[Rockhounds] Micro zeolite associates from Centreville, VA

[R. Peter Richards]

In material said to be from the Virginia Crushed Stone Co.,  
Centerville, VA, I have vugs lined with prehnite.  Late stage  
crystallization has produced loose stacks of microcrystals, or in  
some cases, crystals impaled on byssolite fibers.  These crystals  
show nearly all of their faces, making them a delight for someone  
like me who likes crystal form first and foremost.  They seem to be  
of two sorts, though both are platy with a six-sided morphology that  
is consistent with orthorhombic symmetry.  One can think of a diamond  
with the blunt angles cut off symmetrically.  (Man, it's hard doing  
this without graphics!!!!)

On one type, the truncating faces are shorter than the faces of the  
basic diamond, the un-truncated points of the diamond are formed by  
faces that meet at a bit less than 90° (from a photo I measure 82°),  
and the faces are striated parallel to lines running between these  
untruncated points.  If you're trained, think tabular on {001},  
bounded by {110} truncated by {100} or {010}.

On the other type, the crystals are thinner tabular, the truncating  
edges are longer, there are no striations, and the angles at the un- 
truncated points are more nearly 120° (I measure 100°).

Can anybody say if these are both the same mineral?  Are they both  
prehnite (my guess)?  Does anybody have any additional information  
about these beautiful microcrystals and their presumed mode of  
formation?

And can anybody offer any advice about how to remove the brown gunk -  
smectite or something similar - without ruining these crystals?

I can send digital images to anyone who is interested - contact me  
offline.

Thanks,
Pete Richards