Two types of coarse-grained 
inclusions — type A and type B — 
with different mineralogical 
compositions have been found in the 
Allende meteorite. Type A, shown 
here in a scanning electron 
micrograph, is irregularly shaped. 
These inclusions may be aggregates 
of the original solids that condensed 
from the solar nebula. 
posed, on the basis of this similarity 
to the minerals that thermodynamic 
calculations predicted would be the 
first to condense from a cooling gas 
of solar composition, that these white 
inclusions must be high-temperature 
condensates from the solar nebula. 
To study the mineralogy of the in- 
clusions, meteorite pieces containing 
them are sliced into sections so thin 
that light passes through. The min- 
erals are then identified by their char- 
acteristic colors in a polarizing mi- 
croscope and analyzed chemically by 
bombarding the polished surfaces of 
the sections with electrons. Two kinds 
of coarse-grained inclusions — type A 
and type B — with different minera- 
logical compositions exist. In 1972, 
I pointed out that, except for one min- 
eral, hibonite, the mineralogical com- 
position of the type A inclusions is 
precisely the mineral assemblage pre- 
dicted to be in complete chemical 
equilibrium with a gas of solar com- 
position in the temperature range 
1,175°C to 1,250°C at a total pressure 
of one-thousandth of an atmosphere, 
conditions thought to be representa- 
tive of the inner solar nebula. No ther- 
modynamic data exist for hibonite but 
it was thought to have condensed first, 
instead of the mineral corundum pre- 
dicted by the<calculations. Many type 
A inclusions are irregularly shaped 
and suggest that, while floating freely 
in the solar nebula prior to 
incorporation into the meteorite, they 
resembled “fluff balls,” just what 
would be expected when small par- 
ticles strike one another and stick to- 
gether. 
In contrast to fluffy type A inclu- 
sions, the type Bs are often nearly 
spherical, the shape that would be 
taken by a suspended liquid droplet, 
and their mineralogical composition 
is different from the type As. Their 
coarse crystals are tightly intergrown, 
a texture typical of materials that crys- 
tallized from melts. Some possess 
outer mantles of the mineral gehlen- 
ite, in which it can be shown that 
the crystals grew from the outer mar- 
gins of the inclusions inward with fall- 
ing temperature. This can be readily 
explained if such inclusions were once 
molten droplets that cooled and so- 
lidified from the outside in by radi- 
ating heat away from their surfaces. 
On the other hand, such inclusions 
cannot be explained by the conden- 
sation of solids from a vapor like the 
solar nebula, as it is very difficult 
to see why this would lead to formation 
of a hollow spherical shell of gehlenite 
that was filled in by later condensates. 
Thus, it appears that fluffy type A 
inclusions could be aggregates of the 
original solids that condensed from the 
solar nebula, while many type B in- 
clusions melted either during or after 
condensation. 
Further evidence that both types 
of coarse-grained inclusions are high- 
temperature condensates comes from 
their chemical compositions. Suffi- 
cient material can be dug from a single 
inclusion that it can be analyzed for 
a large number of elements present 
in major or trace amounts by a tech- 
nique known as neutron activation. 
This technique involves the conversion 
of stable isotopes into radioactive ones 
by bombarding the sample with neu- 
trons in a nuclear reactor and then 
determining the concentrations of ele- 
ments from the amounts of gamma 
rays emitted at different energies. 
Such experiments show that a large 
number of elements that exhibit a 
wide range of chemical properties are 
enriched by the same amount (a factor 
of 17.5) in the inclusions relative to 
their abundances in an average sam- 
ple of solar system matter. Conden- 
sation calculations show that the 
chemically diverse elements analyzed 
by neutron activation share one char- 
acteristic: they all condense from a 
gas of solar composition above or 
within the range of condensation tem- 
peratures of the major minerals in the 
inclusions. It must therefore be this 
common feature that enriched all of 
these elements to the same degree in 
the inclusions. Although all refractory 
elements separated themselves from 
the rest of solar system matter by 
forming the inclusions, the fact that 
the enrichment factors for all these 
elements are the same indicates that 
they did not separate from one another 
in the condensation process. 
When viewed in thin section with 
a polarizing microscope, most inclu- 
sions contain dark regions. These areas 
are filled with many crystals so small 
that they cannot be resolved under 
ordinary magnification. This dark ma- 
terial tends to fill veins that cross- 
cut gehlenite crystals, indicating that 
it formed after the gehlenite. The 
same dark material also fills cavities 
on the edges of the gehlenite crystals, 
suggesting that the material in the 
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