For the first time in Brazil, researchers have identified a field of tektites. These are natural glasses formed by the high-energy impact of extraterrestrial bodies against the Earth’s surface. These structures, named geraisites in honor of the Brazilian state of Minas Gerais, where they were first discovered, constitute a new strewn field. This expands the incomplete record of impacts in South America.
The discovery was described in an article published in the journal Geology by a team led by Álvaro Penteado Crósta , a geologist and senior professor at the Institute of Geosciences at the State University of Campinas (IG-UNICAMP). Crósta collaborated with researchers from Brazil, Europe, the Middle East, and Australia.
Until now, only five large tektite fields had been recognized on the planet: in Australasia, Central Europe, the Ivory Coast, North America, and Belize. The Brazilian field now joins this select group.
The geraisites were initially located in three municipalities in northern Minas Gerais – Taiobeiras, Curral de Dentro, and São João do Paraíso – in a strip about 90 kilometers long. Since the article was submitted, new occurrences have been recorded in the Brazilian states of Bahia and, more recently, Piauí. According to Crósta, this expands the known area to more than 900 kilometers in length. “This growth in the area of occurrence is entirely consistent with what is observed in other tektite fields around the world. The size of the field depends directly on the energy of the impact, among other factors,” the researcher explains.
By July 2025, the authors had collected approximately 500 specimens, a number that has since grown to over 600 with the most recent findings. The fragments range in size from less than 1 gram to 85.4 grams and reach about 5 centimeters on the longest axis. Their shapes are typical of aerodynamic tektites: spherical, ellipsoidal, drop-shaped, discoid, dumbbell-shaped, or twisted.
Although they appear black and opaque at first, they become translucent under intense light and display a grayish-green color. This color is distinct from that of European moldavites, which have been used in jewelry since the Middle Ages due to their characteristic intense green color. Their dark surfaces are marked by many small cavities. “These small cavities are traces of gas bubbles that escaped during the rapid cooling of the molten material as it traveled through the atmosphere, a process also observed in volcanic lava but especially characteristic of tektites,” says Crósta.
Geochemical analyses show that geraisites have a high silica (SiO₂) content ranging from 70.3% to 73.7%. The combined content of sodium (Na₂O) and potassium (K₂O) oxides ranges from 5.86% to 8.01%, which is slightly higher than in other tektite fields. Small variations in trace elements, such as chromium (10-48 parts per million) and nickel (9-63 ppm), were identified, indicating that the original material was neither pure nor homogeneous. The presence of rare inclusions of lechatelierite, a form of glassy silica produced at extreme temperatures, further supports an impact origin.
“One of the decisive criteria for classifying the material as a tektite was its very low water content, as measured by infrared spectroscopy: between 71 and 107 ppm. For comparison, volcanic glasses, such as obsidian, usually contain from 700 ppm to 2% water, whereas tektites are notoriously much drier,” Crósta points out.
Dating based on the ratio of argon isotopes (⁴⁰Ar/³⁹Ar) indicates that the event occurred approximately 6.3 million years ago, at the end of the Miocene epoch. Three groups of very similar ages were obtained (6.78 ± 0.02 Ma, 6.40 ± 0.02 Ma, and 6.33 ± 0.02 Ma), which is consistent with a single impact event. “The age of 6.3 million years should be interpreted as a maximum age since some of the argon may have been inherited from the ancient rocks targeted by the impact,” the researcher comments.
To date, no associated crater has been identified. According to Crósta, this is not unusual; only three of the six large classical tektite fields have known craters. In the case of the largest field, located in Australasia, the crater is believed to be oceanic. In Brazil, isotopic geochemistry indicates that the molten material originated in Archean continental crust between 3.0 and 3.3 billion years old. This directs the search to the São Francisco craton, an ancient and geologically stable portion of the continental crust and one of the oldest regions of the South American continent. “The isotopic signature indicates a very ancient continental, granitic source rock. This greatly reduces the universe of candidate areas,” says Crósta. In the future, aerogeophysical methods such as magnetic and gravimetric surveys may reveal circular anomalies associated with a buried or eroded crater.
While it is not yet possible to accurately estimate the size of the impacting body, researchers consider it unlikely that it was small. The large amount of molten material and the wide area of dispersion indicate a significant impact event, albeit smaller than the event responsible for the Australasia field, which extends for thousands of kilometers.
The team is currently working on a mathematical model of impacts to estimate parameters such as the energy released, the velocity, the angle of entry, and the volume of molten rock. They are doing this as new data on the spatial distribution of geraisites becomes available. The discovery of the geraisites fills an important gap in the record of impacts in South America. Only about nine large impact structures are known there, and almost all of them are much older and located in Brazil. This discovery also reinforces the idea that tektites may be more common than previously thought, but often go unnoticed or are mistaken for ordinary glass.
To combat sensationalist interpretations of cosmic impacts, Crósta manages the @defesaplanetaria Instagram profile with undergraduate students. The profile is dedicated to scientific dissemination and differentiating real risks from irresponsible speculation about meteorites and asteroids. Impacts were frequent during the formation of the solar system when a large amount of debris was scattered and planetary orbits were undefined. Large bodies migrated from one position to another, projecting smaller bodies in various directions. However, today, with the system stabilized, impacts are incomparably less frequent. “Understanding these processes is essential to separating science from speculation,” the researcher concludes.
Crósta has been studying structures formed by meteorite impacts since his master’s project in 1978. During this time, he has received several research grants from FAPESP ( 08/53588-7 , 12/50368-1 , and 12/51318-8 ).
About São Paulo Research Foundation (FAPESP)
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Geology
Geraisite: The first tektite occurrence in Brazil
2-Dec-2025