Tektite with a Large Fluid Inclusion
Tektites are a form of natural glass related to extraterrestrial impact. When a meteorite hits Earth’s surface, the impact site is subjected to tremendous heat and pressure. This causes the rocks to instantly melt and quickly cool, creating a glass with a bulk composition similar to that of the original host rock. Only the most volatile elements will be removed, because their gaseous phases escape easily. This is the case for fluids and gases such as water and carbon dioxide, as well as certain metals. This melting and cooling results in a silica-dominated glass called tektite that is often very dark green to black.
Tektites are common in many parts of the world. The most famous type is probably moldavite, related to an impact in southern Germany. The world’s largest tektite area is the Australasian field, which covers Australia (australites), southeast Asia (indochinites), and most of the Indian Ocean. It is related to a single impact 790,000 years ago, although the impact crater has not been identified.
The gem mines in northern Vietnam around Thac Ba Lake, which are famous for ruby and spinel, also yield tektite from secondary deposits. Local dealer Geir Atle Gussiås (BalderGems) procured a large tektite weighing 230.9 g (figure 1) with a very large fluid inclusion. The inclusion contained a mix of gas and liquid, which could be seen under strong illumination (figure 2; see video at http://www.gia.edu/gems-gemology/tektite-inclusion). GIA’s Bangkok lab examined this unique piece by comparing it to six tektite samples collected in the mines during several field expeditions to Vietnam.
We examined the specimen and performed density measurements as well as chemical analysis to determine whether it was a real tektite. The surface features and overall shape of the large piece were very similar to the reference samples, although the reference samples were much smaller. The overall color was black, but when strong light was transmitted through the sample, a brown color was observed. The SG of the reference samples ranged from 2.01 to 2.42. This range can be explained by differences in chemical composition, mainly iron concentration, and the amount of gas bubbles in the matrix, which lowers the SG. The SG of the Vietnamese sample was 1.84, indicating an unusual abundance of fluid and/or gaseous inclusions in the sample. Because this tektite was too large to analyze using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), its chemical composition, along with those of the reference samples, was revealed with energy-dispersive X-ray fluorescence (EDXRF). The large tektite had Fe, Sr, Rb, and Zr contents that are comparable to the other Vietnamese tektites.
We then imaged the internal structure using real-time X-ray microradiography (RTX), a technique commonly used in pearl analysis. Because the device has a small detector, we could not view the whole tektite in one image. The maximum resolution of 4 μm should have been sufficient to pick up any artificial structures such as drill holes. The RTX images show the irregular outline of the tektite’s surface and a large bubble with a smooth outline (figure 3). Several smaller negative inclusions are also visible as dark ovals. When the sample was moved under the RTX, the liquid-gas inclusion was clearly seen because of the motion of the liquid.
Since we did not find a drill hole or any signs of manmade glass, we assume that the liquid in the negative inclusion is a natural phenomenon. Since glass is not porous, it seems most likely that the liquid was included during the tektite's formation. Although this could not be proven, this tektite remains a unique specimen.
