[Rockhounds] Scapolite and kutnohorite

[DonH]

Axel Emmermann wrote:

> Could antiphase domains mean something like the perthite/anorthite
> exsolution but more on a molecular very local scale (and without the
> exsolution ;-))). Something like a "zoning" of Ca and CO3 versus Na And Cl
> over the distance of a few unit cells?

That is how exsolution starts.  It is just a matter of time and rate of 
cooling.

> If I read the paper correctly, the scapolite series "jumps" (or at least:
> accelerates)  towards the meionite end when the Al/(Al+Si) ratio exceeds
> 0.35.  

Axel, look up "Lowenstein alumimum-aluminum avoidance rule."  There are 
reasons why a lot of things change when you being to substitute Al for 
Si in tetrahedra... sorry, no time to go into that now, maybe in a month 
or so.  In brief, Si fits just about perfectly into a tetrahedron 
bounded by O; Al, less well.  Also, Si and Al have different 
size-to-charge ratios and that affects the atoms around them and the 
structures as well.  The shapes of atomic lattices are usually somewhat 
distorted, and I think that is one of the difficult jobs of a 
crystallographer: taking raw data and devising an ideal structure and 
symmetry from that.

Also, remember that charge is not the only factor in forming a series. 
The ionic size and radius-to-charge ratio are also important... search 
for "crystal field stabilzation energy" and "crystal field strength." 
When you consider all the minerals out there, we have very few true 
continuous solid solution series... some combinations, though 
stoichiometrically possbile, are never found in nature.

More later,
Don