meteorite identification

meteorite identification

Meteorite identification

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 Properties of meteorites that are useful in identification

 Meteorites are:

HEAVY: Most meteorites contain a significant amount of Fe-Ni metal, and are
thus heavier (high-density) than rocks typically found at the surface of the Earth.
 There are exceptions to this rule.  Some meteorites contain no metal at all, and
are about as heavy as the dark volcanic rocks found in Hawaii and the
Columbia Gorge.
SOMETIMES MAGNETIC, SOMETIMES NOT:  Most meteorites contain a
significant amount of Fe-Ni metal, and are attracted to a magnet.  But there are
many exceptions of stony meteorites that contain no metal and are not attracted
to a magnet.  Conversely, many terrestrial rocks and artificial rocks do show
magnetic tendency.  Therefore, although often quoted by laypeople as the main
reason they think a rock is a meteorite, magnetic property alone is not an
indicator for a meteorite. 
IRREGULAR IN SHAPE: Meteorites aren't round.  If a meteorite has entered
the Earth's atmosphere without rotating, it can develop a conical shape similar to
the reentry capsules used in the Apollo space missions, although this is not
typical.  Most meteorites are irregularly shaped, as shown by the five views of
CML 0023 (an unclassified North African meteorite) below.  The second image
from the right shows the start of what could be considered an aerodynamic shape.

COVERED BY A FUSION COATING: When a meteorite enters the Earth's
atmosphere, friction raises the surface of the meteorite above its metling temperature.
As the meteorite descends, it slows down, frictional heating
decreases, and the melt quenches to form a fusion coating, a thin layer of dark
glass. The fusion coating may be black or brown, dull or shiny on a recently
fallen meteorite.  After the meteorite has been on the Earth's surface for a while,
the fusion coating may rust, giving the outside of the meteorite a reddish-brown
coloring, or the fusion coating may erode off partially or completely.  The fusion
coating is a thin, discrete layer surrounding an interior that looks quite different
from the fusion coating.  Many Earth rocks can develop a weathering rind (from
chemical weathering) on their exteriors that is similar in appearance to a fusion
coating.  However, there is rarely a sharp boundary between a weathering rind
and the interior of the rock.  In addition, tiny shrinkage cracks (too small to be
visible in the images below) are fairly diagnostic for fusion crusts and are
generally absent for weathering rinds.  The surfaces of many meteorites develop
shallow pits during entry into the Earth's atmosphere.  These pits, known as
regmaglypts, resemble thumb prints, and are usually better developed on
iron meteorites than on stony meteorites.

Forest City (H5 ordinary chondrite, below) is covered by a fusion coating. In the picture below, one tip has been cut off, exposing the lighter gray and speckled interior of the meteorite. It is evident that the fusion coating is very thin.

Another view of Forest City (above), showing that a small ridge of metal is protruding from the fusion coating (slightly to the right of the center of the image). There are small indentations or pits on the surface of the meteorite.