I have two specimens of greigite in my reference collection from the Alacran mine in Chile. In researching the mineral, there are very few photos and none seem to have this crystal form. The ones shown are cubes. These are tetrahedrons, same crystal class but unlike any shown.
This specimen has crystals that are a bit colored on the tarnish.
The other specimen I have not taken photos of is colorful in tarnish, much blue but other colors as well.
Both have nice crystals and I have no idea if the labels are accurate or not.
Anyone have an idea if these are greigite or something else?
Rolf
Is a piece strongly attracted to a magnet? 
Reiner,
Yes it is magnetic.
I see there is no pyrrhotite at the location, thought it may be that but since it is not listed.
The second specimen has much brighter xls than the one in the photo here. Tough call as the list could be incomplete. Greigite is supposed to tarnish blue which pyrrhotite does not do, but then yours does not look blue. Greigite is so rare that unfortunately I would say XRD is needed unless someone has some confirmed material from there that looks like that. If it were me I would put it aside as another ? pending XRD. Too bad you can't get a fresh piece to compare to a piece of pyrrhotite the colors should be distinctively different. Two small grains side by side under the scope would be all it would take.
Reiner,
The second piece I only have in full view with the regular camera and it is also magnetic and this one has the crystals tarnished quite blue in places and other colors in areas. The crystals are the same tetragonal ones as on the first photo. The first photo has the largest crystals so I took that as a close up.
I know what you mean about not posting since there are no photos of similar pieces but when they are pieces for under $10, testing is a bit out. The cost of analysis much outweighs the value of the piece. A big problem in so many specimens that micro mounters end up purchasing.
Someone on mindat once told me that I should only post analyzed specimen photos and it would make the job much easier. I agreed as to it making the job easier to approve a specimen but as just stated, if one spends a few dollars for a specimen, or collects it oneself, the analysis is just not feasible. I told the person who said that I should only post analyzed specimen photos that if that were a criteria of posting, that over 90% of all the photos posted would not be there. How many times has anyone ever given you an analysis sheet along with a ten dollar specimen? Never in my case. I have gotten some for multi hundred dollar pieces but never for those low end reference pieces. Only in the case of some super rare micros and then the cost is at least a hundred dollars.
Wish it were that easy but Reiner is right, best not to post photos of specimens that have no similar ones on the site already.
RolfI have on occassion spent on analysis 10X what I paid for a piece but then to me knowing what I have is often more important than simply having a specimen. I guess it comes down to how important it is for you to actually have a greigite sample. Reiner,
Yes, sometimes it would be worth it but unfortunately I have hundreds, maybe even more, question minerals. We made an estimate one time and to get all the ones we wonder about tested would be about as much or more than we have paid for all the minerals in our collection. So, we hope we live long enough to see a hand analysis unit the average person can afford.
Rolf
