral cluster. The discovery of Vaska's complex ()
opened the door for oxidative addition reactions, a process
fundamental to useful reactions. For example, Crabtree's catalyst, a
homogeneous catalyst for hydrogenation reactions.
Iridium complexes played a pivotal role in the development of
carbon-hydrogen bond activation (C-H activation), which promises to
allow functionalization of hydrocarbons, which are traditionally
regarded as unreactive.
Platinum group
================
The discovery of iridium is intertwined with that of platinum and the
other metals of the platinum group. The first European reference to
platinum appears in 1557 in the writings of the Italian humanist
Julius Caesar Scaliger as a description of an unknown noble metal
found between Darién and Mexico, "which no fire nor any Spanish
artifice has yet been able to liquefy". From their first encounters
with platinum, the Spanish generally saw the metal as a kind of
impurity in gold, and it was treated as such. It was often simply
thrown away, and there was an official decree forbidding the
adulteration of gold with platinum impurities.
In 1735, Antonio de Ulloa and Jorge Juan y Santacilia saw Native
Americans mining platinum while the Spaniards were travelling through
Colombia and Peru for eight years. Ulloa and Juan found mines with the
whitish metal nuggets and took them home to Spain. Ulloa returned to
Spain and established the first mineralogy lab in Spain and was the
first to systematically study platinum, which was in 1748. His
historical account of the expedition included a description of
platinum as being neither separable nor calcinable. Ulloa also
anticipated the discovery of platinum mines. After publishing the
report in 1748, Ulloa did not continue to investigate the new metal.
In 1758, he was sent to superintend mercury mining operations in
Huancavelica.
In 1741, Charles Wood, a British metallurgist, found various samples
of Colombian platinum in Jamaica, which he sent to William Brownrigg
for further investigation.
In 1750, after studying the platinum sent to him by Wood, Brownrigg
presented a detailed account of the metal to the Royal Society,
stating that he had seen no mention of it in any previous accounts of
known minerals. Brownrigg also made note of platinum's extremely high
melting point and refractory metal-like behaviour toward borax. Other
chemists across Europe soon began studying platinum, including Andreas
Sigismund Marggraf, Torbern Bergman, Jöns Jakob Berzelius, William
Lewis, and Pierre Macquer. In 1752, Henrik Scheffer published a
detailed scientific description of the metal, which he referred to as
"white gold", including an account of how he succeeded in fusing
platinum ore with the aid of arsenic. Scheffer described platinum as
being less pliable than gold, but with similar resistance to
corrosion.
Discovery
===========
Chemists who studied platinum dissolved it in aqua regia (a mixture of
hydrochloric and nitric acids) to create soluble salts. They always
observed a small amount of a dark, insoluble residue. Joseph Louis
Proust thought that the residue was graphite. The French chemists
Victor Collet-Descotils, Antoine François, comte de Fourcroy, and
Louis Nicolas Vauquelin also observed the black residue in 1803, but
did not obtain enough for further experiments.
In 1803 British scientist Smithson Tennant (1761-1815) analyzed the
insoluble residue and concluded that it must contain a new metal.
Vauquelin treated the powder alternately with alkali and acids and
obtained a volatile new oxide, which he believed to be of this new
metal--which he named 'ptene', from the Greek word 'ptēnós',
"winged". Tennant, who had the advantage of a much greater amount of
residue, continued his research and identified the two previously
undiscovered elements in the black residue, iridium and osmium. He
obtained dark red crystals (probably of ]·'n') by a sequence of
reactions with sodium hydroxide and hydrochloric acid. He named
iridium after Iris (), the Greek winged goddess of the rainbow and the
messenger of the Olympian gods, because many of the salts he obtained
were strongly colored. Discovery of the new elements was documented in
a letter to the Royal Society on June 21, 1804.
Metalworking and applications
===============================
British scientist John George Children was the first to melt a sample
of iridium in 1813 with the aid of "the greatest galvanic battery that
has ever been constructed" (at that time). The first to obtain
high-purity iridium was Robert Hare in 1842. He found it had a density
of around and noted the metal is nearly immalleable and very hard.
The first melting in appreciable quantity was done by Henri
Sainte-Claire Deville and Jules Henri Debray in 1860. They required
burning more than 300 L of pure and gas for each 1 kg of iridium.
