Telltale Inclusions 
Mineral grains in the Allende meteorite are studied in order 
to understand the transition from a cosmic gas cloud to the early solar system 
by Lawrence Grossman 
Scientists have long recognized that 
a record of events that occurred at 
the very beginning of the solar system 
is preserved in the carbonaceous chon- 
drite meteorites because these objects 
miraculously escaped later processes, 
such as vulcanism, that would have 
erased this information. During the 
1960s, the supposition was that these 
extraterrestrial samples were com- 
posed of the very mineral grains that 
condensed from the solar nebula, the 
hot gas cloud postulated to have given 
birth to the sun and the planets. Direct 
evidence was difficult to obtain, how- 
ever, as existing specimens of these 
precious meteorites were considered 
too small for the extensive studies nec- 
essary to test the hypothesis. This bar- 
rier was broken suddenly on February 
8, 1969, when thousands of pieces of 
a meteorite fell over a large rural area 
in the valley of el Rio del Valle de 
Allende in northern Mexico, providing 
scientists with four tons of material 
from a single carbonaceous chondrite. 
So much material became available 
that chunks weighing several kilo- 
grams were sliced up like loaves of 
bread. The slicing revealed for the 
first time that some types of carbo- 
naceous chondrites are heterogeneous 
mixtures of different types of inclu- 
sions, many of which are large enough 
to be individually sampled and ana- 
lyzed by several different techniques. 
I will concentrate here on the wealth 
of information that was obtained by 
using these techniques on the coarse- 
grained, calcium-rich inclusions that 
constitute about 5 percent of the Al- 
lende meteorite. 
The elements are made in nuclear 
reactions in the stars. Different ele- 
ments and different isotopes of the 
same element are produced in differ- 
ent stars; yet all stable isotopes are 
found in the solar system. The nuclear 
products of the stars are ejected into 
the interstellar medium where they 
mix together in enormous clouds. In 
1969, the prevailing view among cos- 
mochemists was that the solar system 
formed when such an interstellar cloud 
of gas and dust underwent gravita- 
tional collapse and fragmentation and 
when one of the fragments continued 
to collapse into a disk-shaped cloud 
called the solar nebula. It was thought 
that during the collapse, gas mixing 
was thorough enough to erase any pre- 
existing spatial variations in elemental 
and isotopic composition and that tem- 
peratures were high enough in the cen- 
ter of the nebula to evaporate all pre- 
existing interstellar grains. Outer parts 
of the disk remained cold. The planets 
in the inner solar system — Mercury, 
Venus, Earth and its moon, and 
Mars — were viewed as having ac- 
creted from solid materials that con- 
densed in the inner part of the solar 
nebula when the gas cooled off again. 
Hence, what minerals condense from 
a cooling gas of solar composition and 
in what order are questions of obvious 
importance to understanding the 
chemical compositions of the terres- 
trial planets. 
Controlled condensation experi- 
ments are difficult to perform at the 
high temperatures relevant to this 
problem, but calculations based on the 
relative stabilities of minerals and gas- 
eous molecules allow detailed predic- 
tions that form the basis for discussion 
of this subject. These models assume 
that chemical equilibrium was 
achieved during condensation. (Chem- 
ical equilibrium means the tendency 
of the minerals and gas present at 
any given time to become as stable 
as possible.) The models require ther- 
modynamic data for chemical species 
that can exist in such a system, as 
well as estimates of the abundances 
of the elements in the solar system 
and of pressures and temperatures in 
the inner solar nebula. Early versions 
of such calculations predicted that 
minerals rich in aluminum, calcium, 
and titanium would be the first-ap- 
pearing condensates of any of the 
abundant elements in a cooling gas 
of solar composition. In 1972, I pre- 
dicted the sequence in which various 
minerals would condense under con- 
ditions of complete chemical equilib- 
rium. 
A slab surface of the Allende me- 
teorite shows that the coarse-grained 
inclusions are prominent because of 
their large size and light color. Before 
long, these objects had attracted the 
attention of the earliest investigators 
of the meteorite, who discovered that 
the white inclusions are filled with 
calcium-, aluminum-, and titanium- 
rich minerals. The investigators pro- 
A piece of the Allende meteorite 
is exhibited by its finder on 
February 16, 1969, eight days after 
the carbonaceous chondrite fell. 
This photograph was taken in 
the town of Torreon de Mata, near 
the southern end of the large area 
over which the fragments of the 
meteorite fell. 
68 
Roy S. Clarke, Jr. 
