Geriasite

Geraisites are one of the most interesting discoveries in impact geology in recent years. They are tektites – natural glasses formed during the impact of a large asteroid on Earth. Their existence was scientifically confirmed only in 2025, and they represent the first known tektite strewn field in Brazil.

Formation

During the impact of a large asteroid, surface rocks melt and part of the melt is ejected high into the atmosphere. During flight, the molten droplets cool and solidify into natural glass – tektites. Geraisites formed by this process approximately 6.3 million years ago.

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Locality

The first finds come from the northern part of the Brazilian state of Minas Gerais, after which they were named. Later, they were also discovered in the states of Bahia and Piauí. The currently known tektite strewn field extends for more than 900 kilometers, making it one of the largest known tektite strewn fields in the world.

Appearance

At first glance, geraisites are black to dark brown, with a typical pitted surface. When thin edges are backlit, they show a gray-green to olive-green color. They occur in various aerodynamic shapes – spherical, drop-shaped, ellipsoidal, or dumbbell-like – formed during the flight of molten material through the atmosphere.

Chemical Composition

Geraisites are composed mainly of silicon dioxide, approximately 70–74 %. Compared with many other tektites, they contain relatively more sodium and potassium and only a small amount of iron. Another typical feature is their extremely low water content, which helps distinguish tektites from volcanic glasses such as obsidian.

Source Crater

Although the age of geraisites and their impact origin are well documented, the crater that produced them has not yet been found. This is not unusual for tektites – even for several other tektite strewn fields around the world, the parent crater is not known with certainty.

Significance of the Discovery

The discovery of geraisites has expanded the number of known large tektite strewn fields on Earth. They are the first confirmed tektites from Brazil and also important evidence of a previously unknown large impact event in South America. From a scientific perspective, they rank among the most significant tektite discoveries of recent decades.