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Magnetite whiskers and platelets in the ALH84001 Martian meteorite: evidence of vapor phase growth |
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| Authors: | Bradley J P Harvey R P McSween H Y |
| Institution: | a MVA Inc. 5500/200 Oakbrook Parkway, Norcross, GA 30093, USA b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA c Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106-7216, USA d Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA |
| Abstract: | Nanometer-sized magnetite crystals associated with carbonates in fracture zones within Martian meteorite ALH84001 have been examined using analytical transmission electron microscopy. Some of the the crystals exhibit distinctive morphologies: filamentary rods and ribbons, and platelets. The rods and ribbons are elongated along the crystallographic 100] and 111] directions. Some of the rods contain microstructural defects indicating that they grew by spiral growth about screw dislocations. Platelets are flattened along the 100] and 110] directions. These unique morphologies and microstructures constrain the growth conditions of magnetite. The whiskers and platelets most likely formed in the temperature range 500–800°C by direct condensation from a vapor or precipitation from a supercritical fluid, and their properties are inconsistent with a biogenic origin. |
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