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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Flotsam and jetsam

One of the topics I keep coming back to here at Skeptophilia is the possibility of extraterrestrial intelligence.  I have to admit, it's a bit of an obsession with me, and has been ever since I watched Lost in Space and the original Star Trek as a kid.

As with so many things, though, this fascination runs headlong into my staunch commitment to rationality, hard evidence, and the scientific method.  The SETI (Search for Extraterrestrial Intelligence) program has, to date, found no particularly good candidates for a signal from an alien race.  The Fermi paradox -- Enrico's famous question that if the likelihood of extraterrestrial life is so high, then "where is everyone?" -- brings us to the rather depressing answer of the three f's, about which I wrote in detail a couple of years ago.

UFO aficionados point toward all of the sightings of alleged alien spacecrafts, and the more skeptical of them rightly insist that even if it's only a small fraction of them that aren't dismissible because of the usual explanations (hoaxes, camera glitches, natural phenomena mistaken for UFOs, etc.), those few are still worth investigating.  Physicist Michio Kaku, who has gained a bit of a reputation for being out in the ionosphere on the topic, said, "You simply cannot dismiss the possibility that some of these UFO sightings are actually sightings from some object created by an advanced civilization… on the off chance that there is something there, that could literally change the course of human history."

But the fact remains that at present we have zero scientifically admissible evidence for the existence of ET. 

Not so fast, says physicist B. P. Embaid, of Central University in Venezuela, in a paper available at arXiv (but not yet peer reviewed).  Embaid has been studying minerals found in meteorites, and he found two -- heideite and brezinaite -- that he says are superconductors that can only be synthesized in a laboratory.

And therefore, the meteorites in which they were found are fragments of a wrecked spaceship.

In Embaid's favor is the fact that heideite and brezinaite are weird minerals, and have never been found in a single natural terrestrial sample.  Brezinaite was created in 1957 by carefully layering chromium and sulfur; heideite eleven years later, by chemically combining chromium, iron, sulfur, and titanium.  Since their first synthesis, both minerals have been found in meteorites, but they have never been seen otherwise, even in ore samples rich in the constituent elements.

So, Embaid says, these are technosignatures -- relics from a technological civilization.

Predictably, my response is:

But I reluctantly must add that I need a good bit more than this to land myself squarely in Embaid's camp.  There's an important word I left out of my statement regarding heideite and brezinaite never showing up in terrestrial samples -- yet.  Recall that the element helium was first discovered on the Sun, from its characteristic spectral lines, long before it was detected in Earth's atmosphere.  I'm also reminded of the discovery in a meteorite of nonperiodic quasicrystals, a form of matter not thought to be naturally occurring anywhere, by a team led by physicist Paul Steinhardt (and which was the subject of his fascinating book The Second Kind of Impossible, which I highly recommend).  It's always tempting to assume that what we know now represents the final, definitive answer, and forget that nature has a way of surprising us over and over.

So could the discovery of two odd superconducting minerals in meteorites mean that we're looking at the flotsam and jetsam of a wrecked extraterrestrial spacecraft?  Sure.  We shouldn't dismiss that possibility simply because the bent of a lot of scientists is to scoff at UFOlogy; that is in itself a bias.  But based on what we currently have, it is way premature to conclude that the anomalous meteorites are technosignatures.  

Now, if a meteorite contained some superconducting materials laid out in a pattern reminiscent of a circuit board, then you might have me convinced.  That, after all, is how the Tenth Doctor figured out what was going on in "The Fires of Pompeii:"

And hey, if a piece of evidence is good enough for the Doctor, it's good enough for me.

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Forensic geology

I've been interested in rocks since I was a kid.  My dad was a rockhound -- more specifically, a lapidary, who made jewelry from such semiprecious stones as turquoise, agate, and jasper.  The high point of my year was our annual trip to Arizona and New Mexico, when we split our time between searching for cool rocks in the canyons and hills of the southwestern desert and pawing through the offerings of the hundreds of rock shops found throughout the region.

Besides the simple beauty of the rocks themselves, it fascinated me to find out that with many rocks, you could figure out how and when they formed.  A lot of the gem-quality rocks and minerals my dad was looking for -- malachite, azurite, and opal amongst them -- are created by slow precipitation of layers of minerals from supersaturated water; others, such as lapis lazuli, rhodonite, and garnets form when metal-bearing rocks are metamorphosed by contact with magma far underground.

