Hallo Leute,
ich habe gerade vom Bernd Pauli einen Beitrag zum Thema bekommen, den ich hier auf seine Bitte hin veröffentlichen möchte:
KINNUNEN K.A. et al. (1998) Agate as an indicator of impact structures:
An example from Sääksjärvi, Finland (Meteoritics 33-1, 1998, 007):
Abstract - Mineralogical, petrographical and chemical determinations were made for
743 agate (banded variety of chalcedonic quartz) nodules (diameters from 5 mm to 5 cm)
formed during post-impact, low-temperature hydrothermal activity as vesicle fillings in the
melt rocks of the Sääksjärvi meteorite impact structure (diameter 5 km) in SW-Finland.
Other hydrothermal vesicle fillings in the impact melt rocks include chlorite, mordenite,
smectite and kaolinite. The agates were classified into two types, whose mineralogical
properties and chemical compositions fall within the range of volcanic agates (basaltic
and rhyolitic host rocks). The relatively high age (about 510 Ma) of the Sääksjärvi impact
melt rocks, however, is reflected by the presence of recrystallization textures, which are
rare in younger volcanic agates. The Sääksjärvi structure was initially located after
following the fortuitous discovery of agate "path-finders" in the glacial overburden.
It is recommended that wherever volcanic type agates are found as float in Precambrian
shield areas devoid of younger volcanic rocks, the possible presence of impact (or volcanic)
craters in the vicinity should be considered.

Dirk