[Rockhounds] Scapolite

[Axel Emmermann]

Thanks Don,

That cleared up some of my worries ;-)))

I'm glad to hear that you found a new job. Is it in the line of your freshly
mastered skills?

Take care and success in the new job
Axel

> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens DonH
> Verzonden: woensdag 24 september 2008 4:00
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
> Onderwerp: Re: [Rockhounds] Scapolite
> 
> Axel Emmermann wrote:
> 
> > That is why I would like to know whether there is a noticeable
discrepancy
> > between the occurrence of large, gemmy crystals of both species or that
it's
> > apparently not a big deal ;-)
> 
> Well, Pete Richards would be a better person to give a definitive answer
> on this exact question.  As I mentioned previously, the main driving
> factors in crystal growth are the composition of the melt and rates of
> cooling.
> 
> To quote my advanced mineralogy professor, most minerals at the
> nano-scale are a mess; full of defects, dislocations, and substitutions.
>    We studied tourmalines, apatites, and feldspars, among other
> important minerals,and one wonders how these ever form recongizable
> crystals at all, given everything that can go wrong during their
> formation.  Yet they do.  There is a big difference between what happens
> on the molecular scale and the macro scale.  Feldspars in particular are
> an atomic game of musical chairs, followed by outright molecular warfare
> as they cool and try to form stable structures.  Yet, again, we see
> beautiful, sharp, and well-formed large crystals of these.  If you look
> closely at many of them, you can see the exsolution lamellae where the
> incompatible entities separated; yet, on the hand scale, the crystal can
> still have a perfect morphology.
> 
> 
> > It seems also that the ratio of Al-occupied/Si-occupied tetrahedrons is
much
> > higher in meionite than in marialite. Something in the order of 10 to 15
%
> > more Al than Si.  Does this mean that Fe3+ substituting for Al could
also
> > occur more frequently in meionite than in marialite?
> 
> That's easier to answer: Fe3+ will often substitute for Al3+, depending
> of course on the size of the coordination polyhedra.  However, there is
> a limit to the degree of substitution that will actually occur.  Si and
> Al have high surface charges and they like to grab the free oxygens
> first; and, since O, Si, and Al are the most abundant elements before
> Fe, it makes sense that there will be more Si-O and Al-O polyanions in
> the average melt (with "average" being the operative word; plenty of
> exceptions there, of course).
> 
> Well, I am moving to take my new job, so I won't be writing much in the
> next few weeks.  I'm also hoping someone with more knowledge and
> experience on the subject will have a more suitable answer.
> 
> best,
> Don
> 
> 
> 
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