I don't know exactly if is a Tetrahedrite, a Tennantite or another member of Tetrahedrite group. I attached a photo at 20x in plane polarized light and also in cross-polarized light. Is this a zonality feature or are twins? Moreover, if you zoom inside the red-dotted square, it can also be observed a slight color discrepancy between the same Td. The sample is from Coranda-Hondol deposits, a complex gold-polymetallic epithermal ore deposit + Te-rich signature.
To the left of your Td marker there is clear evidence of a mineral that is twinned. This mineral cannot be tetrahedrite/tennantite/goldfieldite which are isotropic. It may be a sulphosalt such as bournonite.
They shouldn't show anisotropy as they are cubic. However there are minerals close to the thd/tnt group such as watanabeite that are weakly anisotropic but these are very rare.
I cannot get your images to display on my computer which makes comment difficult so I only have your comments to go by - also you don't say why you thought it was tetrahedite and give no optical properties but I assume that you have some chemical data (because otherwise it could be so many things). However I assume that you have excluded the possibility of more common species such as luzonite-famatinite (stibioluzonite) and if As-rich, enargite - rather than more obscure sulphosalts. They show distinct to strong anisotropism/bireflectance and the former almost always shows polysynthetic twinning.
I cannot get your images to display on my computer which makes comment difficult so I only have your comments to go by - also you don't say why you thought it was tetrahedite and give no optical properties but I assume that you have some chemical data (because otherwise it could be so many things). However I assume that you have excluded the possibility of more common species such as luzonite-famatinite (stibioluzonite) and if As-rich, enargite - rather than more obscure sulphosalts. They show distinct to strong anisotropism/bireflectance and the former almost always shows polysynthetic twinning.
Thanks for your comment, Sir. Yes! It is really very difficult to figure out the minerals present in that sample without a chemical analyses. That twins with a pink-brown-blue tint can occur in many sulphosalts.
Most probably bournonite (the twinned mineral) - would also fit into the paragenesis in general. Tetrahedrite is isotropic.
They appear to be too broad and too irregular along their boundaries for normal thermal cooling induced twinning. The morphology looks like exsolution or growth zonation. I'd recommend looking at eds spectra on both sides to see what compositional variation exists. It could still be growth related contact twinning but the common twins are still on (111) or parallel to the [111] pole as a zone axis. In that the series is 4 3bar m and isometric, it should not have any bireflectance in reflected polarized light if it is just a twin and not an exsolution of another bireflectant phase. Substitutions and related phases may involve: Ag, Fe, Zn in the cation sites and Bi, As in the Anion sites.
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