"To anyone who has worked with them, tektites are probably the most frustrating stones ever found on earth."

Henry Faul, 1966.

Australite button

Faul’s description of tektites still holds true even today. While more data has been collected and theories on the origin of tektites have been reduced, these small, glassy objects still provide many questions for current tektite researchers. Let us begin our discussion with a brief history lesson.

The name tektite is derived from the Greek tektos, or melted. These glassy objects have fascinated people of the world for centuries. The first known description of tektites comes from China around 900 B.C., during the T’ang dynasty, in a book by Liu Sun, Ling Piao Lu Yi, loosely translated as “Notes on the Wonders Beyond the Nanling Mountains in Kwangtung”. Liu Sun stated that lei-gong-mo, or inkstones of the Thundergods, black stones that made a ringing sound when struck and possessed a brilliant luster, were collected after rainstorms. Barnes (1973)   Until the late 19th and early 20th century, tektites remained an enigma. Little research was conducted until Alfred Lacroix, a researcher from the Musee National d'Histoire Naturelle, Paris, described a large tektite deposit in Indonesia, and later identified the Ivory Coast as another tektite locality. Barnes (1973) These discoveries, and the subsequent publishing of the research about them sparked the interest of other scientists throughout the world. The remaining first-half of the 20^th century was spent locating and identifying additional tektites and their localities across the globe. As more and more tektite localities were found, many different hypotheses were developed to explain the unique chemical and physical properties of tektites.  During the 1950’s and 60’s, tektite research came to the forefront because of the belief that they might have originated from the Moon. This theory was later discounted by most scientists, but the vast body of information collected during this period provided the basis for the tektite research that still continues today.

What is a tektite?

Tektites are rounded, pitted bodies of silicate glass, nonvolcanic in origin, most likely derived by large hypervelocity meteorite collisions with terrestrial rocks. Commonly about the size of a walnut, tektites can vary  from sand grain sized microtektites, weighing grams, to large, blocky, Muong Nong-type specimens weighing up to 12.8 kg (28 pounds).  Tektites can differ  in both color and age, depending on where they are found.  Commonly black color,  tektites can also vary from light green to greenish yellow. Ages vary from ~35.5 million to ~750,000 years old. Chemically, tektites are uniquely characterized by extremely high silica contents ranging from ~70% in Australasian tektites to ~98% in Libyan Desert Glass.

(Click on images to enlarge.)

Ablation vs. Abrasion

Ablation is the removal and reshaping of molten surface layers of meteorites and tektites by vaporization during flight through the Earth's atmosphere.

Abrasion is the mechanical wearing or grinding away of rock surfaces by the friction and impact of rock particles transported by wind, ice, waves, running water, or gravity.

Bates and Jackson (1984)

With tektites closely resembling obsidian, many early researchers believed that they were indeed products of terrestrial volcanic eruptions. Their various shapes were theorized to result from abrasion by wind-blown sand or shaping by water.
Chemical properties and the subsequent determination that tektite surfaces were ablated NOT abraded, later disproved this theory. (Obsidian is found in nature as a black glass, a product of the ash trapped within the crystal, the presence of the green and yellowish-green moldavites and Libyan Glass, as seen below, provides an easy example of why this theory can be discounted.)

Many other theories were developed to try to explain the existence of tektites, some of these included:

The suggestion that tektites were actually man-made objects, created during the smelting process of ancient civilizations. 
However, the chemical and physical property of tektites, as well as the stratigraphic ages of their localities completely discounts this theory. (Humans developed rudimentary smelting techniques as early as 6000 B.C., however, the youngest tektite are around 100,000 years old.)

What is a stratigraphic age?

The length of geologic time during which the rocks of a particular type, differentiated by characteristic properties and attributes, were deposited. These rock units are often used to date materials within them, or that occur before or after the rock was deposited.

