FMF - Friends of Minerals Forum, discussion and message board

[John Hodgson]

I have been a collector for many years because crystals are beautiful. However, until recently I have not paid much attention to crystal habits. I know that what I am asking is a very simple thing for "you guys" to answer. I believe the the galena I have posted shows spinel law twining. Is this true? Also, I purchased this specimen many years ago, but where it was collected was unknown. Any ideas where it could be from? Thank you!

[Pete Richards]

Please provide a picture from above relative to the first picture you posted. These are crystals combining the octahedron and the dodecahedron (I think), but it is not clear from these views whether they are twinned or not. Possible but not certain.

Also a picture showing the surface detail on the big face in the front of the first picture would help.

[John Hodgson]

This is a top view.

[Bob Carnein]

This doesn't look like the galena spinel twins I have in my collection, which are usually thin plates. I agree with Pete Richards--they appear to be cubes modified by octahedron faces.

[Amir Akhavan]

Crystal faces that belong to different forms often differ in their surface features, like luster, growth hillocks, steps, dents or other patterns.
These features vary a lot between different specimens, but on the individual specimen the faces of one form often have very similar surface features.
This can help to identify twins, because you can "group" faces by their surface patterns and see if they form an "imaginary crystal form" that matches what you would expect too see in an individual crystal or twin.
I mention this because the differences between the faces are very obvious on your specimen.

[John Hodgson]

This is photo of the larger crystal with a view of the bottom.

[Pete Richards]

No indication of twinning is indicated by the new views of this specimen. It is a group of crystals, or one incompletely developed crystal, combining faces of the cube and the octahedron. Cube faces are relatively flat, sometimes showing square plates or thin layers suggestive of the cleavage of galena; octahedral faces are more irregular in texture and are equilateral triangles, often truncated.

Spinel twins of galena are usually flattened parallel to the octahedral twin plane, as Bob Carnein said; no evidence of this twinning-altered morphology is seen here.

Galena spinel twins, in my experience, rarely occur singly, but occur in multiples, often as clusters of parallel or near parallel twins. They often have hexagonal outlines. One example is shown here, poorly photographed by my phone.

[John Hodgson]

Thank you for the help, explanation and photos!

[Tobi]