Did an Impact Make the Mysterious Microscopic
Magnetite Crystals in ALH 84001?
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Tiny crystals of magnetite found in the
carbonates in ALH 84001 are under
investigation because of the debate over how
they formed.
Some investigators suggest the crystals are
evidence of past life on Mars, citing magnetite
crystals of similar chemical compositions and
sizes made by magnetotactic bacteria on Earth.
Others use experimental results to show
magnetites formed entirely by non-biological
processes, hence the crystals say nothing about
life on Mars.
One of those processes is the decomposition of
iron carbonate (the mineral siderite), which
occurs in ALH 84001.
www.psrd.hawaii.edu/Oct07/magnetite-origin.html
Did an Impact Make the Mysterious Microscopic
Magnetite Crystals in ALH 84001?
A Brief Geologic History of Martian Meteorite ALH 84001
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ALH 84001 is an old rock that
crystallized in a large body of
magma deep in the Martian crust 4.5
billion years ago.
An impact event excavated the rock
to a location near the surface, where
water evaporated and precipitated
carbonate minerals, including iron
carbonate (siderite), along grain
boundaries.
The rock was shocked after that
event, causing assorted deformation
of the carbonates, possibly including
decomposition of the siderite.
www.psrd.hawaii.edu/Oct07/magnetite-origin.html
Did an Impact Make the Mysterious Microscopic
Magnetite Crystals in ALH 84001?
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Mary Sue Bell’s laboratory
experiments show shock
decomposition of siderite and its
transformation to magnetite when
shocked to 49 GPa.
Magnetite crystals produced in Bell’s
shock experiments have the same
size range, composition, and shapes
as some of the magnetites found in
ALH 84001.
The tiny crystals of magnetite in
ALH 84001 might have been made
when shock waves decomposed iron
carbonate, as during an impact
event.
This transmission electron microscope image
shows the transformation of siderite to magnetite
when shocked to 49 GPa pressure in the lab. In
the upper right area, labeled "S", is siderite
containing nucleating crystals of magnetite. The
other crystals in this image, labeled “a,b,c” are
magnetites of different shapes.
www.psrd.hawaii.edu/Oct07/magnetite-origin.html