[meteorite-list] Resolution of a Big Argument About Tiny Magnetic Minerals in Martian Meteorite

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:48 2004
Message-ID: <200205140419.VAA17861_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May02/ALH84001magnetite.html

Resolution of a Big Argument About Tiny Magnetic Minerals in Martian Meteorite

     --- Magnetic minerals in Martian meteorite
     ALH 84001 formed as a result of impact
     heating and decomposition of carbonate; they
     were never used as compasses by Martian
     microorganisms.

Written by Edward R. D. Scott (Hawai'i Institute of Geophysics and Planetology)
and David J. Barber (Cranfield University and University of Greenwich)

Planetary Science Research Discoveries
May 13, 2002

Tiny grains of magnetite, an iron oxide mineral, from a Martian meteorite
are markedly similar in size, shape, and composition to the little oxide
magnets used by bacteria on Earth and different from other naturally formed
magnetites. Is this good evidence for life on Mars? Or did the Martian
magnetite grains form by another process? Our studies reveal that the planes
of atoms in the Martian magnetites are aligned with atomic planes in the
carbonate in which the magnetites are embedded. This shows that the
magnetites formed in the rock and not inside microorganisms.

     References:

     Barber, D. J. and Scott, E. R. D. (2002) Origin of supposedly biogenic
     magnetite in the Martian meteorite Allan Hills 84001. Proceedings of
     the National Academy of Sciences 99, 6556-6561.

     Thomas-Keprta, K. L., Clement, S. J., Bazylinski, D. A., Kirschvink, J.
     L., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr.,
     McKay, M. F., and Romanek, C. S. (2001). Truncated hexa-octahedral
     magnetite crystals in ALH 84001: presumptive biosignatures. Proceedings
     of the National Academy of Sciences 98, 2164-2169.

             --------------------------------------------------

Magnetite crystals in ALH 84001

Since the startling report in 1996 of possible evidence for life in the
Martian meteorite ALH84001 [See PSRD article: Life on Mars?] attention has
increasingly focused on the origin of the meteoritic magnetite, as this
mineral appears to provide the most compelling evidence for biogenic
activity. The magnetite grains reside in tiny grains of
iron-magnesium-calcium carbonates, which are typically 50-200 micrometers
across and are dispersed throughout the ALH 84001 meteorite as disk-shaped
or spheroidal grains, or, in the case of carbonates that enclose silicate
fragments, as irregularly shaped grains.

       [crossed polarized image of carbonate disk]
       Mosaic of transmitted light images of a thin section of
       ALH84001 between crossed polarizing filters. The pyroxene
       crystals are transformed into a kaleidoscopic display of
       colors that help us to decipher the history of the rock. The
       gray criss-crossing bands are shattered pyroxene crystals
       that formed when an impact on Mars squeezed the rock,
       momentarily twisting and tearing the crystals. Black chromite
       crystals were also wrenched apart by the impact. The white
       arrow near the bottom of the mosaic marks a carbonate disk
       (shown below) that formed in a fracture. Maximum width shown
       of this mosaic is 8.5 millimeters.

The magnetites are too small to be seen using optical microscopes as they
are 4-100 nanometers in size. (The smallest magnetites are about as wide as
15 oxygen atoms.)

                        [focusing on carbonate disk]
       This simple movie of an incomplete carbonate disk with a pale
       orange core was made from thirteen separate micrographs of a
       thin section of ALH 84001. The carbonate is about 130 x 4
       micrometers in size and is embedded inside a greyish-white
       pyroxene crystal. The movie shows how a change in focus of
       the microscope reveals that the carbonate disk formed in an
       inclined fracture in the pyroxene crystal. The magnetite
       crystals are concentrated in the two concentric black lines
       near the rim of the carbonate disk. (Image credit: Ed Scott.)

