MAGNETITE WHISKERS AND PLATELETS IN THE ALH84001 MARTIAN METEORITE - EVIDENCE OF VAPOR-PHASE GROWTH

Nanometer-sized magnetite crystals associated with carbonates in fract ure zones within Martian meteorite ALH84001 have been examined using a nalytical transmission electron microscopy. Some of the the crystals e xhibit distinctive morphologies: filamentary rods and ribbons, and pla telets. The rods and ribbons are elongated along the crystallographic [100] and [111] directions. Some of the rods contain microstructural d efects indicating that they grew by spiral growth about screw dislocat ions. Platelets are flattened along the [100] and [110] directions. Th ese unique morphologies and microstructures constrain the growth condi tions of magnetite. The whiskers and platelets most likely formed in t he temperature range 500-800 degrees C by direct condensation from a v apor or precipitation from a supercritical fluid, and their properties are inconsistent with a biogenic origin.