Thursday, September 5, 2013

Why is Platinum More Expensive than Gold?

During a discussion at dinner today, I asked a question that seems silly and naive:  why is platinum more expensive than gold?  The answer, I think, is interesting and surprisingly complicated.

I read an article recently that said that the majority of the universe's gold and platinum originates from neutron star collisions, and in fact that 7 to 10 times more platinum comes from these collisions than gold.  Figure 1 shows a reproduced and modified figure from the article where they show the expected mass fraction ejected of a variety of atomic mass numbers.  It's noteworthy that gold is stable only at mass number 197, whereas platinum is stable at mass numbers 194, 195 and 196.  Roughly 1/3 of the platinum around today is atomic mass 194, 1/3 is 195, about 1/4 is 196, and most of the rest is atomic mass 198.

Figure 1:  Adapted from Goreily et. al. (2011).  Mass fractions of heavy elements ejected from neutron star collisions.  This is believed to be the major source of heavy elements in the universe.
If we assume that all of the ejecta at each mass number ends up being the corresponding element, then summing the mass fractions for platinum 194, 195, 196 and 198 in figure 1 we see that the mass fraction of platinum ejected from the collision of two neutron stars is about 0.015, and the mass fraction of gold ejected is 0.002.  That means that about 7.5 times more platinum coming out of our collision than gold.

Now our chunks of platinum and gold hurtle out of a gravity wave inducing neutron star collision into space and eventually land in our solar system in the form of meteors.  There's no reason to believe that platinum and gold have separated out significantly on their way to Earth since the abundance of these elements on Earth are 1.9 micrograms/gram for platinum and 0.16 micrograms/gram for gold--so there is roughly 11.8 times more platinum than gold in our planet as a whole by mass.

However, the platinum in the core and the mantle is all but inaccessible to us.  The abundance of platinum and gold in the crust on the other hand is roughly equal at 0.0037 parts per million for platinum and 0.0031 parts per million for gold.  It is still not worth mining either unless some natural process concentrates it, and it seems that there must be fewer such natural processes that lead to the concentration of platinum since only 179 tons of platinum were mined in 2012 compared to 2,700 tons of gold.  For instance, gold deposits form at fault lines when rapid pressure changes during earthquakes cause dissolved gold to precipitate from water underground.  I could not find any articles on natural mechanisms for concentration of platinum.

Finally, there is human history working in favor of gold.  Gold has been known and collected for as long as recorded history, whereas platinum was only recently discovered in the mid-1750s.  Nearly all of the gold that humans have ever collected is still with us in one form or another, and it is estimated that all of the gold that humans have ever possessed equals about 283,000 metric tons, compared to only about 6,000 metric tons of platinum.

So, thanks to geology and human history, we have gone from platinum being 7.5 times more common than gold in neutron star collisions to being substantially scarcer than gold in our possession.  Without considering global demand, it's still fair to say this scarcity in supply is why platinum is more expensive than gold.