[Rockhounds] Scapolite

[DonH]

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