To the left you see a logarithmic graph comparing the physical abundances, by
weight in the earth’s crust, of what many would recognize to be the six
most precious or “noble” metals. Ir, Au, Rh, Pd, Pt, and Ag stand
for iridium, gold, rhodium, palladium, platinum, and silver respectively.
These figures are at best educated guesses, but according to most sources iridium is the
clear winner. It occurs at about four-tenths of a part per billion, a ratio
similar to that of a grain of salt to a Clydesdale, making it about half
again as rare as gold.
The silver shouldn’t surprise anybody, but look at all that platinum
we’re wallowing in. It’s a dozen or more times as abundant as
gold, though that’s largely offset by its greater difficulty and
expense of extraction. Its supply is also pretty dicey, as
80% of it comes from South Africa.
Before going further, we should probably qualify our rare metal
quest with “non-radiological.” The Periodic chart arranges
elements in rows and columns according to their chemical properties. About
80% are metals. As far as anyone knows, only elements with atomic numbers of
1 through 92 — hydrogen through uranium — exist in nature.
Anything transuranic, numbers 93 and above, has to come from a reactor [up to and including #100, fermium] or
particle accelerator. As of late 2006, physicists at Russia’s Dubna facility
had synthesized three atoms of element 118, eka-radon. All transuranics are
radioactive and many are very short-lived, so none would be practical for
coinage or jewelry.
Several naturally occurring metals such as francium, polonium, and astatine
are also radioactive and ultra-rare. You’ll frequently read that the
total astatine supply in the earth’s crust at any given moment is about an
ounce (28 grams).
Ir
Pt
Au
Rh
Pd
Ag
Density
Precious metals are heavy. Here’s a chart comparing the
original six. Iridium is the densest known terrestrial substance at 22.65
grams/cm3. That’s
twice the density of lead or 8 times that of granite. A cube of iridium 6
inches on a side (15 cm) would weigh as much as an average
adult human.
Some of the superheavy elements will probably turn out to be much denser,
providing any can last long enough and exist in sufficient
quantities to be weighed. Physicists speculate that element 108, known as
hassium, may be almost twice as heavy as iridium. Dr. Burkhard
Fricke, an editor of Physics Letters A, suggested in a paper that
densities might peak at element 164, provisionally called dvi-lead, at around
46 grams/cm3.
I recall an old Mission Impossible episode in which the crew smuggled a
gigantic quantity of platinum out of some country by casting it into a shiny
new bumper and installing it onto the front of a car. In view of the
metal’s extreme weight, I wondered how such a car might handle.
Ir
Rh
Pt
Pd
Au
Ag
“Tenacity”
To plot a characteristic we could call tenacity, I combined the hardness,
stiffness1, and melting point of each of the previous six metals. Gold and silver
melt easily and when pure are so soft you can push your fingernail
into them. You need to alloy them with copper or other metals to make them
durable enough for coins or jewelry. Not so with rhodium and iridium. Iridium is
the hardest and stiffest of all six and has the highest melting point at 4435°F
(2446°C). It’s also the most incorruptible metal of any,
resisting all acids including even aqua regia — a bubbling, fuming, 3-to-1 mixture
of hydrochloric and nitric acids worthy of any mad scientist.
Many years ago in Hollywood I pitched a science fiction screenplay whose characters
used holographically ornamented iridium coins. They would be spectacularly
durable. They would also be impossible to counterfeit, since nothing else would
be heavy enough. Osmium comes close, but it’s smelly and toxic (it readily forms
a tetroxide that can blind or even kill you) and similarly scarce. The legendary British firm of John
Pinches is said to have once struck an iridium medallion.
Jewelry makers already wince at the prospect of working in platinum. Because of
its high melting point and quick hardening as it cools, it usually requires a
centrifugal cast. But iridium presents even graver challenges. Its melting point
is 30% higher, and despite its hardness it will crack if you try to hammer it.
Alloying it with a smidgen of platinum would probably boost its resilience
without appreciably debasing its value, though other
problems remain.
One solution is to powder the iridium as finely as possible and mix in a
moist binder to create a paste. You then form that into whatever shape you
desire and bake it in a kiln. This is called sintering. The particles will weld
themselves together into a mass at temperatures far cooler than the
melting point and the binder will cook away. This is how they make tungsten light
bulb filaments.
Other possibilities for iridium crafting include carving it like a stone with
diamond or cubic boron nitride abrasive, electroplating with one of its many
colorful salts dissolved in a liquid, or performing chemical vapor deposition using
iridium hexafluoride (IrF6).
Ir
$ 730 (-1%)
Au
$ 1220 (+1%)
Rh
$ 2270 (-11%)
Pd
$ 481 (+2%)
Pt
$ 1530 (steady)
Ag
$ 18.16 (-1%)
USD / Troy Ounce (% Monthly Change)
Here are the approximate prices for these six metals, from top to bottom in
order of increasing physical abundance, plotted logarithmically, last updated in mid
August 20102.
