octahedrite

50 Shades…. Of Science


Sometimes I sit in the dark and touch my meteorite.
A firm and heavy iron meteorite belonging to the chemical group IIAB, its coarse octahedrite structure feels rough beneath my eager fingers, its strange regmaglypted surface the result of hot and heavy atmospheric ablation at 5.6km above the Earth’s surface.

I think about the 93% iron, 5.9% nickel, 0.42% cobalt, 0.46% phosphorus, and 0.28% sulfur that lends it most of its mass. How deeply that mass must have plunged into the fertile Earth waiting below, the impact crater filling with hot and gooey tektites and the Earth releasing her dirty, dirty meteoric dust with a shudder and groan.

I can almost taste the trace amounts of germanium and iridium on my wanting tongue, my bottom lip quivering as I think about other minerals present: taenite, plessite, troilite, chromite, kamacite, and schreibersite. Mmmmm.
Sometimes I sit in the dark and touch my meteorite.

Campo del Cielo iron meteorite. Structural classification is coarse octahedrite. The slice displays the classic Widmanstatten pattern (a.k.a. Thomson structure) found in some iron-nickel meteorites. This pattern results from the interweaving of crystals from two iron alloys, kamacite (low nickel) and taenite (high nickel). The Widmanstatten pattern is diagnostic of meteorites as this pattern cannot be duplicated in laboratories on Earth. To form this pattern the iron core of the asteroid must cool no faster than a couple of degrees every million years

Muonionalusta iron meteorite from Norrbotten , Sweden. Structural class is fine octahedrite. Weight 84 grams. Photo is a cut, polished and acid etched slice of a single meteorite. The slice displays the classic Widmanstatten pattern (a.k.a. Thomson structure) found in some iron-nickel meteorites. This pattern results from the interweaving of crystals from two iron alloys, kamacite (low nickel) and taenite (high nickel). The Widmanstatten pattern is diagnostic of meteorites as this pattern cannot be duplicated in laboratories on Earth. To form this pattern the iron core of the asteroid must cool no faster than a couple of degrees every million years. In the gaps between the lamellae, a fine-grained mixture of kamacite and taenite called plessite can be seen.

Sikhote-Alin iron meteorite. Structural classification is coarsest octahedrite type IIB. Weight is 130.1 grams. The Sikhote-Alin Meteorite was an observed fall on the Sikhote-Alin mountains in Siberia in 1947. An estimated 70 Tonnes of this iron-nickel meteorite made it to the ground.  The “Shrapnel” form consists of torn and twisted fragments that formed when the meteoroid exploded near the ground or upon impaction. 

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Sikhote-Alin individual iron meteorite. Structural classification is coarsest octahedrite type IIB. Weight is 85 grams. Well developed regmaglypts (thumb prints) resulted from ablation of the meteoroid as it melted during flight. Stippled areas, on an otherwise smooth surface, are splash fields where molten iron-nickel splashed across the surface during flight through the atmosphere, marring what would have become a  pristine fusion crust. The Sikhote-Alin Meteorite was an observed fall on the Sikhote-Alin mountains in Siberia in 1947. An estimated 70 Tonnes of this iron-nickel meteorite made it to the ground.

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Campo del Cielo iron meteorite. Structural classification is coarse octahedrite. Upper photo shows wonderfully developed regmaglypts, also referred to as “thumbprints”. The regmaglypts formed while the meteoroid ablated during passage through the atmosphere. The Campo del Cielo meteorites come from Argentina, where they are believed to have fallen between 4,000 and 5,000 years ago. Nickel-iron meteorites, such as this one, are fragments of the cores of larger ancient asteroids (similar to Earths iron core), that have been shattered by impacts with other asteroids. After spending 4,500 million years in space this piece of ancient iron core found its way to Earth from the asteroid belt between Mars and Jupiter. The lower photo shows the cut, polished and acid etched sliced side of the upper photo. The slice displays the classic Widmanstatten pattern (a.k.a. Thomson structure) found in some iron-nickel meteorites. This pattern results from the interweaving of crystals from two iron alloys, kamacite (low nickel) and taenite (high nickel). The Widmanstatten pattern is diagnostic of meteorites as this pattern cannot be duplicated in laboratories on Earth. To form this pattern the iron core of the asteroid must cool no faster than a couple of degrees every million years

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Sikhote-Alin iron meteorite. Structural classification is coarsest octahedrite type IIB. Weight is 183 grams. The Sikhote-Alin Meteorite was an observed fall on the Sikhote-Alin mountains in Siberia in 1947. An estimated 70 Tonnes of this iron-nickel meteorite made it to the ground. Two forms of this meteorite exist. The “Individual”, that is shown in a previous posting, is a broken piece of the main mass that continued to “burn up” in the atmosphere before hitting the ground. The “Shrapnel” form consists of torn and twisted fragments that formed when the meteoroid exploded near the ground or upon impaction. The sample shown above is a classic example of the “shrapnel” variety.

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Gibeon iron meteorite from Gibeon, Namibia. Structural class is fine octahedrite. Weight 57 grams. Photo is a cut, polished and acid etched slice of a single meteorite. The slice displays the classic Widmanstatten pattern (a.k.a. Thomson structure) found in some iron-nickel meteorites. This pattern results from the interweaving of crystals from two iron alloys, kamacite (low nickel) and taenite (high nickel). The Widmanstatten pattern is diagnostic of meteorites as this pattern cannot be duplicated in laboratories on Earth. To form this pattern the iron core of the asteroid must cool no faster than a couple of degrees every million years. In the gaps between the lamellae, a fine-grained mixture of kamacite and taenite called plessite can be seen. Upper photo was taken with a binocular microscope at 10x to show the detail of intergrown crystals.

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Sikhote-Alin individual iron meteorite. Structural classification is coarsest octahedrite type IIB. Weight is 100 grams. Well developed regmaglypts (thumb prints) resulted from ablation of the meteoroid as it melted during flight.  The Sikhote-Alin Meteorite was an observed fall on the Sikhote-Alin mountains in Siberia in 1947. An estimated 70 Tonnes of this iron-nickel meteorite made it to the ground. Two forms of this meteorite exist. The “Individual” that is shown in this picture is a broken piece of the main mass that continued to “burn up” in the atmosphere before hitting the ground. The “Shrapnel” form consists of torn and twisted fragments that formed when the meteoroid exploded near the ground or upon impaction. The individuals of the Sikhote-Alin fall are one of the most sought after meteorites by collectors because of their exquisite shapes and character. 

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Campo del Cielo iron meteorite.  Structural classification is coarse octahedrite. The slice displays the classic Widmanstatten pattern (a.k.a. Thomson structure) found in some iron-nickel meteorites. This pattern results from the interweaving of crystals from two iron alloys, kamacite (low nickel) and taenite (high nickel). The Widmanstatten pattern is diagnostic of meteorites as this pattern cannot be duplicated in laboratories on Earth. To expose this pattern I first sliced the meteorite in half. Using various grades of emery cloth I polished the face of the sliced piece to a mirror finish. After polishing, the surface was etched with a 1:2 ratio mixture of 10% hydrochloric acid and 3% hydrogen peroxide. This creates a strong oxidizer in the presence of an acid, which is able to slowly eat away the iron. The varing rates of corrosion between the different iron alloys (kamacite and taenite) creates the Widmanstatten pattern of inter-locking iron crystals. The lower photo shows the intact meteorite. The middle photo shows the polished slice. The top photo shows the iron crystals that appear after etching.