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.