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Osmium and Iridium are the two most dense elements in pure form. They are also very pretty metals, iridium particularly. Unlike osmium, iridium does not tarnish in air, and is in fact impervious to many forms of chemical attack, much like platinum. An alloy of 90% platinum and 10% iridium is widely used in both the jewelry and chemical industries, because it is strong, hard, beautiful, and impervious to almost all chemicals even at very high temperatures.
Which is the most dense element, osmium or iridium? That honor has changed hands a couple of times over the years. How, you might ask? Because measuring the density of a pure element isn't nearly as straightforward as you might think. For one thing, it can be different for different crystalline forms (allotropes) of the same element. For example, diamond is much denser than graphite: Which is the true density of carbon? Both. In the case of metals, it may be impossible to grow single crystals, so you're measuring the density of a polycrystalline mixture, and the density may depend on details of how the material cooled. Minute impurities can also have a big impact on the density.
So, as measurements and purities were refined over the years, the current best values of osmium and iridium kept changing, and that's how they traded places as most dense of all. I solved this dilemma by only having room to engrave three decimals places of the density, and to three decimal places their density is identical, so I didn't have to pick sides.
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 Real iridium.
Iridium metal is hard to get, and very expensive. In fact, it was in the very last group of elements I was able to acquire to complete my collection. It was donated by the extremely kind Max Whitby of the The Red Green & Blue Company, which sells a complete collection of elements.
Osmium and Iridium are the two densest elements in the world (they are in fact so close in density that which one is considered the densest has switched a couple of times over the years). Even though this is a quite small lump, you can feel its weight when you shake the bottle: Quite surprising. Having a large block of this would be remarkable, but the closest I'm likely to come to that is my large blocks and cylinders of tungsten, which are only about 15% less dense.
To learn more about the set you can visit my page about element collecting for a general description or the company's website which includes many photographs and pricing details. I have two photographs of each sample from the set: One taken by me and one from the company. You can see photographs of all the samples displayed in a periodic table format: my pictures or their pictures. Or you can see both side-by-side with bigger pictures in numerical order.
The picture on the left was taken by me. Here is the company's version (there is some variation between sets, so the pictures sometimes show different variations of the samples):
Source: Max Whitby of RGB
Contributor: Max Whitby of RGB
Acquired: 20 January, 2003
Text Updated: 11 August, 2007
Price: Donated
Size: 0.2"
Purity: 99.95%
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Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.
To learn more about the set you can visit my page about element collecting for a general description and information about how to buy one, or you can see photographs of all the samples from the set displayed on my website in a periodic table layout or with bigger pictures in numerical order.
Source: Rob Accurso
Contributor: Rob Accurso
Acquired: 7 February, 2003
Text Updated: 29 January, 2009
Price: Donated
Size: 0.2"
Purity: >99%
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Museum-grade sample.
In early 2004 Max Whitby and I started selling individual element samples identical or similar to the samples we use in the museum displays we build. These are top-quality samples presented in attractive forms appropriate to the particular element. They are for sale from Max's website and also on eBay where you will find an ever-changing selection of samples (click the link to see the current listings).
This bottle contains about 50 grams of arc-melted buttons made in Max's reduced-pressure argon-arc furnace.
Source: Theodore Gray
Contributor: Theodore Gray
Acquired: 24 February, 2004
Text Updated: 11 August, 2007
Price: See Listing
Size: 2"
Purity: >99%
Sample Group: RGB Samples
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 Ion source.
This was described as a "Varian 0981-82850-302 Thoria / Iridium Ion Source". Whether this means it contains thoria (a name for thorium oxide) and/or iridium I'm not clear. I'm also not clear whether it's meant to emit iridium ions, or other kinds of ions with the iridium being the material out of which some component of it is made. I think probably the latter. It looks like all plain metal to me, but I could be mistaken. In any case, it's a fascinating bit of tangled metal obviously intended to be used inside a vacuum chamber to emit some kind of ions, and it probably contains some iridium, which makes it good enough for me to list.
