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Glass bombs - part 3 of 3

WOLF VON ENGELHARDT (1967) Chemical composition of Ries glass bombs (GCA
67-8, 1967, pp. 1677-1689, excerpts, cont.):

2. Petrographic characteristics and occurrence

By microscopic investigation of the larger bombs three types (I, II,
III) of glass bombs can be distinguished, II glasses forming a
transition group between the main classes I and III. A detailed
petrographic description of these glasses will be published elsewhere.
Following is a short description of their main features:

I. Glasses

Greenish-brown, brown or black; translucent in small pieces; vitreous
lustre on fresh surfaces of fracture. Under the microscope: thin streaks
of different colour and refraction; no recrystallization products;
pieces of fused SiO2 glass (lechatellerite); fragmental grains of quartz
and little feldspar accumulated together with vesicles into bands or
layers.

II. Slightly recrystallized glasses

Brown, black or gray; vitreous lustre or dull appearance of fresh
surfaces of fracture. Under the microscope: similar to type I glasses,
but hair-like pyroxene crystals and laths of feldspar just beginning to
grow.

III. Highly recrystallized glasses

Black, gray, bluish gray; not translucent; dull appearance of fresh
surfaces of fracture. Under the microscope: recrystallization of a
considerable portion of the glass (feldspar, quartz, pyroxene,
montmorillonite); SiO2 glass, fragmental minerals and vesicles as in
type I glasses.

The differences in appearance of types I, II and III are due to
different cooling processes. All glass bombs were deposited at high
temperatures before crystallization started. The type I glasses were
cooled so fast that they consolidated without reerystallization. The
cooling rate of type III glasses was slower so that different kinds of
crystals could grow. This origin of the I, II and III glass types can be
deduced from the uniformity of chemical composition and the pattern of
distribution of the glasses. They occur in the suevite in such a way
that larger blocks contain either only type III glasses or only type I
(and II) glasses. Where vertical sections in suevite outcrops are
exposed, boundaries can be drawn separating areas containing type III
glasses from those containing type I (and II) glasses.

In the suevite quarries of Otting and Aumühle the contact between
suevite and the underlying Bunte Breccie (coloured breccia) can be seen.
The lowest, apparently chilled, layer of suevite, about 1 m thick,
contains type I (and II), but no type III glasses. In the drill hole of
Wörnitzostheim above a substratum of Bunte Breccie, a 83 m thick layer
of suevite was found, covered with Tertiary lake sediments. In this
profile, lower and upper chilled layers both containing only type I
glasses were observed (Förstner, 1967). Of the two layers, the lower,
above the Bunte Breccie was 17 m thick and the upper, beneath the lake
sediments was 6 m thick. The middle section of the suevite (60 m)
contains only highly recrystallized glasses of type III.


Best wishes,

Bernd




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