[Rockhounds] Scapolite

[Axel Emmermann]

Thanks Pete,

I was trying to write something for the clubmagazines of both my clubs
(MKA-FMS) about a peculiar red fluorescing scapolite crystal is acquired
some years ago.
I'm trying to figure out what it is most likely to be in terms of "position"
within the marialite-meionite series. 
I found a study which shows that meionite is unstable at temperatures below
850 °C. 
Below that it would appear that forming of marialite is favoured.
Another paper showed that the area in which the specimen was found belongs
to an uplift (a Gondwana-collision-thing) and that occurrence of minerals
that are used as geo-thermometers indicate that metamorphism happened at
temperatures not exceeding 850 ° and locally as low as 600°C.
>From both I was hoping to infer that my specimen is likely to be closer to
the marialite side of the series than on the meionite side.

Why is this so important to me:
In Na- and Cl-rich marialite, there is a higher degree of order in the way
that SiO4 and AlO4 tetrahedrons alternate. 
In meionite there is an increase of AlO4 tetrahedrons. As a result the rings
of tetrahedrons that make up the anion cage (with the CO3) in it are less
regular (some adjoining AlO4 tetrahedrons). If I understand correctly what
's in those papers then there would be a slight increase in the length of
some Al-O and Si-O bonds (among other effects)
Now, IF trivalent iron replacing Al3+ were the cause of the red fluorescence
under SW-UV, like it is in some feldspars, it would show a similar spectrum
since it is in the same kind of SiO4 and AlO4 tetrahedral configuration.
However, if the bonds between some of those atoms starts to deviate from the
"normal" lengths, then there may be some crystal field effects spoiling my
attempts (grin).
Therefore I would very much like to know on which side of the scapolite
series my specimen resides....
Mpwapwa, Mormoro, Tanzania is in the Mautia region. Apparently the scapolite
crystal was found in the marble.

Cheers
Axel


> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens Pmodreski at aol.com
> Verzonden: woensdag 24 september 2008 5:10
> Aan: rockhounds at lists.drizzle.com
> Onderwerp: Re: [Rockhounds] Scapolite
> 
> That was a great batch of question you asked about scapolite, Axel--lots
of
> good thoughts there, and I kind of doubt whether ANYONE has all the
answers to
>  the questions you are posing.  (Don made a good start at it though, in
the
> response he posted.)
> 
> I believe that a lot of scapolite is pretty near 50:50 in composition
> between the endmembers, so, I think a lot of it might not really vary much
in
> composition and also in its content, or capacity for containing, iron and
other
> things such as you're thinking about--it's a difference, if one were
really
> comparing minerals that are 95% marialite say vs. 95% meionite, vs.
comparing
> "marialite" that is only 55% to something that's 55% the other.  So, a
start
> would be to just look at a source of data giving the comparative range of
> composition of scapolites from the assorted best known localities for
specimen  and
> gem scapolite.  (No, I do not immediately know where to find such data;
one
> would have to survey the literature.)  But when you found it, you would
also
> probably find the companion data for iron content, etc., in those
scapolites,
> and then you would have an empirical answer to many of the things  you are
> asking.
> 
> Pete
> 
> 
> 
> 
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