Brazilian Geraisites: Evidence of a 6-Million-Year-Old Meteorite Impact Discovered
A groundbreaking discovery by an international team of researchers has unveiled a field of tektites in Brazil, offering compelling evidence of a powerful meteorite impact that occurred approximately 6 million years ago. This field, containing natural glasses formed by the high-energy collision of extraterrestrial bodies with Earth's surface, has been named geraisites in honor of the Brazilian state of Minas Gerais, where the tektites were first identified.
Research Collaboration and Initial Findings
According to a study published in the journal Geology, the research was spearheaded by geologist Álvaro Penteado Crósta, a senior professor at the Institute of Geosciences at the State University of Campinas (IG-UNICAMP). The project involved collaboration with scientists from Brazil, Europe, the Middle East, and Australia. Researchers identified dark glassy fragments, now known as geraisites, 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 produced when earthly material is melted at extremely high temperatures due to meteor impacts and is propelled into the atmosphere before rapidly solidifying. Unlike volcanic glasses, tektites have very low water content and distinct chemical signatures. The Brazilian geraisites were initially found across three municipalities—Taiobeiras, Curral de Dentro, and São João do Paraíso—in a region spanning roughly 90 kilometres. Following the study's publication, additional specimens were reported in the neighbouring states of Bahia and Piauí, expanding the known distribution of the tektite field to over 900 kilometres.
Sample Collection and Characteristics
Researchers have collected more than 600 samples to date. The fragments vary significantly in size, ranging from less than one gram to over 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 Confirms Meteorite Impact
Laboratory analysis revealed that geraisites have a high silica content, between 70 and 73 percent, along with detectable sodium and potassium oxides. More significantly, their water content, ranging from 71 to 107 parts per million, is remarkably low—a key distinction from volcanic glasses like obsidian, which typically have much higher water levels. The presence of lechatelierite, a rare glassy form of silica produced at extreme temperatures, further confirms the material's impact origin.
Dating the Impact Event
Using argon isotope dating (⁴⁰Ar/³⁹Ar method), researchers determined that the tektites formed around 6.3 million years ago, during the late Miocene epoch. Three closely grouped age results—6.78, 6.40, and 6.33 million years—strongly suggest a single impact event rather than multiple episodes. Isotope signatures indicate that the molten material originated from the San Francisco Craton's ancient continental crust, one of South America's oldest geological formations, dating back more than three billion years.
The Search for the Crater
Despite extensive analysis, no related crater has been found yet. According to Crósta, this is not unusual, as only three of the six large classical tektite fields worldwide have known craters. The crater in the largest field, located in Australasia, is believed to be oceanic. Isotopic geochemistry points to the molten material originating from the Archean continental crust between 3.0 and 3.3 billion years ago, focusing the search on the São Francisco craton. In the future, aerogeophysical techniques like magnetic and gravimetric surveys may help identify circular irregularities linked to a buried or eroded crater.
Importance of the Discovery
Only a few major tektite fields are known globally, 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 relatively few documented large impact structures, most of which date back earlier than the Miocene epoch. This discovery fills a critical gap in the continent's geological timeline. Scientists are currently modelling the event to estimate the meteorite's size, velocity, and energy, reinforcing evidence of a substantial—though not unprecedented—cosmic collision.
