hunting for meteorites

expeditions

Hunting for meteorites is a fun hobby in which nearly anyone can take part.  One should familiarize oneself with terrestrial rocks first, as these are common and often share traits not seen in (most) meteorites.  Rock-hounding is a good way to ease into meteorite hunting.  Learning to tell the difference between a fossil and host rock by looking at fine differences in color and texture is similar to what one does when looking for meteorites.

Superior Valley (East)
Our eighth find from Superior Valley (~2 grams); note the meteorite’s characteristic rusty brown color. On some lake beds, meteorites tend to be darker in color than the surrounding rocks (e.g. this image). On other lake beds, the opposite is true. It depends upon the local geology.

It is important to know what a meteorite will look like.  One should look at all of the images/information available, and visit websites like this one before trying one’s luck and hunting in order to have a fair chance of finding a meteorite in the field.  Here’s a page with photos from some of our meteorite-hunting trips.  Teaming up with an experienced hunter can also help.  Learning in the field is perhaps more productive than looking at pictures ahead of time, but early trips can be daunting and discouraging if you’re ill-prepared.

It is important to be familiar with the geology of a given area to know what not to pick up.  In some places, iron-bearing sedimentary or igneous rocks can confound even the best hunters, but in others, a visually similar rock would almost certainly be a meteorite.

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Novato #3, an impact melt (formed in an asteroidal collision in space) — that does not resemble a typical chondrite. If hunting for a meteorite that has lain in the desert for thousands of years, I would not expect to find a meteorite that looked like this.

Many people believe that meteorites are simply black-grey-brown magnetic rocks.  Without additional testing, they will claim that their stones are meteorites.  Take, for example, the following two images:

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Terrestrial hematite, with a classic river-worn shape.  Listed on eBay as a meteorite as of 11/2013.  From China.
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Similar, but less weathered, river-tumbled hematite listed on eBay as hematite as of 11/2013. From China.

“Mekong River Iron” specimens (hematite) are commonly misrepresented as iron meteorites.  Why?  The rocks are magnetic and vaguely resemble the oxidized Nantan meteorites common elsewhere in China.  If the finders/sellers were more familiar with the local geology and/or appearance of meteorites, they could have avoided making this mistake.

One detail that’s particularly useful is fusion crust; as it weathers, it tends to leave a characteristic texture behind.  The above specimens of hematite have rounded high points, but lower areas that were protected from river-tumbling are still rough.

Ablation smooths even protected surfaces:

Playa3Puzzle5
A ~600 gram ‘puzzle’ comprised of 4 fragments. Note the uniformly smooth exterior despite the cracks that cross it. This smooth surface is remnant fusion crust. While most meteorites do not exhibit a complete fusion crust such as this, it pays to be able to tell the difference in texture between this (weathered) ablated surface and a water-worn rock.  The visible cracks are secondary; they formed via thousands of years of weathering on Earth.

The above meteorite started out as a completely smooth rock.  Cracks aside (formed via weathering), every face of the rock is smooth; even concave faces.  It’s unusual for terrestrial rocks to be weathered in such a fashion, but it does sometimes happen.

A fine example of the importance of knowing local rocks well can be seen in the following image.  The lake bed upon which the meteorite was found is covered with lava.  Vesicular basalt is the most common rock on the lake, and the meteorite lacks fresh fusion crust and resembles the basalt.  However, most of the basalt on this playa does not exhibit rounded corners or internal fractures.  It is also generally not magnetic, though there are rare exceptions.  A quick poke with the magnet cane confirmed high iron content, and a closer examination revealed protruding chondrules.  Why the small mud-filled dimples?  Some iron grains had weathered away, and some chondrules had been removed by erosion.  Again, not the large, ~smooth, slightly concave surface.

Playa3Find6
A ~200 gram shock-darkened chondrite. Sand-blasting on this lake bed has removed many chondrules, resulting in pitting that makes the stone look like a piece of basalt. Basalt is very common on many playas in the Mojave.

 

Here’s another example:

Playa2Find6
A highly weathered ordinary chondrite. Fe-Ni grains have weathered out, leaving tiny pits. A few chondrules are visible as grey bumps. 86 grams.

 

Few meteorites are this red/orange in color, but it sometimes happens when they’ve been on Earth for long enough.  This particular lake bed is covered with siliceous igneous rocks; they, too, lose some of their more reactive minerals with weathering, resulting in rocks with similar pseudo-porous textures (e.g. take a rock, weather the feldspars out, and you wind up something that appears to have small vesicles/holes).  The difference?  The above stone is more strongly attracted to a magnet and lacks the small rectangular crystals/vugs that can be seen in the local terrestrial rocks.  It also exhibits a few chondrules and is a slightly different color.  Note: no known meteorites contain sedimentary banding, though shock veins are sometimes present (see below).

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A ~180 gram fragment of an ordinary chondrite.

Details to note: smooth front due to remnant fusion crust. Top side is broken and weathered; it has a different, rougher texture. An uneven shock vein crosses the front of the stone. A large rusty spot is visible on the top of the stone. A large chondrule (light circle) is visible on the right edge. Not all rocks that contain spherical/round inclusions are meteorites. Most are not. But most Earth rocks do not contain similar inclusions, and most meteorites do, so they are a good discriminant. The same goes for rust; many terrestrial rocks oxidize or contain minerals that oxidize.  But whereas 90%+ of meteorites will rust over time, a minority of Earth rocks will exhibit rust; it can pay to give such rocks a second look.

Here’s another good example emphasizing color:

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A ~7 gram chondrite from Superior Valley East, now fenced off due to the expansion of Fort Irwin.

Sure, there are a few dark rocks in the image, but the meteorite (dark/round stone ~1-2 inches from end of cane) is clearly one of the darkest objects in the image.  If you can stop for a moment and rule out 95%+ of the rocks within sight, it’s a good thing to note.  Few meteorites are actually black (the ~200 gram shock-darkened stone pictured above is an exception), and one has to watch out for dark, homogeneous pieces of hematite or other FeO(H) combinations that are often pretty decent (and magnetic) “red”-herrings.  Meteorites usually differ in color and texture from local rocks.  It pays to keep a sample stone on hand for reference; depending on the lake, you may have to calibrate your eyes for the lightest, darkest, smoothest, or otherwise different rocks.

Rusty or chocolate brown (e.g. old tin can color) is generally a good guideline.  Chondrules: great.  Euhedral crystals (minerals with clearly geometric or flat crystal faces): probably bad news.  Parallel veins or layers: probably bad news.  Fusion crust: great, but watch out for deceptive desert varnish or weathering rinds.  Crystals that look like quartz: probably bad news.  There are exceptions to most of these ‘rules,’ but they’re good guidelines if you’re not familiar with every type of meteorite and hope to find something out there.

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These two rocks were found a few yards apart. Which one’s the meteorite? Here’s a clue!

If in doubt, bring it home and either cut it or send it to an institution capable of analyzing it (e.g. UCLA).  If you’d like to know whether or not your rock is worth sending in, I’d be glad to have a look at a photo, but I can’t promise a conclusive answer.