title: Magnetite whiskers and platelets in the ALH84001 Martian meteorite:  evidence of vapor phase growth.
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 crystals exhibit distinctive morphologies: filamentary rods and ribbon, 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 degrees C by direct condensation from a vapor or precipitation from a supercritical fluid, and their properties are inconsistent with a biogenic origin.
authors.affiliation: MVA Inc., Norcross, GA 30093, USA.
authors.forename: J P
authors.initials: JP
authors.lastname: Bradley
authors.forename: R P
authors.initials: RP
authors.lastname: Harvey
authors.forename: H Y
authors.initials: HY
authors.lastname: McSween
mesh_terms: D002254:Carbonates / Q000032:analysis*; D018534:Earth, Planet; D052203:Ferrosoferric Oxide; D006358:Hot Temperature; D007501:Iron / Q000032:analysis* / Q000737:chemistry; D018535:Mars; D019110:Meteoroids; D008854:Microscopy, Electron; D010087:Oxides / Q000032:analysis* / Q000737:chemistry
chemical_list: D002254:Carbonates; D010087:Oxides; D007501:Iron; D052203:Ferrosoferric Oxide