Dear Mike, Chris, Elton, Bernd, and list,
At 05:09 10-12-10, MikeG wrote:
Can someone please tell me which mineral constituent gives Moldavite
it's green coloration? Is it iron?
There is some useful info about moldavite colors on p. 10 of this
20-page document:
Milan Trnka and Stanislav Houzar, "Modavites: a review"
Bulletin of the Czech Geological Survey, Vol. 77, No. 4 (2002), pp. 283-302
http://www.geology.cz/bulletin/contents/2002/vol77no4/04trnkafinal.pdf
Most relevant to Mike's question are these paragraphs:
"The main colouring components are Fe(II), Fe(III) and perhaps also
Mn(II). Their content increases in the direction to brown moldavites
(Bouska and Povondra 1964, Bouska and Cilek 1992), together with the
ratio of Fe(III)/Fe(II) (Bouska et al. 1982). Deviation from this
scale is found in the so-called "poisonous" green moldavites (HCa/Mg
moldavites). Their colour can be explained by a slightly higher
content of Ni, in combination with a high content of alkaline earths
(CaO, MgO) and a low content of K2O (Bouska et al. 1990a).
Some other components, which have low or no colouring capability,
control the effect of colouring elements themselves. The colouring
influence of Fe(III) in glass substantially increases with the
increasing content of Ti and Mn (Volf 1978). The relatively low
content of titanium, nearly half content, in moldavites, rare
georgianites, bediasites from Muldoon (and also in urengoites) is in
all probability the cause of their higher translucency in comparison
with other tektites."
[end of quotation from the article]
The article is intended to "sum up the existing knowledge about their
strewn fields and geology, about their properties, and their origin."
It does a good job of succinctly laying out the basics.
Best wishes to all,
Piper
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