Specific gravities of tektites from Guangdong, China

Georgia Journal of Science, 2002 by Schmude, Richard W Jr

Abstract

The mean specific gravity of 288 tektites from Guangdong, China was measured to be 2.426. The specific gravities of 49 disc-shaped Guangdong tektites with masses between 10.2 and 246.9 grams obey the equation: SG-20 deg C = 2.4419 - 0.00914 log^sub 10^M^sub A^ where M^sub A^ is the tektite mass in grams. The density of the tektites is in between that of quartz-2.648 and of vitreous SiO^sub 2^-2.196. This equation is consistent with the higher specific gravities of microtektites and it may show the role that gas bubbles play in determining specific gravity. Bubbles are more likely to be found in larger tektites and probably reduce the specific gravity of larger tektites.

Key Words: Guangdong tektites, Specific gravity.

INTRODUCTION

Tektites are glassy rocks that may have formed from giant impact events on the Earth in the past (1, 2). Tektites are found in the four areas: Southeast United States, West Africa (near Ivory Coast), Southeast Asia-Australia, and Central Europe. Tektites from different parts of the world have special names; for example, tektites from Southeast Asia are called Indochinites whereas tektites from Central Europe are called Moldavites. All tektites contain large amounts of SiO^sub 2^ along with other metal oxides (1, 3-6). One distinguishing characteristic of tektites is their low abundance of water, which is typically ~ 0.01 percent (7,8); this is much lower than the amount of water found in obisdian.

Current models cannot account for all tektite characteristics (1, 2, 9), which include: size, chemical composition, chemical bonding, and physical shape. Specific gravity is a property, which can yield information on the bulk composition and volume percent of gas bubbles. In this paper, the specific gravities of tektites from Maoming City, Guangdong Province, China is reported. These objects are part of the large group of tektites from Southeast Asia-Australia, and are around 800,000 years old. A source crater for these tektites has not been located but two possible places for it are the Tonle Sap Lake in Cambodia (3) and the Laos-- Thailand border (4). The tektites measured in this paper were purchased from Bob Mitterling who is a meteorite dealer. The relationship between specific gravity and tektite mass is examined in this paper.

Baker and Forster (10) carried out an extensive study of over 1000 tektites from Australia. They found that the tektite specific gravity does change with shape. For example, the mean specific gravity of 32 oval shaped tektites from Port Campbell, Australia was 2.399 whereas the average specific gravity of 9 teardrop shaped tektites from the same location was 2.415.

McNamara and Bevan (2) point out that tektite shape is determined by two factors, which are: 1) rotation of the tektite while it is molten and traveling through the atmosphere and 2) fricitional heating due to interaction of the tektite and the atmosphere as it is falling. The first of these factors, rotation, may influence the movement of air bubbles through the molten tektite due to centrifugal forces. Essentially the rotating tektite would act like a centrifuge forcing more dense material away from the axis of rotation.

RESULTS

All tektites were divided into one of seven groups based on shape: teardrop, dumbbell, disc, log, sphere, ellipsoid and irregular. The shapes are shown in Figure 1. A table listing the mass and specific gravities for all specimens was prepared and is available from the author upon request. The mean and bulk specific gravities for each group are listed in Table II. The mean specific gravity is determined by giving each tektite equal contribution whereas the bulk specific gravity is determined by assigning a weighting proportional to the tektite mass. The mean specific gravity including the two very low specific gravity specimens is 2.426 with a standard deviation of 0.0373. If the two very low specific gravity objects are not considered then the mean specific gravity of the remaining 286 is 2.428 with a standard deviation of 0.0157; the bulk specific gravity for all specimens is 2.423.

*The two tektites having very low specific gravities of 1.975 and 2.072 were not included in the average.

Discussion

Figure 2 shows the distribution of specific gravities (or population polygon) for the 286 tektites from Guangdon, China; the two very low specific gravity specimens (SP-20 deg C of 1.975 and 2.072) are not included. These two speci- mens probably have unusually large gas bubbles and are about 20 standard deviations below the average specific gravity; they are considered to be outliners and are not included in Figure 2. About 85% of the tektites measured in this study had specific gravities of between 2.410 and 2.449. The population in Figure 2 is similar to the one for tektites from Dalat, Vietnam (figure 6 of ref. 12). Figure 3 shows population polygons for six of the seven tektite groups (the dumbbell was not included because of the small number of samples). The distributions in specific gravities among the groups are similar to one another and to the overall plot in Figure 2.