[Rockhounds] Fw: green obsidian

[DonH]

Carol Carter-Wientjes wrote:

> While we're on the topic of obsidian, what's the difference between 
> man-made glass and natural glasses like obsidian, moldavite, tektites?, 
> others?

Hi,

In very simple terms, natural glasses are composed of whatever elements 
are in the magma--mostly silica of course, but other elements such as 
iron, aluminum, magnesium, and all sorts of trace elements.  The mix is 
variable.  Volcanologists spend their careers analyzing the composition 
of volcanic glasses and lavas to determine the source of the flow and 
the origin of magmas, among other things.

Manufactured glasses are made from specific recipes for specific 
purposes.  One interesting fact is that much of the plate glass made 
today is float glass, prepared by floating the mixture on a bed of 
molten tin.  Due to the absorption of tin by the "down side," that side 
of the glass will fluoresce an unsaturated mustard-yellow or straw color 
under SW UV.  As part of my forensics talk, I tested the fl. side and 
the non-fl. side of a piece of float glass, figuring that if there is 
enough tin in one side to fluoresce, there should be enough to detect. 
The bottom line is that the EDS (the instrument that is usually attached 
to the SEM) barely detected tin, and only when we tuned the software to 
look for it.  On the other hand, my favorite instrument the microprobe 
detected it quite easily.  It is important to know this for forensics, 
since two evidence packages may contain broken glass that is actually 
from the same pane, but an analyst may conclude that they are different 
if one fragment is from the tin side and the other one is not.  Of 
course, forensic analysts are aware of this issue, but it is a good 
example to use in geology to show migration of elements in a contact 
zone and how one specimen that appears homogeneous may not be, 
chemically.  Glasses like Pyrex have their own special compositions and 
manufacturing processes, and while there might be minor variation from 
batch to batch, industrial glasses are made using carefully controlled 
processes and recipes.

Don