"The Mystery of the Snow Head Spheres and Thundereggs".
Dr. Manfred Jäkel, 14.09.2023

The lecture is in connection with the special exhibition "Wunder.Welt.Achat" in the terra mineralia in Freiberg.

Snow head spheres, porphyry spheres and thundereggs as well as the scientific term lithophysis are synonyms for one and the same phenomenon.

The lecture penetrates into the secrets of the scientific phenomenon of lithophysis, so-called high-temperature crystallization domains. These are nodular rock formations in silica-rich volcanites, which form mainly in marginal and fluid-rich parts at temperatures of 600 to 700 °C.
An insight is given into

- the complex process of formation of lithophyses,
- the internal structures of the lithophyses (crescent and star-shaped cavities),
- the subsequent filling of the internal cavities.

The focus of the lecture is the wonder world of the lithophysen agates as a result of the mineralization of the cavities by alteration and weathering, which takes place after cooling of the lava at temperatures of below 200 °C. On the basis of geological-mineralogical characteristics and associated pictures statements are made about the

- mineralized cavity forms,
- agate types as well as crystallization sequences,
- macro- and microstructures in the lithophysen agates.

With the presented classification commonalities but also locality-specific differences can be determined for the lithophyses and their agates, because color, form and structure are often so characteristic that a connoisseur can already indicate the locality by sight alone. This is shown by examples of finding places from all over the world.

In the special exhibition examples of lithophysae agates from all over the world are exhibited, which the visitor can admire as fascinating natural works of art.

The Freiberg Colloquium is a lecture series organized by the Saxon State Office for the Environment, Agriculture and Geology, the Saxon State Archives/Mining Archives Freiberg, the Saxon Upper Mining Office, the TU Bergakademie Freiberg, the Helmholtz Institute for Resource Research, terra mineralia and the Geocompetence Center Freiberg e.V.

Place: TU Bergakademie Freiberg, Dietrich-von-Freiberg-Bau, Prüferstraße 4 in Freiberg

Admission is free of charge

The lecture can also be listened to via livestream.