260 Encyclopedia of the Solar System
During substantial heating, noble gases and other atmophile
elements—like carbon and nitrogen—are vaporized and
lost from metallic or siliceous regions. Chalcophilic ele-
ments that form sulfides like troilite (Table 2) include Se,
Te, Tl, or Bi. Chalcophiles and a few siderophiles and
lithophiles are also often quite easily mobilized (i.e., vapor-
ized from condensed states of matter) so that they may be
enriched in sulfides in the parent body or lost from it. Con-
centrations of these elements in specific meteorites then
depend in part on the fractionation histories of their par-
ents and are markers of heating.
2.2 Characteristics of Specific Classes
It is obvious, even to the naked eye, that most iron me-
teorites consist of large metallic iron crystals, which are
usually single-crystal, bccα-Fe (kamacite) lamellae 0.2–
50 mm thick with decimeter to meter lengths (Fig. 8e).
These relatively wide Ni-poor lamellae are bounded by thin,
Ni-rich fccγ-Fe (taenite). The solid-state nucleation and
diffusive growth process by which kamacite grew at slow
cooling rates from taenite previously nucleated from melt
is quite well understood. The 1-atm Fe–Ni phase diagram
and measurement of Ni-partitioning between kamacite and
taenite permits cooling rate estimation between∼900 and
400 ◦C. These typically are a few degrees or so per Ma,
depending on the iron meteorite group, consistent with
formation in objects of asteroidal size. The Ni concentra-
tion in the melt determines the temperature of incipient
crystallization, and this, in turn, establishes kamacite ori-
entation in the final meteorite. These orientations are re-
vealed in iron meteorites by brief etching (with nitric acid
in alcohol) of highly polished cut surfaces: Baron Alois von
Widmanst ̈atten discovered this in the 18th century, and the
etched structure is called the “Widmanst ̈atten pattern.” (An
Englishman, G. Thomsen, independently discovered this,
but his contribution was unrecognized.)
Meteorites containing<6% Ni are called hexahedrites
because they yield a hexahedral etch pattern of large, single-
crystal (centimeter-thick) kamacite (Fig. 7a). Iron mete-
orites containing 6–16% Ni crystallize in an octahedral pat-
tern and are octahedrites. Lower-Ni meteorites have the
thickest kamacite lamellae (>3.3 mm) and yield the very
coarsest Widmanst ̈atten pattern, while those highest in Ni
are composed of very thin (<0.2 mm) kamacite lamellae and
are very fine octahedrites. Iron meteorites containing>16%
Ni nucleate kamacite at such low temperatures that large
single crystals could not form over the 4.57 billion years
(Ga) of solar system history: they lack a Widmanst ̈atten pat-
tern and are called Ni-rich ataxites (i.e., without structure).
The Ni-poor ataxites are hexahedrites or octahedrites that
were reheated either in massive impacts or artificially after
they fell on Earth.
As noted earlier, when primitive parent bodies differ-
entiated, siderophilic elements were extracted into molten
FIGURE 9 Contents of Ni and Ga in iron meteorites. (Some
larger chemical groups are indicated by Roman numerals and
letters.)metal. During melt crystallization, fractionation or sepa-
ration of siderophiles could occur. About 50 years ago,
Ga and Ge contents of iron meteorites were found to
be quantized, not continuous: They could then be used
to classify irons into groups denoted as I to IV. Origi-
nally, these Ga–Ge groups, which correlate well with Ni
content and the Widmanst ̈atten pattern, were thought to
sample core materials from a very few parent bodies. Sub-
sequent studies of many additional meteorites and some ad-
ditional elements, especially Ni and Ir, modified this view.
At present, the chemical groups (Fig. 9) suggest that iron
meteorites sample perhaps 100 parent bodies, although
many, if not most, irons derive from but 5 parents (Fig.
9) represented by the IAB, IIAB, IIIABCD, IVA, and IVB
irons. (The earlier Roman numeral notation for Ga–Ge
groups was retained to semiquantitatively indicate the me-
teorite’s Ga or Ge content. However, a letter suffix was
added to indicate whether siderophiles fractionated from
each other.) In addition to the major minerals (kamacite,
taenite, and mixtures of them), minor amounts of other min-
erals like troilite, and graphite may be present. Also, silicates
or other oxygen-containing inclusions exist in some iron
meteorites.
In most cases, chondrites contain spherical millimeter-
to centimeter-sized chondrules or their fragments. These
chondrules were silicates that melted rapidly at tempera-
tures near 1600◦C and cooled rapidly at some∼ 1000 ◦C/h
early in the solar system’s history; others cooled more slowly
at 10–100◦C/h. Rapid heating and cooling are relatively easy
to do in the laboratory but are difficult on a larger, solar
system–sized scale. Yet, large volumes of chondrules must