Kathie Thomas-Keprta (Lockheed Martin) and her colleagues at the NASA
Johnson Space Center have used electron microscopes to analyze the
composition and morphology of the magnetite crystals in considerable detail.
Studies of thin slices of the rock show that the magnetites are found at the
rims of carbonate grains in optically opaque regions and throughout the
interior of the carbonate grains. By dissolving 600 magnetite crystals out
of the carbonate, they found that about 25% were faceted crystals with
width/length ratios of >0.4 (called elongated prisms), ~65% were irregularly
shaped and 7% were more elongate with width/length ratios of <0.4.
Thomas-Keprta and her colleagues showed that the elongated prisms were
remarkably similar in size, shape, and composition to the magnetites made by
one strain of bacteria that uses chains of single-domain crystals of
magnetite as a compass to aid in navigation. Since the bacterial compasses
are made with great precision and efficiency and appear to differ from
abiogenic magnetites in shape and composition (but not size), they
interpreted the elongated prism magnetites as Martian fossils.

Although all workers agree that the magnetites in ALH 84001 were formed on
Mars and may contain an important record of an ancient Martian magnetic
field, many have argued against a biological origin for the magnetites.
Peter Buseck (Arizona State University) and coworkers have questioned
whether the putative Martian biogenic magnetites and the bacterial
magnetites are identical in shape. John Bradley (Georgia Institute of
Technology) and colleagues concluded that magnetites with width/length
ratios of ~0.1 to 0.2 grew on the surface of the carbonate because their
crystal lattices are aligned where they make contact. They inferred that
these so-called whisker-shaped magnetites could not have been made by
bacteria but had condensed from a vapor above 120oC. Thomas-Keprta and
colleagues did not dispute that these magnetites were probably abiogenic.
However, they argued that whisker-shaped magnetites formed at lower
temperatures in the interior of the carbonate grains whereas the elongated
prisms were concentrated in the opaque rims by a different process.

At the March 2002 Lunar and Planetary Science Conference in Houston, many
speakers discussed possible origins for the magnetites in ALH 84001. D. C.
Golden (Hernandez Engineering Inc.), Douglas Ming, and their colleagues at
the NASA Johnson Space Center heated synthetic Fe-rich carbonates at 470oC
and made faceted magnetites, which resembled those in ALH 84001. However
Thomas-Keprta and colleagues presented many arguments against the formation
of the Martian magnetites by thermal decomposition of carbonate. In
particular they argued that the laboratory-grown magnetites grown by D. C.
Golden and colleagues display cubo-octahedral faces whereas magnetites
produced by one strain of bacteria and the presumptive Martian biogenic
magnetites have six additional dodecahedral faces. (Thomas-Keprta and
coworkers now call the second variety, "truncated hexa-octahedrons" but
Buseck and colleagues note that this term is incorrect according to accepted
crystallographic nomenclature.) Andrea Koziol (University of Dayton) and
Adrian Brearley (University of New Mexico) also made nanometer-sized
magnetites in the laboratory by briefly heating iron-magnesium carbonates to
470oC under different conditions. They did not grow any whisker-shaped
crystals like those in ALH 84001 but argued that their faceted magnetites
resembled the supposedly biogenic variety in ALH 84001 in many ways.
Animated discussions at the conference suggested that arguments based on the
crystal shapes of the meteoritic magnetites, both for and against biogenic
origins, are not currently persuasive.

To help understand how the minerals in ALH 84001 formed and how they were
modified by impact we have been studying a section of ALH 84001 using
transmission and scanning electron microscopy. Our discoveries illuminate
the origin of the magnetites in two ways.

             --------------------------------------------------

A second oxide in ALH 84001 carbonate

Our first discovery was that magnetite is not the only oxide in the
carbonate: a mineral called periclase, magnesium oxide (MgO), forms crystals
in various locations in the carbonate and is most abundant in magnesium-rich
carbonate. Like magnetite, the periclase forms 30-50 nanometer crystals,
frequently associated with voids. Both types of crystals are shown in the
images below.

             [oxide crystals in carbonate]
                       Scale bars are 100 nanometers.

