FMF - Friends of Minerals Forum, discussion and message board

[Yannick Cosson]

Twinned Topaz from Brazil. 4x3 cms. Twin in V like a bertrandite crystal ? But the twins for topaz crystals are rare and poorly documented: is it a topaz crystal?

[Bob Morgan]

If there is a point of attachment at the boundary between the two crystals, perhaps this is a twin, If not, probably no twin.

[Josele]

Have never read about topaz twinning, so I would tend to distrust.

Yannick, in addition to the hardness you can easily check density to confirm topaz: Specific Gravity Test.

[Pete Richards]

Your photo shows no faces. It appears that the free ends of both crystals are damaged. Are there good sharp crystal faces anywhere, and could you show them (as many as possible) in a photograph?

The problem is always that a single specimen like yours might be a twin or it might be some kind of coincidental intergrowth. When we find many examples with the same relationship (angle between the individuals, orientation of faces on the crystals, as demonstrated by measurements), in fact too many to be explained by coincidence, the argument for twinning is strong. There are subordinate criteria as well, such as how the faces of the two individuals meet each other, the comparable size (or lack of it) of the two individuals, etc. But without at least seeing crystal faces, the case for this being a twin is weak, especially since topaz twinning is rare at best.

The known twins of most minerals are well known to the community of mineralogists past and present, and are almost always described in reference texts. The only reference to topaz twinning I found among my usual go-tos is vague about the nature of the twinning, but says it is on (010). Without going into complex details, this would probably produce a twin that would not be onvious visually (like typical Dauphiné twinning in quartz), and certainly would not produce the angle that your specimen shows.

If you can find a reputable reference that illustrates a topaz twin that looks like yours and/or describes the twin law explicitly, you will have a stronger case, Without such backing, not so much.

[Yannick Cosson]

[Bob Carnein]

The new photo shows an apparent cleavage (which would be (001) if it's topaz), suggesting that the twin plane (if that's what it is) would be (hk0). The first photo might allow an estimate of the angle between the two parts of the "twin", allowing calculation of the actual parameters for the (hk0) plane. That said, it surely looks like a twin! If it is indeed topaz, this is certainly an impressive crystal.

[John S. White]

I had a topaz in my collection that I wanted so much to be a twin but I could find no reference to topaz twinning in that direction or, in fact, in any direction so I had to conclude that topaz does not twin. After all not every mineral can twin and it is very rare for someone today to demonstrate twinning in a mineral long thought not able to twin. A rule often applied when someone thinks they have a twinned crystal heretofore believed not to twin is that there has to be more than just one example of the suggested new twin before it can be accepted. A very good rule, in my opinion.

[Bob Carnein]

A topaz crystal that looks a lot like the subject of this thread is now for sale on eBay. It came from the Bill and Ann Cook collection and is described as twinned.

[Yannick Cosson]

Hello, I made the test SG specific gravity: the result is 3,4 and it is the SG of TOPAZ.

In fact I have another smaller sample (2x1) from another locality: Air Niger mountains, collected in the 70s. In these mountains are found topazes and cassiterite in annular granites. it's exactly the same "twin" but the sample is too light to do the SG test with the kitchen weighing machine.

Until now I thought it was a bertrandite...