Though iridium is the scarcest, its price is modest because
its utilities are minor and people don’t crave the straw-colored
metal on an emotional level like they do gold and platinum. All it
needs is some good marketing. One selling point might be that most if not all
mined iridium ultimately comes from meteorites.
Rhodium has really run amok. Until 1985 it never traded above $1000, but it
rose to $5350 in 1991, sunk to $183 in 1997, then spiked to $10,000 in
mid 2008. Analysts cite the boom in the use of rhodium for automobile
catalytic converters in the late 1980s combined with chronic work stoppages at the
South African mines where most of it comes from. Aside from the converters,
rhodium serves to harden platinum and palladium and appears frequently in
jewelry, especially as a plating over white gold (which raises the question of why they’d
bother with the gold at all if you never get to see any).
The Guinness Book of World Records presented Paul McCartney with a rhodium-plated disc in 1979.
Here’s an uncommonly enterprising mint
that now sells solid 1-gram (about 31 to the troy ounce) rhodium medallions, though since the metal is
hard and brittle like iridium and thus murder to work with you’ll need to cough up $35 or so
over its bullion price.
So...
what’s all this business about $1 billion per troy
ounce? Are there metals far scarcer than iridium and enormously more expensive
than rhodium — while at the same time, non-radioactive or very, very
nearly so? As it happens, yes.
Each element comes in varieties called isotopes whose atoms differ in the number
of neutrons in their nuclei. You’ve probably heard of uranium-235 and,
well, the polonium-210 that did in former KGB officer Alexander Litvinenko. Both
are at least moderately radioactive and thus off limits for our quest. But there
are all sorts of stable3 isotopes, too. For example, silver comes in two of
them, 107 and 109. Their natural proportions are 51.85% and 48.15% respectively.
Gold and rhodium are rather unusual in that they occur in only one stable
isotope each, gold-197 and rhodium-103. Tin has the most, ten.
So what you’re looking for is an element that’s extremely scarce in
parts per billion, and an isotope of it that’s of such a tiny proportion
that the product of both numbers is the smallest of any earthly substance.
Osmium comes in seven stable isotopes, and among them osmium-184 is the rarest
at 0.02%. That times the element’s 1.8 parts per billion equals about a
half part per trillion. But as mentioned above, osmium’s not the nicest
stuff to have lying around. Plus, although you can correct me if I'm wrong, no one
seems to have a credible price for 184Os.
For a far more serviceable candidate we don’t have to look far.
Platinum comes in stable isotopes 190, 192, 194, 195, 196, and 198. Among those,
the scarcest is 190, whose natural occurrence is 0.014%. If platinum as a whole
exists in 37 ppb in the earth’s crust, 190Pt would be 0.014% of
that: 0.005 ppb or 5 parts per trillion. Therefore, isotopically pure
platinum-190 is the most precious metal in the world.
Robert A. Freitas Jr., author of “Tangible Nanomoney” in Issue 2 of
the Nanotechnology Industries Newsletter, speculates a figure for
190Pt of $1,347,960 per gram for 4.19% enrichment. This would come
out to $32 million per gram in its pure state, or about $1 billion per
troy ounce.
The Coin Page Engelhard Industrial Bullion Prices Index of Isotope Products, Oak Ridge National Laboratory Platinum Today by Johnson Matthey Precious Metals Marketing Webelements.com Scholar Edition by Mark Winter, University of Sheffield, UK.
Notes:
1. For this I’m using the Young’s modulus, a ratio
of applied stress to resulting strain (change in dimension).
2. As they say, for informational purposes only.
3. I’m considering any isotope with a half-life exceeding a billion years or so to be stable.
Before going further, we should probably qualify our rare metal
quest with “non-radiological.” The Periodic chart arranges
elements in rows and columns according to their chemical properties. About
80% are metals. As far as anyone knows, only elements with atomic numbers of
1 through 92 — hydrogen through uranium — exist in nature.
Anything transuranic, numbers 93 and above, has to come from a reactor [up to and including #100, fermium] or
particle accelerator. As of late 2006, physicists at Russia’s Dubna facility
had synthesized three atoms of element 118, eka-radon. All transuranics are
radioactive and many are very short-lived, so none would be practical for
coinage or jewelry.
providing any can last long enough and exist in sufficient
quantities to be weighed. Physicists speculate that element 108, known as
hassium, may be almost twice as heavy as iridium. Dr. Burkhard
Fricke, an editor of Physics Letters A, suggested in a paper that
densities might peak at element 164, provisionally called dvi-lead, at around
46 grams/cm
Jewelry makers already wince at the prospect of working in platinum. Because of
its high melting point and quick hardening as it cools, it usually requires a
centrifugal cast. But iridium presents even graver challenges. Its melting point
is 30% higher, and despite its hardness it will crack if you try to hammer it.
Alloying it with a smidgen of platinum would probably boost its resilience
without appreciably debasing its value, though other
problems remain.
So...
what’s all this business about $1 billion per troy
ounce? Are there metals far scarcer than iridium and enormously more expensive
than rhodium — while at the same time, non-radioactive or very, very
nearly so? As it happens, yes.
