Accretionary Wedge #49: Optical Mineralogy in Space!

Time for another Accretionary Wedge, this month hosted by En Tequila es Verdad, the topic being Out of This World - extraterrestrial geology. For this post I'm looking back to my undergraduate optical mineralogy/petrology and planetary geology classes, when we had the unique treat of participating in NASA's Lunar and Meteorite Petrographic Thin Section Program. This allowed us to examine thin-sections and small samples of space rocks, ranging from meteorites collected on Earth (mostly from Antarctica) to Moon rocks and regolith brought back from the Apollo Program. With the recent passing of astronaut Neil Armstrong, I thought this would be a relevant subject.  As a pilot I also have a thing for the whole NASA/Space Travel thing. But for even longer than I've been a geologist and pilot I've been a space nerd, so pardon while I nerd out. The following specimen pictures were taken by myself and fellow classmates.

As most of you probably know (but some might not), you can learn a lot about a rock by gluing it to a piece of glass and polishing it down very thin until it is transparent (a thin-section). I recall botching a few of these...



One of the grinding machines tended to throw the small block of rock being sectioned (the billet) across the lab...

I couldn't imagine the pressure on whoever was preparing the Lunar Samples! Once they're in thin-section, you take a look at them through a petro-scope, a fancy microscope with special filters for distinguishing minerals by their optical properties.

Petro-scope and colored pencils - the tools of the optical mineralogy student.



On the high-magnification lens it is possible to accidentally focusing "into" and break the thin-section, so the NASA samples were prepared on extra-thick glass.

Now, to the good stuff!


Meteorite Samples

The kit came with some hand samples of meteorites for us to look at to learn about the different meteorite types. The range usually has to do with the iron and carbon content.

Some meteorite varieties in the NASA educational kit.

One of the samples was of the highly prized high-iron variety showing Widmanstätten patterns, the result of large iron crystals growing in the parent body which the meteorite originated from. It takes millions of years of cooling to grow iron crystals this size. We just had fun trying to pronounce it (vit-min-shtot-in?)

Iron meteorite with Widmanstätten patterns.

Also high in iron, but also high in olivine crystals, was the pallasite meteorite. While the iron meteorites above are thought to have cooled in the iron core of a small sort-of planets early in the history of the solar system which then broke apart, these pallasite meteorites represent the boundary between the iron core and the olivine-rich mantle of that planet. This is my favorite meteorite type.

Pallisite metoerite sample containing iron (silver and red) and olivine crystals (green). This formed near the boundary between the iron core and olivine mantle of a small planet early in the solar system's history which then broke apart. Which then fell to Earth. And someone found it, and then it made it's way to our classroom. Quite a journey.

Ok, I lied. The next meteorite is probably my favorite. It was a very small sample but it continually blew everybody's mind:




This inconspicuous rock is a Shergottite meteorite, aka a Martian Meteorite. No, this was not brought back from some sample mission to Mars. It was found in Libya in 1998. These rocks from the Red Planet make it to Earth by asteroids impacting Mars and ejecting rocks into space, which eventually makes it to Earth. I knowwwww!

As for the scopes, I spent most of this time looking at chondrules. These are spheres in the meteorites from molten droplets which cooled in space during the early formation of the solar system. Just looking at little crystals which formed when the Earth did ~4.5 billion years ago, no big deal. Since they haven't changed much since their formation, these are the samples which geologists use to understand the age and chemistry of the early Earth.



Meteorite thin-section, showing two chondrules of radial pyroxene; the left chondrule is still in tact, while the bottom chondrule has shattered. XPL, 2mm across.

It was apparent to us that the meteorites were mafic in composition; quartz was not present in any samples. This was an important thing to bring up, since having felsic rocks containing quartz is unique in the solar system, and requires the geologic refining process of plate tectonics. Another interesting thing was seeing how minerals which formed in low pressure and zero gravity look.




Meteorite thin-section with a chondrule containing barred olivine; the yellow and red bunches which intersect at a right angle. Terrestrial olivine crystals are usually more amorphous. XPL, 2 mm across.

One interesting topic of planetary geology is the presence or absence of liquid water. We generally thought of liquid water as being unique to earth, so this next thin-section came as a bit of a surprise to all of us:




Meteorite cross-section; the mineral grain in the cross-hair is serpentine, a mineral which forms from the reaction of mafic minerals with water. Liquid water must have then been present in space to form this serpentine. The dark matrix is due to the high carbon content, making this a carbonaceous chondrite. PPL, 2 mm across.



