Kamal Ghorui
A team of researchers has discovered a field of tektites in Brazil, providing evidence of a powerful meteorite impact that occurred approximately 6 million years ago. This field contains natural glasses formed by the high-energy impact of extraterrestrial bodies against the Earth’s surface. These structures, named ‘geraisites’ in honour of the Brazilian state of Minas Gerais, are where the tektites (geraisites) were first found.According to a study published in the journal Geology, the research was led by geologist Álvaro Penteado Crósta, a senior professor at the Institute of Geosciences at the State University of Campinas (IG-UNICAMP), in collaboration with researchers from Brazil, Europe, the Middle East, and Australia.Researchers identified the dark glassy objects known as ‘geraisites’ fragments in northern Minas Gerais and confirmed their extraterrestrial impact origin through detailed geochemical and isotopic analysis.
What are geraisites and how do they form
Tektites are naturally occurring glasses that are produced when earthly matter is melted at extremely high temperatures resulting from meteor impacts and is propelled into the atmosphere before quickly solidifying. Unlike other glasses that form from volcanic activity, tektites have very low water content and chemical signatures.The Brazilian geraisites were initially found across three municipalities- Taiobeiras, Curral de Dentro, and São João do Paraíso – in a region stretching roughly 90 kilometres. After the publication of the study, additional specimens were reported in the neighbouring states of Bahia and Piauí, expanding the known distribution of the tektite field to more than 900 kilometres.Researchers have collected over 600 samples so far. The fragments vary significantly in size, ranging from less than one gram to more than 85 grams. Many display aerodynamic shapes such as spheres, teardrops, discs, and dumbbells- classic forms created as molten material travels through the atmosphere at high speed.Although they appear black and opaque at first glance, the glass becomes translucent and greyish-green under intense light. Their surfaces are marked by tiny cavities formed by escaping gas bubbles during rapid cooling.
Chemical evidence shows a meteorite impact
Researchers have collected over 600 samples so far. The fragments vary from less than one gram to over 85 grams and display aerodynamic shapes such as teardrops, discs, and dumbbells. Though appearing opaque black, they turn greyish-green under strong light.Laboratory analysis revealed that geraisites have high silica content (between 70 and 73 percent), as well as detectable sodium and potassium oxides, according to laboratory analysis. More significantly, their water content, which ranges from 71 to 107 parts per million, is remarkably low. This is an important distinction: the water content of volcanic glasses, like obsidian, is usually much higher.The presence of lechatelierite, a rare glassy form of silica produced at extreme temperatures, further confirms the material’s impact origin.The tektites formed around 6.3 million years ago, during the late Miocene epoch, according to research using argon isotope dating (⁴⁰Ar/³⁹Ar method). A single impact event rather than several episodes is strongly suggested by three closely grouped age results: 6.78, 6.40, and 6.33 million years.According to isotope signatures, the molten material came from the San Francisco Craton’s ancient continental crust, which is one of South America’s oldest geological formations and dates back more than three billion years.
The crater has not been found yet
Despite extensive analysis, no related crater has been found as of yet. Only three of the six large classical tektite fields have known craters, so this is not unusual, according to Crósta. The crater in the biggest field, which is in Australasia, is thought to be oceanic. According to isotopic geochemistry, the molten material in Brazil originated between 3.0 and 3.3 billion years ago in the Archean continental crust. This focuses the search on the São Francisco craton, one of the oldest areas of the South American continent and an old and geologically stable section of the continental crust. In the future, circular irregularities connected to a buried or eroded crater might be discovered using aerogeophysical techniques like magnetic and gravimetric surveys.
Why is this discovery important
Only a few major tektite fields are known worldwide, including those in Australasia, Central Europe, Côte d’Ivoire, North America and Belize. The newly identified Brazilian field now joins this rare group, significantly broadening the global map of confirmed meteorite impacts. South America has comparatively few documented large impact structures, and most date back far earlier than the Miocene epoch. This discovery fills a critical gap in the continent’s geological timeline. Scientists are modelling the event to estimate the meteorite’s size, velocity and energy, reinforcing evidence of a substantial – though not unprecedented – cosmic collision.