In addition, aggregates of periclase crystals ~3 nanometers in size are
oriented preferentially with respect to the carbonate crystal lattice,
similar to what happens when calcite, calcium carbonate (CaCO3), decomposes
to lime, calcium oxide (CaO). To understand why these oxides are oriented
preferentially in partly decomposed carbonate, we need to understand the
crystal structures of these minerals.

             --------------------------------------------------

Crystal structure

We found that the carbonate crystals in ALH 84001 have the structure of the
mineral calcite (or the closely related mineral, dolomite, CaMgCO3) and are
at least 5 micrometers in size, very much larger than the crystals of
magnetite or periclase. Although calcite, lime, and periclase have very
different chemical compositions, their crystal structures can be related in
simple ways by considering how the largest atoms, the oxygen atoms, are
arranged in each structure. Lime and periclase both have the structure of
sodium chloride (common salt), and this structure can be related to that of
calcite by replacing the oxygen atoms in lime by CO3 groups. The CO3 groups
are arranged in planes so that the oxygen atoms are close to the positions
of oxygen atoms in the close-packed planes of oxygen in the lime or
periclase structures. When calcite decomposes during heating to release
carbon dioxide, it is therefore not surprising to find that the crystals of
lime tend to form with their close-packed layers of oxygen atoms oriented
nearly parallel to those in the parent carbonate crystal. We found the same
relationship for the periclase crystals in magnesium-rich carbonate in ALH
84001. Such relationships are common in minerals and inorganic solids when
one solid forms on the surface of, or within another that is structurally
related. The orientation and location of the periclase crystals in ALH 84001
provide strong evidence that they formed inside the carbonate crystals as a
result of thermal decomposition and loss of carbon dioxide. Since we found
much evidence for localized shock melting of silicate minerals in ALH 84001,
and there is additional evidence that the rock was heated by impact 4 Byr
ago, it is probable that the carbonate was decomposed by impact heating, at
this time, rather than by deep burial in the planet.

             --------------------------------------------------

How magnetite crystals are oriented in the ALH 84001 carbonate

Our initial studies of periclase and magnetite crystals in ALH 84001
suggested that we should investigate in more detail the relative
orientations of the magnetite and carbonate crystals. Although magnetite
(Fe3O4) has more oxygen atoms per molecule than periclase (MgO) and lime
(CaO), the oxygen atoms in all three minerals are arranged in almost
identical close-packed layers. Thus any magnetites that form within the
carbonate lattice following thermal decomposition of carbonate may be
crystallographically oriented. We therefore decided to look more carefully
at the crystal lattices of adjacent magnetite and carbonate crystals.

We first showed that groups of magnetites in small voids and in
microfractures are commonly oriented with respect to each other and the
carbonate, so that their atomic lattices are aligned at their contacts.
Mineralogists call this alignment epitaxy. John Bradley and colleagues found
that the whisker-shaped magnetites exhibited epitaxy. An epitaxial
relationship indicates abiogenic growth in or on the carbonate. We then
studied the magnetites that were fully embedded in the carbonate crystal
using techniques to image the lattices of both minerals, as individual
magnetite crystals are too small to give good diffraction patterns. This
required considerable patience and care because the carbonate crystal
lattice is heated and easily damaged by the beam of electrons in the
microscope.

                     [transmission electron microscope]
             David Barber at the helm of a JEOL 2010
             analytical transmission electron microscope
             operating at 200 kV at the Hong Kong University
             of Science and Technology.

                              [aligned atoms]
           Aligned atoms in carbonate and magnetite -- this
           electron microscope lattice image shows part of a
           faceted magnetite crystal (above the dashed line)
           which is embedded in a carbonate crystal. The
           horizontal lines (as shown by arrows) in the carbonate
           and the magnetite crystals are sets of atomic planes
           in the two crystals which have almost identical
           spacings and orientations. (The near vertical lines in
           the magnetite crystal are caused by a tiny mismatch in
           orientation and spacing between different atomic
           planes in the two minerals.) Such images show that the
           planes of oxygen atoms in the two minerals are closely
           aligned and that the magnetite crystal must have
           formed inside the carbonate crystal. The 5 nanometer
           scale bar is ~20 times the diameter of an oxygen atom.

