FMF - Friends of Minerals Forum, discussion and message board

[Bob Carnein]

Here's a challenge (?) for the crystallography folks. I recently bought this specimen of hambergite from Stak Nala, Baltistan, Pakistan. The crystal is 3.4 cm long, and, although it looks like it's convex in the photo, the "midline" of the crystal marks a slight reentrant. I believe it's a twin, which isn't unusual in hambergite. My question relates to the forms shown. Hambergite is orthorhombic, but I have never been able to figure out the orientation and forms on crystals like this or, especially, in the "boomerang"-shaped crystals like those from California shown on Mindat. If anybody on the forum has some insight, I'd appreciate hearing it.

[Bob Morgan]

I'm not very good at identifying orthorhombic crystal faces, but looking though the photos in Mindat, I came across photo 6MK-DUA which shows the orientation of 'boomerang' crystals in relation to regular prismatic crystals, They are double ended with striations front and back and long sloping smoother faces that meet from both ends, but only of one side of the crystal.
It's hard to tell what faces your crystal has, Perhaps the faces toward the camera are sets of sloping faces like on the 'boomerang' crystals. If twinning is involved it would be on faces parallel to striated sides of the prism, which would be the reported twin plane for hambergite.
Photos taken from different angles might help.

[Johan Kjellman]

Check,
Richards 1996 Canadian Mineralogist 34, pp. 615-621.
It's on twinned hambergite from Stak Nala and should help you a bit.

cheers

[Pete Richards]

It is difficult to impossible to decipher crystallography from a photograph, except in the most general terms. The slight re-entrant shown does suggest twinning, but the faces involved appear to be curved. Because they are not parts of planes, they do not have well defined Miller indices.

It would certainly help to have views of the specimen from other directions, especially looking down on the top (relative to the orientation shown here) and looking at the specimen from the right side.

Since the Canadian Mineralogist paper cited is mine, I think I can post the drawings below, as published there, without violating copyright. The main point I would make is that twinning is typically on {110}, and can be repeated. The morphology of the main twin I studied was strongly influenced by its double twinning, and is unlike the one Bob posted here.

I just noticed an omission which has been undiscovered for 26 years since the paper was published(!) The faces labeled X in the drawings belong to the form {341}; this is not mentioned in the caption. This is important because a this form, or one close to it in orientation, might be the form of which Bob's crystal is composed.

[James Catmur]

It is always great when the author of a paper can use it to help answer questions

[Bob Carnein]

Thanks to Johan, Pete, and Bob M. for your comments.

Attached are several additional photos of the hambergite. The first is looking down from "above", i.e. looking down the long axis as it exists in the first two photos I posted. The third shows the "right side" of the crystal (note: unfortunately, it is broken, so only a small portion of the right side exists). The second photo shows the "back" of the crystal, which clearly exhibits 2 directions of cleavage that appear to intersect at an obtuse angle (resembling the cleavages in an amphibole). The most prominent cleavage appears to be parallel to what I'm calling the composition surface of the twin.

[Bob Carnein]

I just looked at the hambergite crystal again and did a little digging. I believe the "composition plane" may be parallel to the cleavage, but I'm not sure that makes sense,. The big faces probably are a dipyramid. I can't accurately measure the angles, so I'm not sure about the Miller indices of the dipyramid. Thanks to Pete's input and diagrams, I think I have partlly answered my own question.