Chemistry Reference
In-Depth Information
Ma ago, and as an implication that matrix-mediated biomineralization (BCM) appeared on
Earth as early as 2000 Ma ago (Chang et al. 1989).Because the cubooctahedral crystal habit
is the equidimensional form of magnetite, it is commonly found in inorganically-produced
magnetites (Palche et al. 1944). However, the elongated, non-equidimensional, pseudo-
hexagonal prismatic forms of magnetite are very unusual and those found in sediments and
soils may represent true fossils of magnetotactic bacteria.
McKay et al. (1996) included the presence of ultrafine-grained magnetite, pyrrhotite,
and greigite in the rims of carbonate inclusions in the Martian meteorite ALH84001 as
one of the lines of evidence for life on ancient Mars. This meteorite, which is estmated to
be approximately 4.5 billion years old (Nyquist et al. 2001; Thomas-Keprta et al. 2002),
contains magnetite crystals that are cuboid, teardrop, and irregular in shape with sizes
ranging from about 10 to 100 nm (McKay et al. 1996; Thomas-Keprta et al. 2000). About
25% of the magnetite crystals present have an unusual morphology referred to as a
truncated hexa-octahedral, essentially the same morphology as that found in the marine
magnetotactic vibrio, strain MV-1 (Thomas Keprta et al. 2001).
The magnetite crystals have been examined to a great degree using a number of
different techniques. The intracellular magnetite crystals produced by strain MV-1
display six distinctive properties that allow them to be distinguished from any known
population of inorganically-produced magnetites (Thomas-Keprta et al. 2000, 2001,
2002). These six properties are: (i) narrow size range (a non-log-normal size distribution
with the mean centered in the single-magnetic-domain size range); (ii) restricted width-
to-length ratios (iii) high chemical purity (crystals are essentially stoichiometric
magnetite); (iv) few crystallographic defects (crystals are defect-free with the exception
of occasional twinning perpendicular to the [111] axis of elongation; (v) crystal
morphology with unusual truncated hexa-octahedral geometry consisting of a
combination of the three crystallographic forms for the
mm
point group, the cube
{100}, the octahedron {111}, and the rhombic dodecahedron {110}, with only 6 of the
12 possible {110} faces being expressed, namely, those that satisfy the relationship
{1
1
0} •
[1
11] = 0, with
e
longati
on
defi
n
ed to be along the [111] ļ [111] axis [i.e.,
(110) , (110) , (10 1 ) , (101) , (011) , (011) ] (the term truncated hexa-octahedron should
not be confused with hexoctahedron, which is another crystallographic form in the
isometric system with 48 equivalent faces in the
42
mm
point group denoted by the symbol
{321}); and (vi) elongation along only one of the possible four threefold rotation axes of
a regular octahedron. These six properties have been referred to as the magnetite assay
for biogenicity and used as a set of standards for the determination of whether magnetite
crystals have a biogenic origin or not (Thomas-Keprta et al. 2002).
The 25% of the magnetite crystals in ALH84001 that resemble those of strain MV-1
appear to fit the above criteria (Thomas-Keprta et al. 2001, 2002). However, some
disagree that that these criteria are robust enough to distinguish between biogenically-
and abiogenically-produced magnetite crystals (Buseck et al. 2001) and so the debate
continues. An abiogenic hypothesis is based on the low temperature precipitation of
carbonates (Golden et al. 2000) and thermal decomposition of iron-bearing carbonate to
produce magnetite (Golden et al. 2001, 2002, 2003; Barber and Scott 2002) with the
implication that in ALH84001 such an event occurred through impact shock heating.
Evidence for this process comes from the observation that in addition to magnetite, nano-
dimensional periclase (MgO) crystals are also associated with the carbonate globules in
ALH84001, particularly the Mg-rich carbonate (Barber and Scott 2002). Both magnetite
and periclase crystals are frequently associated with voids in the carbonate, suggesting a
mineralization process in which CO
2
is released. Some faceted magnetite and periclase
crystals in carbonate are crystallographically oriented with respect to the carbonate
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