These extreme difficulties in melting the metal limited the
possibilities for handling iridium. John Isaac Hawkins was looking to
obtain a fine and hard point for fountain pen nibs, and in 1834
managed to create an iridium-pointed gold pen. In 1880, John Holland
and William Lofland Dudley were able to melt iridium by adding
phosphorus and patented the process in the United States; British
company Johnson Matthey later stated they had been using a similar
process since 1837 and had already presented fused iridium at a number
of World Fairs. The first use of an alloy of iridium with ruthenium in
thermocouples was made by Otto Feussner in 1933. These allowed for the
measurement of high temperatures in air up to 2000 C.
In Munich, Germany in 1957 Rudolf Mössbauer, in what has been called
one of the "landmark experiments in twentieth-century physics",
discovered the resonant and recoil-free emission and absorption of
gamma rays by atoms in a solid metal sample containing only 191Ir.
This phenomenon, known as the Mössbauer effect resulted in the
awarding of the Nobel Prize in Physics in 1961, at the age 32, just
three years after he published his discovery.
Occurrence
======================================================================
Along with many elements having atomic weights higher than that of
iron, iridium is only naturally formed by the r-process (rapid neutron
capture) in neutron star mergers and possibly rare types of
supernovae.
Iridium is one of the nine least abundant stable elements in Earth's
crust, having an average mass fraction of 0.001 ppm in crustal rock;
gold is 4 times more abundant, platinum is 10 times more abundant,
silver and mercury are 80 times more abundant. Osmium, tellurium,
ruthenium, rhodium and rhenium are about as abundant as iridium. In
contrast to its low abundance in crustal rock, iridium is relatively
common in meteorites, with concentrations of 0.5 ppm or more. The
overall concentration of iridium on Earth is thought to be much higher
than what is observed in crustal rocks, but because of the density and
siderophilic ("iron-loving") character of iridium, it descended below
the crust and into Earth's core when the planet was still molten.
Iridium is found in nature as an uncombined element or in natural
alloys, especially the iridium-osmium alloys osmiridium (osmium-rich)
and iridosmium (iridium-rich). In nickel and copper deposits, the
platinum group metals occur as sulfides, tellurides, antimonides, and
arsenides. In all of these compounds, platinum can be exchanged with a
small amount of iridium or osmium. As with all of the platinum group
metals, iridium can be found naturally in alloys with raw nickel or
raw copper. A number of iridium-dominant minerals, with iridium as the
species-forming element, are known. They are exceedingly rare and
often represent the iridium analogues of the above-given ones. The
examples are irarsite and cuproiridsite, to mention some. Within
Earth's crust, iridium is found at highest concentrations in three
types of geologic structure: igneous deposits (crustal intrusions from
below), impact craters, and deposits reworked from one of the former
structures. The largest known primary reserves are in the Bushveld
igneous complex in South Africa, (near the largest known impact
structure, the Vredefort impact structure) though the large
copper-nickel deposits near Norilsk in Russia, and the Sudbury Basin
(also an impact crater) in Canada are also significant sources of
iridium. Smaller reserves are found in the United States. Iridium is
also found in secondary deposits, combined with platinum and other
platinum group metals in alluvial deposits. The alluvial deposits used
by pre-Columbian people in the Chocó Department of Colombia are still
a source for platinum-group metals. As of 2003, world reserves have
not been estimated.
Marine oceanography
=====================
Iridium is found within marine organisms, sediments, and the water
column. The abundance of iridium in seawater and organisms is
relatively low, as it does not readily form chloride complexes. The
abundance in organisms is about 20 parts per trillion, or about five
orders of magnitude less than in sedimentary rocks at the
Cretaceous-Paleogene (K-T) boundary. The concentration of iridium in
seawater and marine sediment is sensitive to marine oxygenation,
seawater temperature, and various geological and biological processes.
Iridium in sediments can come from cosmic dust, volcanoes,
precipitation from seawater, microbial processes, or hydrothermal
vents, and its abundance can be strongly indicative of the source. It
tends to associate with other ferrous metals in manganese nodules.
Iridium is one of the characteristic elements of extraterrestrial
rocks, and, along with osmium, can be used as a tracer element for
meteoritic material in sediment. For example, core samples from the
Pacific Ocean with elevated iridium levels suggested the Eltanin
impact of about 2.5 million years ago.
Some of the mass extinctions, such as the Cretaceous extinction, can
be identified by anomalously high concentrations of iridium in
sediment, and these can be linked to major asteroid impacts.