We found that the embedded magnetite crystals give good lattice images
whenever the carbonate crystal is aligned so that the electron beam is
parallel to rows of atoms in the carbonate crystal. This finding implies a
fully three-dimensional relationship between the two atomic structures,
which is called topotaxy. But to prove it beyond doubt we had to repeat the
exercise after turning the carbonate crystal and its embedded magnetite
crystal to a new orientation relative to the electron beam. Invariably the
first image showed good continuity of lattice planes across the
magnetite-carbonate boundary, though with some distortion. The second image
was usually inferior because of the cumulative electron damage to the
carbonate crystal, despite the precautions taken to minimize heating and
beam damage. Nevertheless, we were able to confirm that the second image
also showed lattice continuity across the magnetite-carbonate boundary. Many
but not all magnetites are oriented with their close-packed layers of oxygen
atoms aligned with those in the carbonate, consistent with what we observed
for periclase. This demonstrates that the embedded magnetite crystals formed
within the carbonate crystal by diffusion of atoms and loss of carbon
dioxide.

             --------------------------------------------------

Implications for the supposedly biogenic magnetites

We surveyed all the different locations in carbonate where magnetite
crystals occur and found that the diverse sizes, shapes, and abundances were
consistent with the nucleation and growth of magnetite in carbonate.
Magnetite, like periclase, nucleated on voids, microfractures, and other
defects. The magnetites display all the features of precipitates that form
in cooling crystalline solids when atoms can diffuse far enough for new
crystals to grow but not far enough to reach the boundaries of the parent
crystals. Small crystals that form under these conditions tend to have
well-developed faces.

We could not identify the crystal faces of the topotactically oriented
magnetites. Trying to infer the exact external shape of embedded
nanocrystals from conventional electron microscope images is like trying to
guess the shape of an object suspended in a glass of water from its
silhouette. But we did find that magnetite crystals with well-developed
external faces, like the supposedly biogenic crystals, occur as individual
crystals within the carbonate and commonly show topotactic orientations.
Magnetites in the optically-opaque rim are crowded together and appear to be
more irregular in shape, consistent with growth in a highly strained
carbonate crystal.

These results are not consistent with those of Thomas-Keprta and colleagues
who concluded that the supposedly biogenic magnetites are not preferentially
oriented with respect to the carbonate lattice. They inferred that the
presumptive biogenic magnetites are located in the opaque rims where they
are surrounded by randomly oriented carbonate crystals as small as 10
nanometers across (cf. 4-100 nanometers for the magnetites). This would
preclude a crystallographic relationship between the carbonate and magnetite
lattices. However, we find that some of the magnetites in the opaque rim are
preferentially oriented and that the adjacent carbonate usually consists of
slightly misoriented regions ~100 nanometers in size. The strain fields in
the carbonate crystal around the abundant magnetites appear to have
interacted to produce this microstructure during magnetite growth. There was
no evidence that the carbonate crystals increased in size during our
experiments.

             --------------------------------------------------

Some arguments that have been advanced against formation of magnetite by the
thermal decomposition of carbonate

Several arguments have been offered against the formation of magnetites in
ALH 84001 by thermal decomposition of carbonate and many were discussed at
the March 2002 Lunar and Planetary Science Conference. Below we offer
responses to these arguments.

     1. Many magnetites are entirely embedded in carbonate and are not
     accompanied by voids contrary to what would be expected if the
     carbonate decomposed by simply breaking down into oxides and
     carbon dioxide.

