Identity HelpIf this Topaz crystal twinned?
I've had this Topaz/Muscovite specimen for several years; it comes from the Shigar Valley in Pakistan. It has an unusual feature in that the prism face on one side seems to be split in half. The opposite face is continuous and does not exhibit the split (see the last photo). Is this possibly a twinned crystal? Any help would be appreciated.
Nik Nikiforou
My guess (note that....GUESS) based on the first photo, a parallel growth. Something was in the way, which got removed when the specimen was extracted. But I could be wrong.... 
parallel growth - doesn't change the apparent symmetry, which I assume twinning would
cheers 
Doug's thought about 'something in the way' chimes a bell for me.Here are some pics of a hydroxylapatite specimen that seems twinned - and the cause of the twinning (?) 'something in the way', is still in place (translucent pink with no obvious habit, ID unknown). I'd really like to understand more about what goes on in these situations.
Nik,
From the photos that you post it appears, based on morphology and a guess at the forms present, that the reentrant (notch) divides the crystal in two by a plane that is parallel (010). If this was a contact twin with the two components related by reflection across that plane then the two component crystals could not have a mirror plane (m) in the same orientation. The point group symmetry of topaz in 2/m2/m2/m. The second m in this symbol is oriented parallel to (010) in topaz. Thus, if I am correct with my orientations, this is not a twin. The general rule is the symmetry element that describes the twin relationship (e.g. m, 2, etc.) cannot exist in the twin components in the same orientation.
Cheers
John 
Not all xls with re-entrant angles are twins!!! Thank you all for the information, it has been very helpful.
Cheers,
Nik
