[Rockhounds] when volcanoes drop bombs: giant crystal payload

[DonH]

John Junkroski wrote:

> This sounds very much like a site we visited a few weeks ago after  
> digging at the Spencer opal mines.
> 
> A local gentlemen suggested that we drive out and assured us that  
> access was no problem since the local road commission was using the  
> cinder cone as a source for road-fill.
> 
> We spent the better part of an hour exploring three or four recently  
> active pits and found nothing other than a few 1/4 inch  yellowish,
> opaque crystals that I could not identify. I picked up an awful lot  of 
> (boring) black basalt and found nothing worth bringing home.
> 
> If anyone has any suggestions on how to locate the "sunstone"on the  
> cindercone I would greatly appreciate hearing them, as we intend to  go 
> back to Spencer early next Summer.


The majority of what the professor had were in fact from 1/4 to 1/2 inch 
long.  That bomblet was the biggest specimen found on that trip by far, 
according to him.  I suppose like anything else, it's luck.

Well the x-ray diffractometer provided a nice crisp pattern; the crystal 
was almost 100% feldspar, with just a little amorphous material, i.e. 
the red volcanic matrix.  I ground the material to a fine consistent 
pwoder in a micronizing mill and packed it very carefully into a 
mount--I realize that doesn't mean much to most people, but rest assured 
that I prepared the specimen very well.  Then I ran a moderate 
resolution scan that took about 2 hrs. (a really nice high resolution 
scan can take 8 hrs. or more).  The best pattern match was for 
"00-041-1481, Anorthite, sodian, disordered (Ca,Na)(Si,Al)4O8."  There 
was no labradorite pattern on file!  Truth be told, the peaks matched a 
few of the patterns on file, and in the time I had to work on it, it was 
too close to call.  Besides, feldspars can vary continuously over a 
fairly wide range, relatively speaking--if I had the time I would do the 
math by hand to get a better description of the structure.  However, in 
this case, we will learn more from the chemistry, when he gets it done! 
  He had the fragments all mounted and polished so I hope he gets the 
data soon.

More later,
Don