Some chondrules were twinned, such as this one, containing two seperate optically continuous grains of olivine.

An example of a relic chondrule - the original chondrule has overprinted by recrystallization, so it is not really visible in PPL (top), but can be seen in XPL (bottom).

Lunar Samples

These were samples of Lunar regolith, the fine-grained sandy material on the surface of the moon (Moon dust). This regolith forms from the constant bombardment of meteorites on the Moon's surface combined with a bit of strange volcanism. The mineral grails were usually large enough to identify. For anyone who has done some sand petrology, you'll probably notice how extremely angular these grains are (as explained in this clip from the "Moon Hoax" episode of Mythbusters).

The first set of samples were mare soil collected from Apollo 17 on December 11, 1972, landing at Taurus-Littrow. This was the last Apollo mission to visit the Moon, and also the only mission to have an astronaut who had a PhD in geology and didn't serve in the armed forces, Harrison "Jack" Schmitt. I saw Jack Schmitt present at the National GSA Meeting in Denver in 2010 - it was strange hearing someone describe Moon craters from "having stood in one".




Apollo 17 landing site (left) and collection site for our regolith (right).
 

Apollo 17 mare soil slide.



Regolith grains in the microscope. Lunar regolith at this site consists mostly of plagioclase feldspar (good sample just to the left of the cross-hair with twinning), breccia grains (messy blue/yellow/opaque grains), and volcanic glass (yellow). 2 mm across, PPL.

 

 

High-magnification view of a volcanic glass grain. The yellow portion is the "glass", while the clear spaces are olivine and ilmenite crystals. The fuzzy black feathery-looking things are some kind of strange oxide which grew into the glass from the olivine and ilmenite.

 

Some samples of the regolith had these orange volcanic glass spheres which were visible as orange soil by the Apollo 17 astronauts. These spheres formed from "fire fountains", volcanic vents which spewed magma droplets which cooled into spheres before falling back to the Moon. The other grains are plagioclase and breccia.

 

Some samples were lousy with the orange soil (orange grains). Apparently the regolith on the Moon only falls on one extreme or the other of the angularity classification.

The next collection site was from Apollo 16 on April 27, 1972 (I'm not going in chronological order). The type of regolith here is referred to as highland soil.




Apollo 16 landing site (left) and the Apollo 16 collection site for the following samples (right).

 

Apollo 16 highland soil regolith slide.



Apollo 16 regolith, consisting of plagioclase feldspar (magenta/yellow twinned grain toward the bottom), volcanic glass (yellow), lunar breccia (dirty opaque grains), and even a chondrule (center). XPL, 2mm across.

 

Magnified view of the chondrule from the previous image. The circular shape means this cooled and formed in zero-gravity before returning to the surface of the Moon. The angular crystals inside grew inward from the edges.

The set of samples was from Apollo 15, July 30th, 1971. These samples were more regolith which showed high deformation due to impacts.




Apollo 15 landing site (left) and sample collecting (right).



Apollo 15 regolith slide.

Apollo 15 regolith. In the cross-hair is a volcanic glass sphere which has been shattered due to impacts on the Lunar surface. The rest of the sample is breccia. PPL, 2mm across.



Apollo 15 regolith with a large volcanic glass grain (yellow). This sample was unique because within the volcanic glass grain were these strange ninja-star shaped iron oxides (hard to see in the photo) which were only present in some of the glass. Although our sample guide pointed out the ninja-stars, it said why they were present in some and not all wasn't fully understood. The grain is surrounded by more breccia.

All in all, it was a unique and amazing experience for us to get a chance to examine these space rocks under the petroscope. The minerals were identifiable by us, while at the same time not typically looking like anything we had seen from our Earth rocks. It really was interesting to see how quartz is non-existant in space rocks.

Meteorite minerals summary: mafic minerals (olivine and pyroxene) with varying amounts of iron, carbon, and serpentine. The age of these meteorites (~4.5 billion years old) and chemistry are generally regarded as the snapshot of the age and makeup of the early solar system.

Lunar minerals summary: regolith made of plagioclase feldspar and volcanic glass. Plagioclase is very common on the surface of the Moon, and is also the most abundant mineral of the Earth's crust, which is one of the reasons behind the current idea regarding the formation of the Moon, the Giant Impact Hypothesis.

Yay space!