Theories suggesting that tektites were material ejected from lunar volcanoes or that tektites were formed by the impact of meteorites with the Moon gained wide acceptance during the 1950's and 60's. However, some scientists doubted these theories, suggesting that tektites were actually formed on Earth by meteorite and/or comet impacts.
After the Apollo Moon landings scientists studied and compared the chemical composition of lunar rocks to tektites. Their conclusions showed that there was little to no similarity between tektites and lunar materials, shedding doubt on the lunar origin of tektites, and adding credit to the terrestrial impact theory that is now widely accepted.

The main issues facing both past and current scientists working with tektites is the determination of when, where, and how they originated. To deduce the conditions necessary to produce tektites, it is important to recognize several significant properties about tektites. These properties include;

• Distribution and geological age.
• Physical properties and chemical compositions.
• Shape and melting features of some specimens.
• Internal features.

Tektites are distributed across the Earth in areas called strewnfields. These strewnfields are found on every continent except Antarctica. The main strewnfields found on Earth are the Australasian, the largest of all of the strewnfields, covering an estimated 10% of the Earth’s surface, the  Ivory Coast, Czechoslovakian, and North American strewnfield.

Individual tektite varieties are given distinctive names; derived from the region of their strewnfield occurrence, and/or the place or country in which they are found. Examples of regional nomenclature include “moldavites”, named for the Moldau River in the Czech Republic, and “bediasites”, named for the Bedias Indians, for those found in Texas.
Others, named after the region or country in which they are found, include australites, javanites, indochinites, and philippinites. A few groups, such as the Ivory Coast tektites and Georgia tektites, are not specifically named.

(Images and information about the possible source craters of tektites)

What is a strewnfield?

The term strewnfield, is commonly used to describe a finite area across the globe in which specific materials, in our case, tektites, have fallen. The boundaries of the strewnfield are confined by the extent of the material. Each strewn-field is comprised of specimens similar in age and chemical compositions, including tektites found on land, as well as microtektites, found in deep sea sediments, and are inferred to have been created by impact event of the same age.

(Map of major tektite strewnfields)

What are the varieties of tektites?

• Australites
• Bediasites
• Billitonites
• Darwin Glass
• Georgia Tektites
• Indochinites
• Ivory Coast Tektites
• Javanites
• Libyan Desert Glass
• Moldavites
• Martha's Vineyard Tektite
• Philippinites

The ages of tektites are derived by several means. The first and most direct way of dating a tektite is by measuring the solidification age of the glass through the use of  ⁸⁷Rb/⁸⁷Sr decay measurements. The age of  ablation caused by atmospheric entry or re-entry can be derived by potassium-argon decay measurements. Fission track analysis, another geochronologic tool, can indicate the latest heating episode of the tektite glass . Finally, the stratigraphic age of sediments surrounding tektites can provide a general time frame of formation and/or deposition of the tektites.

Tektites range in age from 35.5 million to 750,000 years old. Using these dates scientists have tried to correlate the ages of tektite strewnfields with impact craters across the globe. For the most part, these researchers have been successful in determining the source craters for tektites, however, the source crater for the Australasian strewnfield, still has not been found, allowing some people to still question the meteorite impact theory.

Geochronology

An explanation of the ideas and methods behind radiometric dating and fission track analysis, both topics in the broad subject of geochronology, can be a little hard  to understand, with entire semesters dedicated to learning the subject matter. If you would like more information on these subjects please click the links below.

Radiometric dating

Radiometric dating and fission track analysis

Thin section of a bediasite.

Normally the first thing mentioned about tektites is their color. Outwardly, tektites appear to be glossy-black, but thin sections of specimens are translucent, and generally appear to be yellowish brown. The black color of most tektites can vary, however, depending on the strewnfield the tektite is located in. Moldavites are dark green and Libyan Desert glass is greenish-yellow to straw yellow in color.

Tektites possess a hardness of between 6 and 7 on the Mohs' scale.

What is hardness?

