[Rockhounds] Puzzle

[R. Peter Richards]

Hi Carolyn,

The textural and fluorescence characteristics you describe would  
probably not survive pseudomorphy.  I don't know why you would have  
determined a hardness of 7.5 if it is a perthitic feldspar, but I  
don't trust hardness tests too much, especially if not done by one  
who has extensive experience with them (I don't know about you in  
this regard...).  Feldspars can have an approximately hexagonal  
crystal cross-section in the right orientation, so your sense of a  
hexagonal crystal shape may be misleading you.  One really has to  
measure the angles between faces, as measured between lines  
perpendicular to the faces, to determine whether they are really  
120°, as the  hexagonal system requires.  What does the termination  
of the crystal look like?  Beryl should have a clear 6-fold aspect to  
its termination, whereas feldspar is unlikely to have more than a  
poor imitation of it.  (I know that is as statement that takes  
experience to interpret and apply, and I regret that, but that is the  
way it is!)

My sense is that your observations support microcline, but your  
hardness measurements are somehow in error.

Another possibility is that what you have is a cast of microcline  
after a mold left by the dissolution of beryl, but this seems a stretch.

Can you provide some photos?

Regards,
Pete Richards


On Jul 12, 2009, at 2:45 PM, Carolyn Reynard wrote:

> To the List: I have and interesting puzzle.
>
> I collected the specimen at the Havey Quarry, Oxford Maine.
>
> It is a crystal that appears to be common beryl as you can  
> determine the hexagonal crystal shape.  The hardness is
> 7.5.  The color is a warm light tan.  The matrix is feldspar.
>
> What doesn't quite convince me it is a  beryl is a texture that  
> looks likes like perthite and runs the complete length of the  
> crystal into part of the matrix and is at right angles to the C  
> axis. The feldspar matrix fluoresces at bright light blue with red  
> perthitic texture as does this crystal..
>
> I am wondering if this specimen is a pseudomorph, with perthitic  
> microcline after beryl.   Would the microcline replacement still  
> retain the hardness of the beryl?
>
> I believe this is the important question.  Certainly the crystal  
> fluoresces like the microcline  but there is the problem of  
> hardness being too great for a feldspar.
>
> Nice specimen, just wish I could figure it out!
>
> Carolyn Reynard
>
>
>
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___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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