EPITAXIAL-GROWTH OF NANOPHASE MAGNETITE IN MARTIAN METEORITE ALLAN-HILLS-84001 - IMPLICATIONS FOR BIOGENIC MINERALIZATION

Crystallographic relationships between magnetite, sulfides, and carbon ate rosettes in fracture zones of the Allan Hills (ALH) 84001 Martian meteorite have been studied using analytical electron microscopy. We h ave focused on those magnetite grains whose growth mechanisms can be r igorously established from their crystallographic properties. Individu al magnetite nanocrystals on the surfaces of carbonates are epitaxiall y intergrown with one another in ''stacks'' of single-domain crystals. Other magnetite nanocrystals are epitaxially intergrown with the surf aces of the carbonate substrates. The observed magnetite/carbonate (hk l) Miller indices orientation relationships are((1) over bar,(1) over bar,3)(m) parallel to (1, (1) over bar,0)(c) and (1, (1) over bar,1)(m ) parallel to (0,0, (3) over bar)(c) with lattice mismatches of simila r to 13% and similar to 11%, respectively. Epitaxy is a common mode of vapor-phase growth of refractory oxides like magnetite, as is the spi ral growth about axial screw dislocations previously observed in other magnetite nanocrystals in ALH 84001. Epitaxy rules out intracellular precipitation of these magnetites by (Martian) organisms, provides fur ther evidence of the high-temperature (> 120 degrees C) inorganic orig ins of magnetite in ALH 84001, and indicates that the carbonates also have been exposed to elevated temperatures.