FMF - Friends of Minerals Forum, discussion and message board

[Josele]

[Josele]

[Mark Holtkamp]

Josele, the parallel extinction doesn't prove it is biaxial, it could still be uniaxial. But it is consistent with Pete's interpretation. If the photograps were taken along the b-axis, you would expect the extinction directions to make a large angle with the polarising directions.
I don't know what's going on in the last photograps but it looks like the polarisation directions of the sunglasses are not perpendicular?

[Josele]

Understood, thank you Mark.
Better we forget last image, these "polaroid" old glasses do very weird things when you turn one of them over facing the polarized computer light.

[marco campos-venuti]

I have an untwinned single crystal of titanite from that area. I had it as from Mohmand Agency, Pakistan, not really near, but you know stones in these mountains walk with nomad peoples. It has many similarities with your crystal, color, elongation and is heavily included with actinolite-type mineral (looking white when inside, but looking black when outside of the titanite). Untwinned titanite crystal are very rare in hydrothermal veins, but common in volcanic miarolitic cavities, despite very small. The shape is simple and clear.