Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Friday, September 22, 2023

In the pink

Sure, diamonds are pretty and sparkly and rare and valuable, but do you know how they form?  Because that's honestly the coolest thing about them.

Diamonds are found in geological formations called kimberlite pipes.  This is a structure shaped like a long, narrow ice cream cone, extending downward into the Earth (how far downward we'll get to in a moment), and characterized by some rocks and minerals you usually don't find lying around -- chromium-rich pyrope garnets, forsterite, and various types of ultramafic (low-silica igneous) rocks that break down to a very specific kind of clay.  Jewel hunters long ago figured out that diamonds were likely to be found in association with these rocks and minerals, and used those as indicators of where to look -- such as the diamond-rich Kimberly region of South Africa (which gave its name to kimberlite), a couple of spots in Greene and Indiana Counties, Pennsylvania, and the Udachnaya area of Siberia.

All of that's just background, though.  Remember how a few days ago, I mentioned how much I'm fascinated with things that are big and powerful and scary and can kill you?  Well, part of the cachet of diamonds is the fact that the way they form is all of the above.

Geologists discovered more or less simultaneously that the composition of kimberlite pipes is consistent with magma found in the (very) deep mantle, and that known kimberlite pipes extend a (very) long way down.  The best models indicate that the eruption that forms them starts on the order of four hundred kilometers below the surface of the Earth, making it the deepest known volcanic feature.

No one knows what triggers the eruption to begin.  It seems to be a rare occurrence, whatever it is.  Fortunately.  Because once it starts, and the magma moves upward through the mantle, the drop in pressure makes dissolved gases bubble out, rather like popping the cork off a bottle of champagne.  This speeds up the movement, which lowers the pressure more, so more gas bubbles out, and so on and so forth.  Also -- gases expand as the pressure drops, so the higher it rises, the more volume it displaces.

The result is what's called a diatreme.  What seems to happen is that with no warning, there's a Plinian eruption -- the same sort that destroyed Pompeii and Herculaneum -- but moving at supersonic speeds.  Imagine what it must look like -- from a distance, preferably -- everything is calm, then suddenly a several-kilometer-wide chunk of land gets blown up into the stratosphere.  The conical hole left behind fills with material from the deep mantle (thus its odd composition by comparison to other igneous rocks).  Give it a few million years, and weathering results in the characteristic clay found in a typical kimberlite.

So what's all this got to do with diamonds?

Well, in the intense heat and pressure of the eruption, some of the carbonate ions in minerals in the magma are reduced to elemental carbon, and that carbon is compressed to the point that its crystalline structure changes to a hexoctahedral lattice.  The result is a transparent crystal that looks nothing like the soft, black, powdery stuff we picture when we think of carbon.  (Further illustrating that bonding pattern is everything when it comes to physical properties.)

Why this all comes up, though, is that not all diamonds are the colorless transparent crystals that usually come to mind in association with the word.  Diamonds actually come in a variety of colors.  Now, on the surface, this isn't that unusual; pure corundum (crystalline aluminum oxide) is colorless, but if it has chromium impurities, it's red (those are called rubies), and if it has traces of iron and titanium, it turns blue (and are called sapphires).  The same is true with beryl (colored varieties include emeralds, heliodor, and aquamarine), spinel, and quartz.

Some diamond colors -- the yellows, blues, and greens -- are due to impurities as well.

The exception is pink.

[Image is in the Public Domain courtesy of photographer Roy Fuchs]

Pink diamonds are really rare, and although they're colored, it's not because they have impurities.  They're pure carbon, just like the colorless ones.  So how do they end up pink?

The clue came from where they're found.  Analysis of pink diamonds showed that they occur in places where kimberlite pipes get caught up in the rupture of tectonic plates.  So it's not just a colossal megaexplosion that's necessary to form them, they then need to get subjected to enormous pressures as supercontinents break up.  Those pressures cause the molecular bonds between the carbon atoms to bend, and that deformation is sufficient to change how the lattice interacts with light, and results in the crystal having a pink color.

Ninety percent of the known pink diamonds come from one place; the Argyle Formation in western Australia.  This area was right on the fault margin during the breakup of the Nuna Supercontinent 1.3 billion years ago.  And the paper that appeared in Nature this week showed that's no coincidence.  Take a colorless diamond, put it in the gigantic vise of a fault during a continental rupture, and it turns pink.

The whole thing is fascinating, not least because producing them requires being in the middle of two of the most violent processes on Earth.  Just goes to show that catastrophic events can result in beauty -- even if you wouldn't want to be in the middle of them as they occur.


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