Cretaceous–Paleogene boundary presence
========================================
The Cretaceous-Paleogene boundary of 66 million years ago, marking the
temporal border between the Cretaceous and Paleogene periods of
geological time, was identified by a thin stratum of iridium-rich
clay. A team led by Luis Alvarez proposed in 1980 an extraterrestrial
origin for this iridium, attributing it to an asteroid or comet
impact. Their theory, known as the Alvarez hypothesis, is now widely
accepted to explain the extinction of the non-avian dinosaurs. A large
buried impact crater structure with an estimated age of about 66
million years was later identified under what is now the Yucatán
Peninsula (the Chicxulub crater). Dewey M. McLean and others argue
that the iridium may have been of volcanic origin instead, because
Earth's core is rich in iridium, and active volcanoes such as Piton de
la Fournaise, in the island of Réunion, are still releasing iridium.
Production
======================================================================
!Year!!Consumption (tonnes)!!Price (US$)
|2001 2.6 415.25 $/ozt
|2002 2.5 294.62 $/ozt
|2003 3.3 93.02 $/ozt
|2004 3.60 185.33 $/ozt
|2005 3.86 169.51 $/ozt
|2006 4.08 349.45 $/ozt
|2007 3.70 444.43 $/ozt
|2008 3.10 448.34 $/ozt
|2009 2.52 420.4 $/ozt
|2010 10.40 642.15 $/ozt
|2011 9.36 1035.87 $/ozt
|2012 5.54 1066.23 $/ozt
|2013 6.16 826.45 $/ozt
|2014 6.1 556.19 $/ozt
|2015 7.81 544 $/ozt
|2016 7.71 586.90 $/ozt
|2017 n.d. 908.35 $/ozt
|2018 n.d. 1293.27 $/ozt
|2019 n.d. 1485.80 $/ozt
|2020 n.d. 1633.51 $/ozt
|2021 n.d. 5400.00 $/ozt
|2022 n.d. 3980.00 $/ozt
|2023 n.d. 4652.38 $/ozt
|2024 n.d. 5000.00 $/ozt
Worldwide production of iridium was about 7300 kg in 2018. The price
is high and varying (see table). Illustrative factors that affect the
price include oversupply of Ir crucibles
and changes in LED technology.
Platinum metals occur together as dilute ores. Iridium is one of the
rarer platinum metals: for every 190 tonnes of platinum obtained from
ores, only 7.5 tonnes of iridium is isolated. To separate the metals,
they must first be brought into solution. Two methods for rendering
Ir-containing ores soluble are (i) fusion of the solid with sodium
peroxide followed by extraction of the resulting glass in aqua regia
and (ii) extraction of the solid with a mixture of chlorine with
hydrochloric acid. From soluble extracts, iridium is separated by
precipitating solid ammonium hexachloroiridate () or by extracting
with organic amines. The first method is similar to the procedure
Tennant and Wollaston used for their original separation. The second
method can be planned as continuous liquid-liquid extraction and is
therefore more suitable for industrial scale production. In either
case, the product, an iridium chloride salt, is reduced with hydrogen,
yielding the metal as a powder or 'sponge', which is amenable to
powder metallurgy techniques. Iridium is also obtained commercially as
a by-product from nickel and copper mining and processing. During
electrorefining of copper and nickel, noble metals such as silver,
gold and the platinum group metals as well as selenium and tellurium
settle to the bottom of the cell as 'anode mud', which forms the
starting point for their extraction.
Leading iridium-producing countries (kg)
Country !! 2016 !! 2017 !! 2018 !! 2019 !! 2020 7,720 7,180
7,540 7,910 8,170
| 6,624 6,057 6,357 6,464 6,786
| 598 619 586 845 836
| 300 200 400 300 300
| 200 300 200 300 250
Applications
======================================================================
Due to iridium's resistance to corrosion it has industrial
applications. The main areas of use are electrodes for producing
chlorine and other corrosive products, OLEDs, crucibles, catalysts
(e.g. acetic acid), and ignition tips for spark plugs.
Metal and alloys
==================
Resistance to heat and corrosion are the bases for several uses of
iridium and its alloys.
Owing to its high melting point, hardness, and corrosion resistance,
iridium is used to make crucibles. Such crucibles are used in the
Czochralski process to produce oxide single-crystals (such as
sapphires) for use in computer memory devices and in solid state
lasers. The crystals, such as gadolinium gallium garnet and yttrium
gallium garnet, are grown by melting pre-sintered charges of mixed
oxides under oxidizing conditions at temperatures up to .