Hardness is a measurement that determines an objects resistance to scratching. Hardness is measured using the Mohs’ Scale, with values ranging from 1 (talc) to 10 (diamond), where a substance with a lower value will be scratched by an object possessing a higher hardness value.

Here are the hardness values of some common objects:

·         Fingernail = 2.5

·         Copper Penny = 3

·         Knife Blade = 5.5

·         Glass = 5.5

·         Steel File = 6.5

What is refractive index?

A technique used to measure the deflection of light as it passes from one medium, normally air, into another medium of differing density. Tektites normally possess refractive index ranges between 1.4600 and 1.5200.

What is specific gravity?

A measurement used to determine the density of an object by finding the ratio of the weight of a given volume of substance to the weight of a volume of water displaced by the object being measured. Tektites have specific gravity ranges between 2.275 and 2.510.

The Relation of Specific Gravity and Refractive Index to Silica Content.

Specific gravity, and refractive index show an inverse relationship to silica; that is, the higher the refractive index and specific gravity, the lower the silica

Lechatelierite is a very rare, fused silica glass, formed by the melting of quartz crystals by extremely high temperatures and pressures. When associated with tektites, this material is normally used as evidence for shock metamorphism, caused by a meteorite impact event.

What does the presence of lechatelierite within tektites indicate?

The presence of lechatelierite  indicates that tektites were formed by the melting of quartz-bearing minerals, and that the temperatures reached during the melting of the parent rock was in excess of the melting point of quartz.    (< ~900º C) Quartz-bearing materials on Earth include terrigenous sedimentary rocks, acid igneous rocks and soils formed by quartz-bearing rocks. Barnes (1940)

Bubbles associated to lechatelierite: What they tell us.

When lechatelierite is formed by the melting of quartz, the gas and liquid filled inclusions found within crystals are volatilized. These volatilized materials are normally lost at very high temperatures, thereby leaving the tektite with no bubbles. The presence of bubbles indicates that temperatures were intermediate. Lower temperatures lead to “frothy” lechatelierite, where the volatilized materials within the crystal expanded, but did not coalesce into bubbles.

What do air bubbles tell us about the tektite?

The abundance, size and shape of bubbles distributed throughout tektite glass are important factors in determining the history of the glass. As a melt becomes hotter it becomes more fluid and more of the vaporization products escape, resulting in the retention of fewer and smaller bubbles. Also, the hotter the fluid, the more likely that the bubbles will be spherical. Irregular bubbles indicate that the melt did not become sufficiently fluid for surface tension to draw the bubbles into spheres; elliptical bubbles indicate that the melt flowed as it cooled.

In addition to their distribution, age, and physical and chemical properties, tektites may be subdivided into three distinct types on the basis of their external form.

(click here to see a thin section of a Type A tektite)

Also known as the Muong Nong-type, these tektites are often chunky or platy in external form, but often show evidence of internal compositional layering as well as variations in bubble content.  Believed to be formed by puddles of molten glass, this tektite type was discovered in 1935 by Lacroix, they are named for the town of Muong Nong, Laos, located near where they were first found.

Or “splash-form” tektites, consisting of specimens, when unbroken, can be shaped as spheres, ellipsoids, rods, teardrops, dumbbells, and boats. Some of these tektites show contoured, bent and rolled internal structures due to fluid flow while molten.  The shapes of splash-form tektites have previously been erroneously associated with aerodynamic shaping. In actuality, these tektites were shaped by variations in rotation rates of partially melted glass droplets. With dumbbell shaped specimens showing the highest rates of rotation and spheroid specimens exhibiting little to no rotation. The size of these “splash-form” tektites is limited by the surface tension of the melt from which they are formed.

These tektites are type B forms that have been subjected to attack by heat on one side (Called the anterior side), so that a portion of the mass has been lost to ablation and/or has been resculpted to form a flange derived from the anterior side’s material. Often known as flanged “buttons”, these forms are most common in Australites, but are also found in some Javanites.