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.
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 1. This is a 2-dimensional X-ray tomography "slice" through an eclogite xenolith from the Udachnaya kimberlite in Siberia. The dark gray shapes near the center and left edge are diamonds. The light gray shapes are garnets, and the small white spots are sulfides. The medium gray matrix is clinopyroxene. A series of these slices, evenly spaced through the sample, provide a 3-D model of the xenolith. These data helped us locate more than 30 diamonds within this rock. The sample is about 4 x 6 cm where this slice was taken.|
|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.
|Figure 3. This is a 0.6 carat diamond extracted from the eclogite xenolith from Siberia. The black spots in the upper left portion of the diamond are mineral inclusions. These inclusions, along with the diamond's irregular shape, make it worthless as a gemstone, but a treasure trove of scientific information.|