Certain long-life aircraft engine parts are made of an iridium alloy,
and an iridium-titanium alloy is used for deep-water pipes because of
its corrosion resistance. Iridium is used for multi-pored spinnerets,
through which a plastic polymer melt is extruded to form fibers, such
as rayon. Osmium-iridium is used for compass bearings and for
balances.
Because of their resistance to arc erosion, iridium alloys are used by
some manufacturers for the centre electrodes of spark plugs, and
iridium-based spark plugs are particularly used in aviation.
Catalysis
===========
Iridium compounds are used as catalysts in the Cativa process for
carbonylation of methanol to produce acetic acid.
Iridium complexes are often active for asymmetric hydrogenation both
by traditional hydrogenation. and transfer hydrogenation. This
property is the basis of the industrial route to the chiral herbicide
(S)-metolachlor. As practiced by Syngenta on the scale of 10,000
tons/year, the complex [Ir(COD)Cl]2 in the presence of Josiphos
ligands.
Medical imaging
=================
The radioisotope iridium-192 is one of the two most important sources
of energy for use in industrial γ-radiography for non-destructive
testing of metals. Additionally, is used as a source of gamma
radiation for the treatment of cancer using brachytherapy, a form of
radiotherapy where a sealed radioactive source is placed inside or
next to the area requiring treatment. Specific treatments include
high-dose-rate prostate brachytherapy, biliary duct brachytherapy, and
intracavitary cervix brachytherapy. Iridium-192 is normally produced
by neutron activation of isotope iridium-191 in natural-abundance
iridium metal.
Photocatalysis and OLEDs
==========================
Iridium complexes are key components of white OLEDs. Similar
complexes are used in photocatalysis.
Scientific
============
An alloy of 90% platinum and 10% iridium was used in 1889 to construct
the International Prototype Meter and kilogram mass, kept by the
International Bureau of Weights and Measures near Paris. The meter bar
was replaced as the definition of the fundamental unit of length in
1960 by a line in the atomic spectrum of krypton, but the kilogram
prototype remained the international standard of mass until 20 May
2019, when the kilogram was redefined in terms of the Planck constant.
Historical
============
Iridium-osmium alloys were used in fountain pen nib tips. The first
major use of iridium was in 1834 in nibs mounted on gold. Starting in
1944, the Parker 51 fountain pen was fitted with a nib tipped by a
ruthenium and iridium alloy (with 3.8% iridium). The tip material in
modern fountain pens is still conventionally called "iridium",
although there is seldom any iridium in it; other metals such as
ruthenium, osmium, and tungsten have taken its place.
An iridium-platinum alloy was used for the touch holes or vent pieces
of cannon. According to a report of the Paris Exhibition of 1867, one
of the pieces being exhibited by Johnson and Matthey "has been used in
a Whitworth gun for more than 3000 rounds, and scarcely shows signs of
wear yet. Those who know the constant trouble and expense which are
occasioned by the wearing of the vent-pieces of cannon when in active
service, will appreciate this important adaptation".
The pigment 'iridium black', which consists of very finely divided
iridium, is used for painting porcelain an intense black; it was said
that "all other porcelain black colors appear grey by the side of it".
Precautions and hazards
======================================================================
Iridium in bulk metallic form is not biologically important or
hazardous to health due to its lack of reactivity with tissues; there
are only about 20 parts per trillion of iridium in human tissue. Like
most metals, finely divided iridium powder can be hazardous to handle,
as it is an irritant and may ignite in air. Iridium is relatively
unhazardous otherwise, with the only effect of Iridium ingestion being
irritation of the digestive tract. However, soluble salts, such as the
iridium halides, could be hazardous due to elements other than iridium
or due to iridium itself. At the same time, most iridium compounds are
insoluble, which makes absorption into the body difficult.
A radioisotope of iridium, , is dangerous, like other radioactive
isotopes. The only reported injuries related to iridium concern
accidental exposure to radiation from used in brachytherapy.
High-energy gamma radiation from can increase the risk of cancer.
External exposure can cause burns, radiation poisoning, and death.
Ingestion of 192Ir can burn the linings of the stomach and the
intestines. 192Ir, 192mIr, and 194mIr tend to deposit in the liver,
and can pose health hazards from both gamma and beta radiation.
External links
======================================================================
* [
http://www.periodicvideos.com/videos/077.htm Iridium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.britannica.com/EBchecked/topic/293985/iridium-Ir
Iridium in Encyclopædia Britannica]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Iridium
