About the spectacular stony-iron meteorite Imilac

Posted by Michael Bakich
on Tuesday, December 29, 2009
This Imilac meteorite the author is about to slice has a coating to protect the meteorite. Michael D. Reynolds photo
Enjoy another installment of Michael Reynolds' series on meteorite:

When I think of beautiful meteorites, the first one that comes to mind is the spectacular stony-iron meteorite Imilac. Prospectors first found this meteorite in the Atacama Desert in northern Chile in 1822. Imilac exteriors look rough; some describe uncut Imilac specimens as ugly or hellish in appearance. But if you happen to find a hellish-looking rock while strolling along in the Atacama Desert — itself a hellish place with a lot of hellish-looking rocks — consider yourself lucky indeed!
Imilac is a stony-iron that meteoriticists group as a Pallasite. Pallasites are meteorites that are mixtures of metal and silicates, usually olivine grains. Imilac is also a “PMG,” which refers to a Pallasite belonging to the main chemical group. Searchers have turned up roughly 900 kilograms of Imilac meteorites over the years. The largest known specimen recovered to date, 198 kilograms, is in the British Museum. A second large Imilac — 95 kg — is in Copiapo, Chile, a mining town and the capital of the Atacama region. Imilac material ranks third in mass among Pallasite meteorites recovered, after Brenham and Huckitta. Imilac is a stable meteorite, having spent most of its “life” in a dry desert environment.

The sources of Pallasites appear to be asteroids that underwent a differentiation process. Thermal processes altered these asteroids, separating their material into a mantle and a metallic (nickel-iron). During the differentiation process, probably caused through collisions in the early solar system, fractional crystallization separated the two major minerals found within the asteroid. This resulted in the olivine forming a layer between the metallic core and the mantle. Further collisions broke apart the asteroids, and, lucky for us, Imilac specimens ended up on Earth as nature’s stained-glass cathedral windows! One theory on the formation of Pallasites is that they formed from the destruction and breakup of a planet in the early solar system. Pallasites get their name from Simon Peter Pallas, a German doctor and naturalist who, in 1772, first described the Krasnojarsk, Siberia, Pallasite.

Imilac comes in two types: fragments and sliceable specimens. The sliceable Imilac specimens, often referred to as Grade or Type 1, contain both the nickel-iron metal and olivine crystals whereas the fragments, Grade or Type 2, are usually all nickel-iron. Over time, weathering removes the characteristic olivine found in Pallasites, leaving behind strangely shaped nickel-iron fragments. Type 2 Imilac specimens remind me of kidney stones! They are irregular and jagged in appearance.

This picture shows the sliced side of the previous specimen. Note the easily visible olivine crystals and the web of nickel-iron. Michael D. Reynolds photo
Imilac meteorites exhibit a simply stunning interior matrix of nickel-iron and olivine crystals when sliced and polished. If the slice is thin enough, a light will easily shine through the greenish, translucent olivine crystals embedded in the metal matrix. You probably know olivine by its gem name of peridot. Sometimes the meteoritic olivine has an angular shape and the crystal color appears green to golden in hue. Overall Pallasite olivine can vary in color from yellow to gold or even brown due to terrestrial weathering.
The olivine within one specific Pallasite is used to set the standard for peridot. In 1902, a meteorite, called the Marjalahti meteorite, fell in Respublika Kareliya, Russia. This observed fall was rare for a couple of reasons. First, people have witnessed only three Pallasite falls. Second, the olivine crystals within the 45-kilogram mass are of such high quality that Marjalahti meteorite olivine is now the official standard for peridot! If you are looking for a piece of the Marjalahti meteorite, good luck. I have never seen any offered on the market and as a researcher I have never come across a specimen.

Note the color and translucent nature of the olivine in this Imilac meteorite — simply spectacular! It takes a lot of cutting and finishing to prepare a slice of Imilac. Michael D. Reynolds photo
When I was at the Chabot Space & Science Center in Oakland, California, I had an engaged couple come to me looking for an unusual astronomical wedding band. After we talked it through, they decided to go with pieces of faceted Imilac olivine. I connected the couple to my friend Bob Haag, who took olivine out of a piece of Imilac meteorite and then faceted it for their wedding bands. What a spectacular set of rings this made for their special day.

When I cut and prepare Imilac, I use a lot of caution. You want the thinnest slice possible, but not so thin that the olivine crystals shatter or fall out of the slice. Too thick of a slice will result in less-than-desirable translucence. I cut Imilac s-l-o-w-l-y and with an oil-lubricating saw. After cutting, I prep at least one of the two sides. I then etch the metal matrix to show the crystalline pattern. What a spectacular showpiece it makes!

Imilac is becoming harder to purchase. The reason is obvious: When you first glance at a nicely cut and lit Imilac, you have got to have a slice! Words and photographs do not adequately describe their beauty. Several years ago during one of my pilgrimages to Tucson, Arizona, for the annual gem and mineral show, I purchased two beautifully prepared slices — at my wife Debbie’s encouragement! (Now how could I go wrong there?) A few years later, I thought to myself, “I really do not need two beautiful slices of Imilac,” and traded one of them for some other meteorites. That was a huge mistake! Imilac is one of my showpieces, and if you own a slice of “the rock” or have seen one, you understand why I say that.

So if you have an opportunity to buy a slice of Imilac, do so with no reservation. Put that high-cost extraordinary eyepiece purchase on hold; the eyepiece will still be there later, but the Imilac will not! And it is a purchase you will treasure for a long time — as will others you share it with.

Do you have a meteorite or cratering question? How – and where – to buy meteorites? Collecting tales? Successful meteorite hunts? A favorite meteorite? Favorite meteorite books and publications? A must-see meteorite exhibit? Please email me at mreynold@fscj.edu. We will explore your meteorite questions and more each month!

Keep looking up!
Dr. Mike Reynolds

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