c. Variations of Stable Isotope Ratios in Nature
I. Extraterrestrial Materials: Meteorites, Tektites,
Lunar Materials
Meteorites may be regarded as specimens of matter from space. They
supply information on the properties and composition of matter from
beyond the earth. However, meteorites also have importance for geochemistry. Because direct observations of the earth's interior are not
possible, we must draw conclusions in an indirect manner. One way to
do this is by analogy with meteorites.
In the solar system there are probably millions of meteorites of all
sizes, from the finest dust particles to hunks measuring kilometers in
diameter. Our knowledge of the composition of meteorites comes from
larger bodies that are observed as they fall, or from objects that are
recognized as meteorites by the special characteristics distinguishing
them from terrestrial rocks.
1. Meteorites
Meteorites are classified into three groups: irons, stony-irons, and
stones (for an exact nomenclature, see KEIL, 1969).
Iron meteorites are classified on the basis of their structure and, to some extent,
their nickel contents, into hexahedrites, octahedrites, and nickel-rich ataxites. Iron
meteorites consist predominantly of metallic iron with 5.5 to 20% of alloyed nickel,
small amounts of other metals, a few percent sulfides, mostly iron sulfide (troilite),
some iron carbide (cohenite), and graphite. Irons form about 5% of observed
meteorite falls.
Stony-iron meteorites are mainly classified into pallasites, and mesosiderites.
Members ofthis group are characterized by appreciable nickel-iron contents.
Stone meteorites have been classified into two groups: achondrites and chondrites. Distinguishing these two groups from each other is in most cases possible on
the basis of the presence of chondrules in chondrites and their absence from achondrites. Chondrules have a spherical or elliptical shape, usually a few tenths of a
millimeter to a few millimeters in diameter, containing essentially the same silicate
minerals as the matrix of the chondrite in which they are embedded.
There is a minor but significant group, carbonaceous chondrites, whose composition differs from that of ordinary chondrites, mainly in water- and carboncontents. The most interesting minor constituents of carbonaceous chondrites for
stable isotope research are carbonates, sulfates, sulfides, and "organic" carbon
compounds.
I. Extraterrestrial Materials: Meteorites, Tektites,
Lunar Materials
Meteorites may be regarded as specimens of matter from space. They
supply information on the properties and composition of matter from
beyond the earth. However, meteorites also have importance for geochemistry. Because direct observations of the earth's interior are not
possible, we must draw conclusions in an indirect manner. One way to
do this is by analogy with meteorites.
In the solar system there are probably millions of meteorites of all
sizes, from the finest dust particles to hunks measuring kilometers in
diameter. Our knowledge of the composition of meteorites comes from
larger bodies that are observed as they fall, or from objects that are
recognized as meteorites by the special characteristics distinguishing
them from terrestrial rocks.
1. Meteorites
Meteorites are classified into three groups: irons, stony-irons, and
stones (for an exact nomenclature, see KEIL, 1969).
Iron meteorites are classified on the basis of their structure and, to some extent,
their nickel contents, into hexahedrites, octahedrites, and nickel-rich ataxites. Iron
meteorites consist predominantly of metallic iron with 5.5 to 20% of alloyed nickel,
small amounts of other metals, a few percent sulfides, mostly iron sulfide (troilite),
some iron carbide (cohenite), and graphite. Irons form about 5% of observed
meteorite falls.
Stony-iron meteorites are mainly classified into pallasites, and mesosiderites.
Members ofthis group are characterized by appreciable nickel-iron contents.
Stone meteorites have been classified into two groups: achondrites and chondrites. Distinguishing these two groups from each other is in most cases possible on
the basis of the presence of chondrules in chondrites and their absence from achondrites. Chondrules have a spherical or elliptical shape, usually a few tenths of a
millimeter to a few millimeters in diameter, containing essentially the same silicate
minerals as the matrix of the chondrite in which they are embedded.
There is a minor but significant group, carbonaceous chondrites, whose composition differs from that of ordinary chondrites, mainly in water- and carboncontents. The most interesting minor constituents of carbonaceous chondrites for
stable isotope research are carbonates, sulfates, sulfides, and "organic" carbon
compounds.