Source: eBay seller dougcarol1987
Contributor: Theodore Gray
Acquired: 11 June, 2005
Text Updated: 31 May, 2006
Price: $16
Size: 3"
Purity: 0%
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K-T Boundary Clay.
The K-T boundary clay is is found in thin layers all over the world at sedimentary levels that indicate it is the same age everywhere: About 65 million years ago. This is the boundary between the Cretaceous (K for some reason) and the Tertiary (T for obvious reasons) periods, and also the time at which there was a mass extinction.
That such a thin layer of similar material should be found all over the globe is strange, but what's even stranger is that it is always highly enriched in iridium compared everything around it. It's as if something dumped a huge quantity of iridium on the earth and spread it around in some kind of giant explosion.
That something was almost certainly a large (ca. 10km diameter) chondritic meteorite, a type known to contain very high levels of iridium compared to the earths crust. All the evidence points to such an object hitting the Yucatan peninsula of Mexico a the same time the clay was deposited and the dinosaurs became extinct.
And the dark layer in this rock is a tiny bit of that iridium-rich clay material.
Source: Jensan Scientifics
Contributor: Theodore Gray
Acquired: 8 April, 2009
Text Updated: 9 April, 2009
Price: Anonymous
Size: 1.5"
Purity: <1%
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Osmiridium (aka Iridosmine).
These are very small granules of native (naturally occurring) osmium-iridium alloy. These two metals are often found combined this way, and because the alloy is actually in many ways more useful than either element on its own, the mixture is often used just as it's found, for example in the tips of expensive fountain pens. Don't be fooled by the picture: This stuff is about the size of fine sand, I've just got a really good macro lens.
Source: eBay seller entropydave
Contributor: Theodore Gray
Acquired: 15 February, 2006
Text Updated: 5 December, 2006
Price: $85
Size: 0.02"
Composition: IrOs
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Big bag of beads. (External Sample)
This is a large ($18,000) order of platinum group metals placed by an excellent customer of my partner Max Whitby's element sales business. I happened to be visiting Max in London just before the order needed to be shipped to our customer in the US, so I hand-carried the precious cargo home rather than risking international shipping. These beads were made by Max in his reduced pressure argon arc furnace.
The customer wishes to remain anonymous, so you'll just have to keep wondering where this remarkable trove of rare metals currently resides: The only thing you can be sure of is that I don't have it.
Source: Max Whitby of RGB
Contributor: Max Whitby of RGB
Acquired: 4 September, 2007
Text Updated: 7 September, 2007
Price: N/A
Size: 0.25"
Composition: ReRuOsIr
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 Oliver Sacks' slab. (External Sample)
This iridium was original sold to Sacks by my friend Max Whitby in the form of marble-sized buttons. Sacks wanted it in the form of a single large slab, and in the summer of 2004 I got a chance to visit Kaistar R&D in New Jersey with him. Kaistar is run by a family of Russians and they have the lovely electron-beam furnace you see here, with Sacks in front of it holding his newly minted 1.7 pound lump of solid high-purity iridium. Read my story about the trip for more information and pictures of what goes on inside the furnace.
Location: Oliver Sacks' Office
Photographed: 6 August, 2004
Size: 2.5
Purity: 99.995%
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Big bag of beads. (External Sample)
This is part of a large ($18,000) order of platinum group metals placed by an excellent customer of my partner Max Whitby's element sales business. I happened to be visiting Max in London just before the order needed to be shipped to our customer in the US, so I hand-carried the precious cargo home rather than risking international shipping. These beads are made by Max in his reduced pressure argon arc furnace.
The order consisted of equal volumes of ruthenium, rhenium, osmium, and iridium. Here is what the whole collection looks like:
The customer wishes to remain anonymous, so you'll just have to keep wondering where this remarkable trove of rare metals currently resides: The only thing you can be sure of is that I don't have it.
Location: Anonymous
Photographed: 4 September, 2007
Text Updated: 6 September, 2007
Size: 0.25"
Purity: 99.99%
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