While on a postdoc at the University of Tennessee-Knoxville I studied diamonds, inclusions in diamonds, and rocks that contain diamonds; all in an effort to understand how diamonds form.
Diamondiferous Rocks
With colleagues Bill Carlson and Cambria Denison at the University
of Texas, Austin, we used a high-resolution X-ray technique to see
inside of diamond-bearing rocks. This technique is called high-resolution
X-ray computed tomography, and is similar to the medical CAT-scan technique.
Because diamonds have a slightly lower density and mean atomic number compared
to silicate minerals, they do not attenuate X-rays as much as the silicate
minerals do. The sample is scanned in a series of two-dimensional "slices"
(Figure 1), which can be assembled into a three-dimensional digital model
of the sample. We used these 3-D models to precisely locate the diamonds
within the sample (Figure 2) so that we could best plan how to cut the sample
to extract the diamonds without destroying the minerals around the diamonds.
We also used volume visualization techniques to determine spatial relationships
between the diamonds and the other minerals in the rock.
Figure 2. This is a 0.2 carat diamond still embedded in the same Siberian eclogite. Comparing the chemical compositions of the minerals in the xenolith to the compositions of the inclusions tells us how the composition of the xenolith changed since the diamonds grew. |
Diamonds and Their Inclusions
Diamonds often contain inclusions of other minerals. These inclusions
were trapped within the diamonds as they grew, and as long as the diamond
remained intact (no cracking), the inclusions are isolated from the rest
of the rock. The diamond is then similar to a time capsule. This makes the
inclusions scientifically valuable because they contain crucial clues to
the chemical and physical characteristics of the environment under which
the diamond formed. Once we have removed the diamonds from the rock (Figure
3) we can begin to study these inclusions. There are basically two ways
to gain access to inclusions inside diamonds: a) destroy the diamond by
crushing or burning it, or b) grind away a corner of the diamond until an
inclusion is exposed. We chose the latter technique because it preserves
the diamond, although it is more time consuming.