Our response: Decomposition of even a pure carbonate mineral is no simple
matter. In calcite, for example, there are no CaCO3 molecules that simply
break into CaO and CO2 molecules as one might suppose. Even though
carbonates have well-defined structures with specific sites for all atoms,
they behave like well-regulated soups of charged atoms, vacancies, and
defects that are all hopping around at speeds that depend on the
temperature. So when calcium carbonate decomposes, calcium oxide and carbon
dioxide can form at different sites and times. In the case of ALH 84001, our
evidence for topotaxy and faceted voids shows that all species of atoms as
well as vacancies could diffuse over hundreds of nanometers through the
carbonate when the magnetite formed. Thus, voids and magnetite need not be
closely associated.

     2. Many experiments in which iron-magnesium carbonates were
     decomposed by heating have generated magnesium-rich magnetite not
     the pure magnetite observed in ALH 84001.

Our response: Decomposition of iron-magnesium carbonate is much more complex
than decomposition of calcium carbonate because different oxides can form
depending on the nature of the gas around the carbonate. Although there are
some inconsistent reports in the literature, Andrea Koziol concluded at the
March 2002 Lunar and Planetary Science Conference that in oxygen-rich
environments magnesium-iron carbonates decompose to form magnesium-rich
magnetite, but under oxygen-poor conditions, pure magnetite is formed.

     3. Some magnetites in ALH 84001 appear to be enriched in aluminum
     and chromium relative to the carbonate, inconsistent with simple
     decomposition of carbonate.

Our response: The magnetites in ALH 84001 did not form by simple
decomposition of carbonate crystals: the process was much more complicated.
For a 100-nanometer sized magnetite crystal to have formed within a
carbonate crystal, atoms must have diffused through the carbonate structure
over distances of hundreds of nanometers. Atoms of magnesium and calcium
would have diffused away from the construction site of the new magnetite
crystal while iron atoms diffused towards it. Aluminum and chromium atoms
fit better into the structure of magnetite than carbonate, so enrichments of
these elements might be expected.

     4. Magnetites in the opaque rims are associated with the iron
     sulfide pyrrhotite, which appears too abundant to have formed by
     precipitation from carbonate.

Our response: We did not find pyrrhotite: in our section, sulfide in the
opaque rims appeared to be present as finely crystalline or possibly
amorphous material. We cannot exclude the possibility that some sulfide
precipitated from the fluid that generated carbonate before magnetite
formed.

     5. Most experiments that have tried to reproduce the magnetite
     population in ALH 84001 by thermal decomposition of carbonate have
     failed in some regard. For example, experiments by Koziol and
     Brearley generated randomly oriented magnetite crystals.

Our response: Differences between synthetic and Martian magnetites should be
expected because the Martian carbonate was heated and may itself have
crystallized during a large impact on Mars. We do not know the detailed
thermal and shock history of ALH 84001 and even if we did it would be
extremely difficult to simulate those conditions exactly in the lab. So we
should not expect simple experiments to reproduce all of the features of the
ALH 84001 magnetite.

     6. Since microorganisms can form magnetites oriented in chains,
     how do we know that they cannot orient magnetites on carbonate?

Our response: Microorganisms can certainly orient magnetite crystals
crystallographically within their cells, but not outside their cells.
Minerals made outside cells by organisms are not well ordered, have wide
ranges of grain sizes, and cannot be crystallographically oriented.

             --------------------------------------------------

Shock heating of carbonate

We think that there is now abundant evidence that most and probably all of
the magnetites in ALH 84001 formed because of shock heating of carbonate.
Faceted magnetites resembling the supposedly biogenic magnetites are
crystallographically oriented in the carbonate lattice and could not have
formed inside bacteria. We infer that ALH 84001 magnetites differ from
abiogenic terrestrial magnetites because terrestrial carbonates never
experienced the unique impact history of ALH 84001.

If any magnetites with the sizes of the supposedly biogenic variety had been
deposited in the ALH 84001 carbonate prior to the impact heating that caused
oxide precipitation, they could not have retained their original properties.
Even supposing that an earlier generation of magnetites were completely
impervious to the shock wave that heated the carbonate above about 450oC and
were totally immune to any reaction with the shock-heated or shock-melted
carbonate [See PSRD article: Shocked Carbonates May Spell N-O L-I-F-E in
Martian Meteorite ALH 84001], they would certainly have acted as seed
crystals during subsequent cooling. Thus any magnetites deposited in the
carbonate prior to impact heating would have been coated with new layers of
magnetite up to tens of nanometers in thickness. Martian organisms cannot
therefore be responsible for the size and shape of any magnetite crystal in
the ALH 84001 carbonate.

