[Rockhounds] mystery mineral

[Carol J. Bova]

My immediate reaction was scapolite, distorted crystals are not  
unusual, and this scapolite specimen reminded me of your description  
as hummocky:
http://www.galleries.com/scripts/item.exe?ENLARGEC+Minerals+Silicates+Scapolite+SCA-2
Weathered pieces will have that frosted exterior and can be  
transparent inside.  But.. the hardness is only 5.5-6, and the  
conchoidal chips are another 'probably not' indication.

I checked Sinkankas for yellow minerals with vitreous luster, and  
besides scapolite, datolite doesn't seem to fit the shape, leaving the  
other two candidates as orthoclase and spodumene.  My GIA chart says  
spodumene often fluoresces (LW weak-strong; SW weaker), but  
fluorescence is weak to inert in orthoclase, and sanidine overlaps  
yellow orthoclase in properties.

Yes, I know these all these have cleavage, but... since this is an  
exercise in guessing possibilities without even seeing it, I'll keep  
going.

John Sinkankas described the fracture in orthoclase as "uneven, often  
stepped, and rarely, conchoidal." Could the chipped edges you describe  
actually be stepped?

He also cites the yellow transparent from Madagascar as trichroic, and  
orthoclase is dichroic, so, just for the heck of it,  pending specific  
gravity readings, any pleochroism? If there isn't, doesn't prove  
anything, but if there is, it proves it's not glass, and is a doubly- 
refractive material.

Carol


On Mar 26, 2009, at 12:59 AM, pmodreski at aol.com wrote:

> How about if I get off this rift zone that we seem to be kind of  
> stuck in
> for the time being, and ask something about a mineral?
>
> I've got one that is really puzzling me.  And I'm not sure if it's   
> really a
> mineral--it could be something synthetic, or just glass. Perhaps   
> someone on
> the List may have ever seen something similar, and have some good   
> suggestions
> for me.
>
> This is something I came across amongst a bunch of very assorted   
> specimens,
> given away by the Colorado School of Mines geology department  
> several  years
> ago; I was looking through the material, picking out specimens  
> either to  give
> away to teachers, or to save for my own use in classroom  
> demonstrations and
> seminars for teachers.
>
> This one, this "crystal" (it looks like a crystal, but I don't know  
> if it
> really is) is rectangular and blocky, about 2 inches long and about  
> an inch
> diameter (it weighs about 100 grams), and is pale yellow.  It is  
> completely
> clear--transparent--internally, but the outside has kind of a  
> slightly frosted
> appearance all around, as if it has been tumbled around a good  
> deal.  There  are
> a great many chips on the edges, and all show a conchoidal fracture-- 
> no
> suggestion of cleavage in any direction. In overall  shape it would  
> be called a
> rectangular prism; the major sides are at right  angles, and the top  
> is a flat
> (basal) termination.  The base is  irregular.  One side has an  
> additional
> "prism" face at about 45 degrees to  the others, and one corner also  
> has a beveled
> face at what appears to be 45  degrees; if these were all true  
> crystal faces,
> their miller indices would  be 001 (the flat top, "c" face), 100,  
> -100, 010,
> 0-10, and these last two,  110 and 111.  Two of the main prism faces  
> are quite
> flat, but the other two  (adjacent, not opposite faces) are not  
> really flat at
> all, but kind of  irregularly rounded or "hummocky" (plus a lot of
> chipped/broken areas).  I  don't see any bubbles or inclusions in it  
> of any kind; it all
> seems to be clear  and gem-quality.  The "crystal" is hard--a knife  
> won't
> scratch it, and a  quartz crystal doesn't readily make a mark  
> either.  It feels
> about the same  weight as an equivalent piece of quartz, and it is not
> fluorescent.
>
> I've seen a lot of strange minerals and things that were not
> minerals--people bring them to me all the time--but this doesn't  
> quite look like  anything
> familiar.  It has the general shape of an orthorhombic crystal,  but  
> is somehow
> just doesn't look "right" for being a real crystal.  The  lack of  
> cleavage
> rules out gem-quality feldspar, or topaz.  I've seen lots  of glass;  
> if this were
> glass, or quartz, someone must have cut it into the  rectangular  
> shape (an
> odd shape to cut it into), and then put it somewhere (a  riverbed?   
> a tumbler?)
> where it acquired a frosted surface.  And if it  were glass, I  
> should be able
> to scratch it readily with a sharp quartz crystal  point, which I  
> can't.
>
> Does this description ring any bells for anyone?  Yes, I'm one of   
> those
> people who is always (well, sometimes) telling other people to "go  
> carry  out
> simple tests on your specimen, such as hardness", so this is their  
> chance to  tell
> me to do the same thing.  I have a balance set up to measure specific
> gravity so I'm going to do that--I suspect it will come out similar  
> to  quartz.
> Does my description sound like any particular kind of gem cutting   
> rough that
> some of you may have seen?  Or (but I don't really expect this  to  
> pan out), does
> it remind you of a crystal of any real mineral that you have  seen,  
> that I
> may not be thinking of?  I can't imagine what it could be, if  not  
> quartz; I
> don't think it could be topaz (specific gravity will confirm  that),  
> and it's
> surely not beryl or apatite; nothing hexagonal looking about  it.   
> If it were
> just a tiny crystal I'd be more inclined to think it was  "real",  
> but, something
> this big--doesn't seem likely.
>
> I haven't had a chance to take any pictures of this, but once I do,  
> I can
> forward one to any who'd like to see it.  And I'll see what the  
> specific
> gravity shows.
>
> Let me know if you have any great ideas about this, thanks,
>
> Pete Modreski
>
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