[Image licensed under the Creative Commons Olga Semiletova, Минералы горных пород, Creative Commons Attribution-Share Alike 4.0 International license]

Once I found out that the "when" part was also often knowable, through such techniques as radioisotope dating and stratigraphy, it was always with a sense of awe that I held pieces of rock in my hand.  Even around where I live now, where there are few if any of the lovely gem-quality stones you find in the southwest, there's still something kind of mind-boggling about knowing the layers of limestone and shale that form the bedrock here in upstate New York were formed in the warm shallows of a warm ocean during the Devonian Period, on the order of four hundred million years ago.

But if you think that's impressive, wait till you hear about the research out of the University of Johannesburg that was published in the journal Icarus last week.

The research centered around a stone in the desert of western Egypt called Hypatia, given the name by Egyptian geologist Aly Barakat in honor of the brilliant, tragic polymath whose career was cut short when she was brutally murdered by a mob on the orders of Cyril, bishop of Alexandria.  (The aftermath, although infuriating, is typical of the time; Hypatia was largely forgotten, while Cyril went on to be canonized as a saint by the Roman Catholic Church.)  The stone, fittingly considering Hypatia's contributions to astronomy, turns out to be extraterrestrial in origin, later falling as a meteorite to the surface of the Earth.

But "extraterrestrial" is a big place, as it were.  Where exactly did it form?  Chemical tests on the rock found that it didn't match the composition of any known asteroid or comet; then, the mystery deepened when it was found to contain nickel phosphide, which has never been found on any solid material tested in the entire Solar System.

Further tests only made the rock seem more anomalous.  Silicon, second only to oxygen as the most common element in the Earth's crust (a little over 28%, to be exact), was almost absent, as were calcium, chromium, and manganese; on the other hand, there was far more iron, sulfur, phosphorus, copper, and vanadium than you'd expect.  The ratios were far off not only from rocks in our Solar System, they didn't match the composition of interstellar dust, either.

The researchers decided to go at it from the other direction.  Instead of trying to find another sample that matched, they looked at what process would create the element ratios that Hypatia has.  And they found only one candidate that matched.

A type 1a supernova.

Type 1a supernovas occur in binary star systems, when one of the stars is relatively low mass (on the order of the Sun) and ends its life as a super-compact white dwarf star.  White dwarf stars have an upper limit on their mass (specifically about 1.4 times the mass of the Sun) called the Chandrasekhar limit, after Nobel Prize winning astronomer Subrahmanyan Chandrasekhar.  The reason is that at the end of a star's life, when the outward pressure caused by the fusion in the core drops to the point that it can't overcome the inward pull of gravity from the star's mass, it begins to collapse until some other force kicks in to oppose it.  In white dwarf stars, this occurs when the mutual repulsion of electrons in the star's constituent atoms counterbalances the pull of gravity.  In stellar remnants more than 1.4 times the mass of the Sun, electrostatic repulsion isn't powerful enough to halt the collapse.  (The other two possibilities, for progressively higher masses, are neutron stars and black holes.)

In binary stars, when one of the members becomes a white dwarf, the gravitational pull of its extremely compact mass begins to siphon material from its companion.  This (obviously) increases the white dwarf's mass.  Once it passes the Chandrasekhar limit, the white dwarf resumes its collapse.  The temperature of the white dwarf skyrockets, and...

... BOOM.

The whole thing blows itself to smithereens.  Fortunately for us, really; a lot of the elements that make up the Solar System were formed in violent events such as the various kinds of supernovas.  But the models of the relatively rare type 1a (only thought to happen once or twice a century in a typical galaxy of a hundred billion stars) generate a distinct set of elements -- and the percent composition of Hypatia matches the prediction perfectly.

So this chunk of rock in the Egyptian desert was created in the cataclysmic self-destruction of a white dwarf star, probably long before the Solar System even formed.  Since then it's been coursing through interstellar space, eventually colliding with our obscure little planet in the outskirts of the Milky Way.

When I was twelve, holding a piece of billion-year-old limestone from the Grand Canyon, little did I realize how much more amazing such origin stories could get.

I think the real Hypatia would have been fascinated.

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