             --------------------------------------------------

The future of ALH 84001

Does this result mean that ALH 84001 will now return to obscurity?
Absolutely not! Meteorites are cosmic gifts that keep on giving, and giving!
It is still the oldest rock we have from any planet: it crystallized over
4.4 billion years ago. We have tiny lunar fragments with comparable ages
that were extracted from breccias but we don't know why the ancient Martian
crust should be so well preserved in ALH 84001 if the Moon and Mars suffered
the same bombardment history. How were the carbonates in ALH 84001 formed?
What was the intensity of the Martian magnetic field when the magnetites
cooled? How and where was ALH 84001 launched from on its journey from Mars
to Earth? An ancient Martian rock is indeed a precious gift, even if it
failed to bring evidence for extra-terrestrial life.

ADDITIONAL RESOURCES

     Barber, D. J. and Scott, E. R. D. (2002) Origin of supposedly biogenic
     magnetite in the Martian meteorite Allan Hills 84001. Proceedings of
     the National Academy of Sciences 99, 6556-6561.

     Golden, D., Ming, D., Lauer H.V. Jr., Schwandt, C., Morris, R.,
     Lofgren, G., and McKay, G. (2002) Inorganic formation of "truncated
     hexa-octahedral" magnetite: implications for inorganic processes in
     Martian meteorite ALH 84001. Lunar and Planetary Science XXXIII,
     abstract 1839.pdf.

     Koziol, A. and Brearley, A. (2002) A non-biological origin for the
     nanophase magnetite grains in ALH 84001: experimental results. Lunar
     and Planetary Science XXXIII, abstract 1672.pdf.

     Scott, Edward R.D., "Shocked Carbonates May Spell N-O L-I-F-E in
     Martian Meteorite ALH 84001" PSR Discoveries. May 1997.
     <http://www.psrd.hawaii.edu/May97/ShockedCarb.html>.

     Taylor, G. Jeffrey "Life on Mars?" PSR Discoveries. Oct 1996.
     <http://www.psrd.hawaii.edu/Oct96/LifeonMars.html>.

     Taylor, G. Jeffrey "Life on Mars--The Debate Continues" PSR
     Discoveries. March 1997.
     <http://www.psrd.hawaii.edu/Mar97/LifeonMarsUpdate.html>.

     Taylor, G. Jeffrey "Fossils in Martian Meteorite: Real or Imagined?"
     PSR Discoveries. Dec 1997.
     <http://www.psrd.hawaii.edu/Dec97/LifeonMarsUpdate2.html>.

     Thomas-Keprta, K. L., Clemett, S. J., Bazylinski, D. A., Kirschvink, J.
     L., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr.,
     McKay, M. F., and Romanek, C. S. (2001). Truncated hexa-octahedral
     magnetite crystals in ALH 84001: presumptive biosignatures. Proceedings
     of the National Academy of Sciences 98, 2164-2169.

     Thomas-Keprta, K., Clemett, S., Romanek, C., Bazylinski, D.,
     Kirschvink, J., McKay, D., Wentworth, S., Vali, H., and Gibson, E.
     (2002) Multiple origins of magnetite crystals in ALH 84001 carbonates.
     Lunar and Planetary Science XXXIII, abstract 1911.pdf.

     Weyland M., Midgley, P., Dunin-Borkowski R., Frankel R., and Buseck P.
     (2002) Advanced TEM techniques for assessing the possible biogenic
     origin of meteoritic magnetite crystals. Lunar and Planetary Science
     XXXIII, abstract 1592.pdf.
Received on Tue 14 May 2002 12:19:22 AM PDT


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