Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 99 (10), 6556-61

Origin of Supposedly Biogenic Magnetite in the Martian Meteorite Allan Hills 84001

Affiliations

Origin of Supposedly Biogenic Magnetite in the Martian Meteorite Allan Hills 84001

David J Barber et al. Proc Natl Acad Sci U S A.

Abstract

Crystals of magnetite (Fe(3)O(4)) and periclase (MgO) in Fe-Mg-Ca carbonate in the Martian meteorite Allan Hills 84001 were studied by using transmission electron microscopy to understand their origin and evaluate claims that the magnetites were made by Martian microorganisms. In magnesian carbonate, periclase occurs as aggregates of crystals (grain size approximately equal to 3 nm) that are preferentially oriented with respect to the carbonate lattice. Larger periclase crystals approximately equal to 50 nm in size are commonly associated with voids of similar size. Periclase clearly formed by precipitation from carbonate as a result of partial decomposition and loss of CO(2). Magnetite occurs in more ferroan carbonate, and, like periclase, it is associated with voids and microfractures and the two oxides may be intermixed. Magnetite nanocrystals that are commonly euhedral and entirely embedded in carbonate are topotactically oriented with respect to the carbonate lattice, showing that they formed as solid-state precipitates. Magnetites in Fe-rich carbonate rims are not well oriented. These magnetites are generally more irregular in shape and diverse in size than the euhedral variety. All occurrences of magnetite and periclase are entirely consistent with in situ growth by solid-state diffusion as a result of carbonate decomposition during impact heating. Biogenic sources should not be invoked for any magnetites.

Figures

Figure 1
Figure 1
TEM images of periclase and magnetite in carbonate and their electron diffraction patterns: (a) nano-crystalline aggregate of periclase (P) in foamy region of magnesian carbonate; (b) diffraction pattern from a: 200 and 220 rings for MgO are prominent and show weak 001 preferred orientation; spot reflections are from carbonate; (c) two MgO crystals (P) associated with voids in magnesian carbonate; (d) diffraction pattern from magnetite-rich rim showing magnetite rings indicating preferred orientation (400 and 440 rings are marked) and spot reflections mostly from carbonate, which form an incomplete <11̄00> zone axis pattern; (e) examples of euhedral crystals of magnetite, the larger ones mostly associated with voids in ferroan carbonate. Scale bars = 100 nm.
Figure 2
Figure 2
Lattice image of part of a small embedded magnetite that was topotactic with the surrounding carbonate lattice (the white lines indicate the periphery of the magnetite). The carbonate close to the magnetite crystal is electron damaged but still shows (112̄6) lattice planes, spacing 0.167 nm (horizontal), that are parallel to (224) planes in the magnetite, spacing 0.171 nm. The simulated diffraction pattern (Inset) from the Fourier transform of a larger area including the magnetite crystal shows an hexagonal array of spots from the magnetite (imaging along a 〈111〉 zone axis) and weak spots (arrows, C) from the carbonate.
Figure 3
Figure 3
Lattice images demonstrating the topotaxy of magnetite crystals that are fully embedded in ferroan carbonate. (a) Image of a magnetite crystal, larger than that of Fig. 2, viewed along a direction very close to the carbonate [1̄100] zone axis, showing alignment and continuity of the (111) lattice planes in the oxide with the (0003) carbonate planes. The broken white lines locate the corresponding phase interface, whereas the arrows indicate where lattice planes within the magnetite align with and correspond to those in the carbonate. The strong 1-nm moiré fringes are caused by the close alignment of (51̄3)mag and (112̄6)carb planes (both tilted slightly off-axis and so not imaged), with interplanar spacings of 0.142 nm and 0.165 nm, respectively. (Inset) Zone axis diffraction pattern from the carbonate. (b) Image of the crystal seen in a, viewed along the carbonate [6̄511] zone axis, which also shows continuity of the lattice planes across the phase interface (marked with broken white lines) where indicated with arrows. The 1.5-nm fringes covering the magnetite crystal are moiré fringes formed by (171)mag and (213̄,10)carb, with interplanar spacings of 0.118 nm and 0.109 nm respectively. (Inset) Zone axis diffraction pattern from the carbonate.

Similar articles

See all similar articles

Cited by 4 PubMed Central articles

LinkOut - more resources

Feedback