FMF - Friends of Minerals Forum, discussion and message board

[Josele]

A friend brought me a small smoky quartz from Madagascar, bought in a road shop near Itremo. Although it is somewhat chipped, it retains some interesting features. In addition to the usual faces of the prism (m) and positive (r) and negative (z) rhombohedrons, the crystal shows what I suppose are two faces of the trigonal bipyramid (s), one of the trapezohedron (x) and another one that I can not recognize. Is a small scalene trapezium placed between adjacent x, s, r and m faces as can be seen in pictures.
Comments are welcome!
Thanks.

[Pete Richards]

Josele, you offer quite a puzzle! I think things are not as they seem, though I can't be sure. I note that the face that is in position to be the s face actually is not smooth, but concave and stepped - in your top-left image in figure 4 this is particularly apparent. Furthermore, I don't think its relationship to the other faces is right for an s face. The s face has the special property that it bevels rhombohedron and prism faces in both directions. This is to say that its opposite edges are parallel to each other in both directions, as can be seen in my first drawing and your second (excellent) image. I don't think the face in your fourth image shows this property, and I propose to ignore it as being not a true face, though I don't have a good explanation for why it is there. Perhaps contact with another crystal?

I suggest that there is no s face on this corner, and that the face illuminated in your figure 3 is an x face with indices (511). The mystery face appears to bevel x(511) and r(101). If so, its indices must be some linear combination of (511) and (101). I chose (511)+2*(101) or (713), which I arbitrarily named ∂. (713) is not a satisfying set of indices because they are too high (especially 7), but I don't think any combination of the indices of the two surrounding faces (i.e. (511) and (101) ) will produce a face that bevels the two and has satisfyingly low indices.

Further progress beyond this somewhat speculative analysis would require measurement of the angles between the various faces.

Hopefully this analysis is of some help in understanding your crystal!

[Pierre Joubert]

'Perhaps contact with another crystal? '

That is what it looks like to me too.

[Josele]

Pete, Pierre, thanks for your help.

After seeing Peter's drawings and spent some more time with the X10 loupe, now I see there is not a s-face in this corner as you pointed, definitely angles don't fit. Although to me it does not seem a contact rather a complex / distorted step followed by a slightly curved plane sub-parallel to the adjacent r-face. There is some other distorted corners at this specimen. Anyway, not a real face.

The supposed x-face really looks like a true x-face. Face is flat but it has the longest side of the triangle somewhat curve because prism face is curved at top.

And weird small face is a bevel between x and r faces, as showed in Peter's figures 4 and 5. Not a very common face!

Pete, I really envy your capability for recognizing faces and even for determining Miller indices (from photos!!!), congratulations and thanks for your patience sharing your knowledge in this didactic and profitable way.

[Pierre Joubert]

Hi Josele. May I make a suggestion to you and the administrators? There has been a number of threads about curious growth patterns and shapes in quartz. Can we not build on this thread and slightly change the topic to accommodate other contributions, as well as with links to all previous contributions? I also have a number of 'strange' forms of quartz to show.

[Josele]

Pierre, of course you can use this thread to show your weird quartz when you want. I'm already looking forward to see them!

[Pierre Joubert]

[Cesar M. Salvan]

[Pierre Joubert]

Wonderful Cesar. I am looking forward to the expansion of a topic close to my heart. Thank you!

[Pierre Joubert]

Hi Josele and other interested FMF members. Here is my first contribution, a quartz with goethite specimen, which I found about a year ago. There were a few similar specimens from this pocket, but not quite like this one. It has (to me) a very strange growth form, which could be partly the result of a broken and rehealed crystal. I find it hard to show the specimen as I would want to. Some of the smaller edges are rounded.

[Pete Richards]

Hi Pierre,

Your specimen definitely qualifies as distorted! I think your second image is closest to being in a standard orientation. You may be right that it involves breaking and healing, but it is not clear that this occurred.

I've taken the liberty of capturing your second image and labeling it with the faces I think are present. r and z are the two rhombohedra that normally terminate quartz crystals, faces labeled m belong to the prism. There is no way to tell for sure which rhombohedron is which - I may have them labeled backwards - but it is usually assumed that the rhombohedron r has the larger faces. The hillocks on the top r face are a common growth feature and don't require breakage to explain; the irregular surface of the bottom r face (seen especially in image 4) is more suggestive of a broken and healed surface, but it appears to be in the proper orientation for an r face. Quartz does have a poor cleavage parallel to the rhombohedron which is sometimes the breakage plane when quartz breaks during growth, so it is possible for this to be a broken surface and still have the right orientation for a rhombohedron face.

The illuminated face in image 3 is a prism face, which is shown by the striations.

Attached is a cartoon that shows how your crystal relates to a more normal shape, in my interpretation.

[Josele]

Nice piece, Pierre, very interesting. Without the help of Pete I would never arrive to determine the faces.
One rhombohedron much more developed than the other and few visible prism faces. Could it be called almost a pseudo cubic quartz crystal?

[Pierre Joubert]

[Pierre Joubert]

[Pierre Joubert]

Sorry Pete, my message should have read like this: 'The crystal forms in your first 3 sketches are not uncommon from this area, so if there were no interferences, it probably would have ended up looking like one of those.'