[Rockhounds] Ad- Blue/green Apophyllite

[R. Peter Richards]

I cannot comment on the reasons for the color, but I can say with  
some confidence that these are not twins.  Bow-tie growths usually  
result from rapid growth or growth in a "dirty" environment, which  
leads to spreading distortions of the crystal lattice.  With further  
growth, the distorted crystal divides into a number of nearly- 
parallel crystals which collectively produce the bow-tie shape.  This  
is not twinning, but a variant of (only approximately) parallel  
growth.  Twinned crystals should have predictable geometric  
relationships between each individual in the twin (e.g. they are at  
right angles to each other, with one prism face of each lying in the  
same plane).  Often, the shape of the crystal is distorted when  
compared to untwinned crystals, because growth is faster in certain  
directions as a direct result of the twinning.  Think of staurolite  
twins as one example where the distortion is minimal but the  
geometric relationship is evident (there are two different types of  
these twins), or twins of spinel or galena or quartz, where  
considerable distortion of form is the norm.

Pete Richards


On Oct 20, 2007, at 3:32 PM, Lawrence Rush wrote:

> I recently got a dozen pieces of an unusual blue-green variety of  
> Apophyllite. Although these are from Jalgoan, India, they do not  
> look at all like the usual blocky form of Apophyllite from there.  
> These are different in color, and are arranged in a parallel habit,  
> with the prism being elongated and frequently twinned. Apparently  
> they never occur as single crystals.The matrix (Natrolite) is  
> mamillary and stalagmatitic.  My Indian friend who sends me  
> minerals from there says that these are occasionally found there  
> and prized by the miners for their color and gem-like appearance.
>
> I would appreciate any any comment on these odd crystals from any  
> mineralogists in the group (and also anyone interested in buying  
> any, of course!).
>
> Larry
> www.ConnRoxMinerals.com
>
> -- 
> _______________________________________________
> Rockhounds at drizzle Mailing List
> Subscription Services:
> http://lists.drizzle.com/mailman/listinfo/rockhounds
> List Home Page, with a link to the List Usage Policy:
> http://www.eclecticlapidary.com/Rockhounds/index.html

___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
---