[Rockhounds] Scapolite

[Axel Emmermann]

Hi Pete

> 
> [To write this here to possibly help all, without attempting to write the
entire formulas, I'll
> just note that Marialite is the Na end-member which also is richer in Si
and Cl, and Meionite
> is the Ca end-member, which is also richer in Al and CO3 .]

[Axel] They are "theoretical" end-members or do they actually exist? 

> For a start, I looked at the pictures posted for both minerals on
mindat.org, and I see that,
> 
> (1) the large gemmy crystals--from Tanzania and Brazil--are listed as
being marialite [not
> saying that this is necessarily for the reasons you suggested in your
post; it could just be
> that the conditions for growing good gem crystals more often exist in
Na-rich rather than
> Ca-rich environments]

[Axel] I read that the scapolite series is used as a "geothermometer". Is
that still the case or do we use more modern methods now?
 
> (2) there are more photos posted on mindat for meionite (32) than for
marialite (17).

[Axel] I noticed that. Mine has an almost cylindrical prism body. All those
on on mindat that have the same habit are labelled Marialite.

 
> Then, for a start to learn about the reported chemical range of scapolite,
I looked at the
> info and analyses given in "An Introduction to the Rock-forming Minerals"
by Deer, Howie,
> and Zussman.? I see that most analyses plotted in a compositional diagram
there are between
> about 15% to 85% meionite

[Axel] which roughly corresponds for the "limits" I founds for wernerite
somewhere ;-))))

--i.e., they are spread over a considerable range throughout the
> middle of the compositional series, with few or none toward the?end-member
compositions--
> the most extreme look to be about 15% meionite (=85% marialite) and about
93% meionite
> (7% marialite).
>
[Axel] Formed in metamorphic conditions in marble (often)... it would be a
surprise to find NO Ca and CO3 (I think even SO4 is possible?)
> 
> 
> The above?book lists 3 complete chemical analyses of scapolites, which
correspond to
> compositions of 19.3%, 65.7%, and 84.8% meionite end-member; and the iron
contents of
> those three samples are given as, respectively (one showing iron as Fe2O3
and two showing it
> as FeO--but who knows if those data are correct or not, because analyses
giving the correct
> oxidation state of iron in minerals are quite difficult to obtain):
> 
> 19.3% meionite? contains 0.07 wt.% Fe2O3
> 65.7% meionite contains 0.30 wt.% FeO
> 84.8% meionite contains 0.23 wt.% FeO
> from this limited data, one does not get any clear-cut relationship
between bulk composition
> (meionite = higher Al content) and iron content.

[Axel] Then there's iron and iron... Fe2+ replacing Ca or Mg would kill
fluorescence, Fe3+ replacing Al in tetrahedral coordination could possibly
do the same as in albite or other felspars. I need a spectrum :-)))
 
[Axel] Thanks for helping, Pete (and Don and Earl and all the others)

Axel