Diamonds in the Deep

Diamond occupies the ideal niche between scarce and rare: it is sufficiently abundant that almost everyone can own one, but rare enough to command millions of dollars for newsworthy large stones.

 

The statement above comes from Robert Hazen’s Symphony in C that I’ve been reading. I’ve been thinking about diamond while doing class prep – looking over my notes on hybridization, pi-bonds and delocalized pi-systems. I usually spend half a class period on the allotropes of carbon (diamond, graphite, buckyballs, nanotubes, graphene) but in a Covid-19 season with a shorter semester, this vignette has become optional rather than essential. Even if we do end up discussing it in class, I will likely not have time to talk about the interesting factoids I’ve learned from Hazen’s book. So to keep the memory alive, that’s the topic for today.

 

While graphite is (thermodynamically) more stable than diamond, high pressure and temperature conditions can convert black graphite to translucent diamond. It’s a mind-blowing transformation if I stop to think about it. Making carbon more dense also makes it less opaque. Wow! So where can we find suitable conditions for diamond formation? In the deep earth! And the churning continental movements bring some of these close to the surface where we humans can dig for it. There are probably a bunch more in the deep, but modern technology allows us to make diamonds synthetically by applying high pressure and temperature in the laboratory. Apparently, there’s even an industry of “memorial” diamonds where you can turn the cremated ashes of a loved one into a gemstone.

 

Previously, “flawless” stones were all the rage, but nowadays “imperfect” stones are what’s in. What are these imperfections? It’s what’s in the stone. Other mineral trapped in the diamond provide new hues and sparkles. They even have fancy marketing names: cognac diamonds, champagne diamonds, etc. Scientists are particularly interested in these inclusions, be they mineral or even trapped gases and fluids, because they reveal something about geochemical processes a long time ago. As someone who does origin-of-life research, it’s important to learn as much as we can about the early earth, so that we can design appropriate experiments under plausibly prebiotic conditions to study how biochemistry came to be!

 

How old are diamonds? The oldest ones measured thus far are over three billion years old, but the wide range allows scientists to make educated guesses about the evolution of Earth’s mineralogy. Apparently, diamonds older than three billion years have noticeably different mineral inclusions compared to younger diamonds, revealing some of the intricacies of plate tectonics. Radioactive dating of trapped elements in diamond (typically with beta-decay of rhenium-187, not carbon-14 which has too short a half-life) allows us to estimate their age. And alas, I had to cut radioactivity and nuclear chemistry in a Covid-semester.

 

But there’s still much we don’t know even if we’d like to play up how much we do know. And I laughed when reading the following vignette Hazen provides from a 1952 publication by geophysicist Francis Birch. “Unwary readers should take warning that ordinary language undergoes modification to take a high-pressure form when applied to the interior of the Earth. A few examples of equivalents follow…” Here’s my reproduction of the table in Hazen’s book.

 

Scientists are generally cautious and use hedging language, likely irritating to politicians, pundits, and the public at large. In a high-pressure world of instant messages and outrage, words take meaning out of the ordinary. Certainty is strongly desired over uncertainty. But uncertainty is likely closer to reality. Such is the nature of complexity, and we live in a complex world. There are more diamonds in the deep. But they might prove difficult to access. And that’s what keeps things interesting. And expensive. Or we could look out to the stars. Apparently there might be a bunch of carbon-rich planets beyond our solar system, at least according to this news report last week. Diamonds in deep space. They’re everywhere!