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An Inclusion Study of the Lake Superior Agates, by Donald Kasper, 2011

Key Features

This book proposes and presents the data to support the model view that the Lake Superior agates formed in metamorphosed basalts in lava flow tops that were highly vesicular, around Cretaceous.  At that time, substantial ash deposits of the Midwest from Western volcanism released silica that filled these voids (amygdules).  Earlier clay deposits of illite dominant in the midwestern rock strata from Ordovician was also released by this local tectonic activity, introduced as inclusions of various types in the amygdules and in some vein agates.  The geochemistry behind these and other inclusion reactions, and their correlation to regional geologic systems is described.


·         The regional geology that contributed to the agates seen is described.  This includes the Keweenaw volcanics, volcanogenic massive sulfide systems (formed deep sea vents), the MidContinent Rift System, and rhyolitic volcanism that alternated with the regional basalt flows.

·         The significant role of halloysite and illite clays are described in the inclusions they make.

·         Many of the inclusions found are related to possible geochemical reactions that formed them.

·         The geologic formation of sericite clay systems is related to the “skip ‘n atom” agate species.

·         The data is reviewed, and the proposal is made that celadonite forming with the agates dates them to Cretaceous (about 150 million years old).

·         The occurrence of shadow banding is related to illite clay.

·         The occurrence of “forests of tubes” found in specimens is related to halloysite clay.

·         The role of supercritical water in the formation of waterline bottom/druzy quartz top agate structures is described, mixed waterlines/wall-banding systems, and others.

·         Substantial nomenclature errors in identifying agate species are revised and corrected.

·         The data about the unusual occurrence of cube structures related to anhydrite and calcite are reviewed.

·         The reason the Lake Superior agates are banded, while those of other regions such as the Western U.S. are not, is proposed related to differences in regional occurrence of the clay species in the agates.

·         Various mineral inclusions often grouped as “moss” agate, are differentiated.

·         The important role of chlorite and its release of clay leading to waterline structures is proposed.

·         The unusual curl structures in some agates are identified by relating them to the chlorite mineral group.

·         Some of the key literature reviewing how agates and their inclusions form with them are related to silica gel chemistry.


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