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MineralsBismuth

3rd Sep 2014 02:09 UTCReiner Mielke Expert

Does anyone know of any studies on the Fe-S-Bi system. I have an intergrowth of pyrite-pyrrhotite-bismuth that I am thinking might tell me something about the temp-pressure and chemistry of it's formation.

3rd Sep 2014 02:48 UTCPaul Brandes 🌟 Manager

Reiner,

Not sure if this will help, but hopefully it's a start:

Phase Relations in the Systems PbS-Fe-S-SbS and PbS-Fe-S-BiS

3rd Sep 2014 12:28 UTCBen Grguric Expert

The nature of the intergrowth is of course very important. For example you might have a pyrite-pyrrhotite intergrowth associated with paragenetically later bismuth. In which case phase relations in the Fe-Bi-S system aren't particularly relevant. The tendency of pyrite to idiomorphism means that it's paragenetic position is often difficult to establish in ore assemblages.

3rd Sep 2014 14:28 UTCReiner Mielke Expert

Thanks Paul!

Hello Ben

I still need to make a polished section to determine the exact nature of the relationship. All I can say at this time is that the bismuth seems to be restricted to Py-Po masses which are in a carbonate-chlorite matrix. Have never seen such a relationship before, it is particularly unusual for the Cobalt-Gowganda area of Ontario where I found the sample yesterday.

4th Sep 2014 16:26 UTCReiner Mielke Expert

I cut a slab of the sample and much to my surprise I don't have Py or Po rather tarnish arsenopyrite and some unknown grey magnetic mineral with a hardness less than stainless steel (5.5?) The arsenides contain disseminated grains of bismuth but there are also large bismuth crystals? that contain arsenides and matrix. See attached scan of the slab. The bismuth "crystal" is 2cm across in it's largest dimension. I plan on doing a magnetic separation of the grey arsenides and getting that XRDed. Anyone have any ideas what is going on here in terms of paragenesis? and what the unknown might be?

4th Sep 2014 16:38 UTCAlfredo Petrov Manager

Arsenopyrite-ferberite-bismuthinite-bismuth is a classic association at several mines in Bolivia, but would seem unlikely for Cobalt.

4th Sep 2014 17:24 UTCRob Woodside 🌟 Manager

Looks like the Bi was the last to come in.

4th Sep 2014 17:30 UTCReiner Mielke Expert

I agree the bismuth was last but I cut several slices and the bismuth mass is more or less oval not a vein somewhat like a porphyroblast. All the bismuth I have seen from the Cobalt-Gowganda area was been as fracture fillings so this is very odd.

4th Sep 2014 17:34 UTCRichard Gunter Expert

I see something similar in my Langis Mine sample with large Bismuth crystals but it has Co-arsenide rather than Arsenopyrite. Bismuth was never an economic mineral so has much less literature on the paragenesis that the Silver.

I read one early theory that said the Bismuth and Silver were part of a high temperature series and at Cobalt they separated out as the temperature fell to be deposited as late-stage phases. I have not seen any re-mobilization of Bismuth the way that Silver can be re-mobilized into leaves in the Calcite from the cores of the dendrites. All the Bismuth I have seen is the late-stage coarse crystal aggregates and cleavages.

4th Sep 2014 18:07 UTCReiner Mielke Expert

Bismuth as fracture fillings in carbonate is very common in Silver Centre and I have often seen it at the Lawson Mine in Cobalt, in fact almost all the bismuth I have found has been in late fracture fillings and that has been hundreds of samples. Unfortunately the fracture fillings are usually so thin that they do not make good specimens, and thick veins are rare. Bismuth coming in at the same time as the arsenides like the silver would make sense in this case, because it seems to have the same relationship to the arsenopyrite that silver commonly does to the Co-Ni arsenides.

4th Sep 2014 18:20 UTCRichard Gunter Expert

Do you have a sense of Bismuth occurring in the low-silver portions of the veins as a possible substitute for Silver?

5th Sep 2014 00:50 UTCReiner Mielke Expert

They say that in Cobalt the presence of bismuth generally indicates the end of silver.

5th Sep 2014 11:49 UTCBen Grguric Expert

Although I normally look at these things a micro-scale on the reflected light microscope, it looks to me like your bismuth was partially replaced by the phase marked As. The texture looks like the so-called caries replacement texture, though you'd need to see it on a micro-scale to be sure. A good reference on the textures and parageneses would be the 1971 special issue of the Canadian Mineralogist entirely devoted to the Cobalt area. Your As phase shows some magnetism which is interesting as there are very few obviously magnetic minerals, e.g. magnetite, ilmenite or monoclinic pyrrhotite. If your As phase it not one of these it is probably intergrown with one of the three. I think you'd get a better diagnosis simply by polishing the slab (final polish should be 1 micron diamond paste) and examining the textures on a reflected light microscope. Unknowns can be marked with a triangle of masking tape and characterised with EDS. If you're serious about understanding the paragenesis this is the only way to do it.

5th Sep 2014 15:09 UTCReiner Mielke Expert

Hello Ben,

Thank you for your comments and suggestions. The large fragment in the middle of the bismuth mass is a piece of matrix ( cb-chl) with As in it which means that the Bi incorporated the matrix with As fragment rather than the Bi being replaced by As. Also what is not visible in the scan are many very small pieces of Bismuth within almost all the As masses. This weekend I am going to do a magnetic separation and send that off for powder XRD to try and determine what the magnetic phase is. After that I plan on making a polished section to try and determine the relationship between the magnetic and non-magnetic phases. The 1971 CM publication does not deal much with bismuth and there is nothing in it that even comes close to what I am seeing here.

5th Sep 2014 18:47 UTCRichard Gunter Expert

One of the other phases in your Bismuth might be disseminated < 1 mm striated sulphide/sulphosalt crystals. You can see them as small, blue-tarnished, non-magnetic, grains on the Bismuth faces in the photo. They match Lindstromite visually but I have not seen any reference to this texture in the literature. The sample was labelled only "Cobalt" so I have no mine name. This phase does not exist in the Bismuth crystals from the Langis Mine.

5th Sep 2014 20:21 UTCReiner Mielke Expert

Could be bismuthinite as I have found those two intergrown on a couple occasions both in Cobalt and Silver Centre. I also recently acquired a sample of bismuthinite that was tarnished bright blue ( confirmed by XRD) but I have no idea what causes the tarnish. I have never seen that with bismuthinite at other mines in the area or in samples I have from Quebec, Manitoba, and Bolivia.

5th Sep 2014 21:08 UTCRichard Gunter Expert

I will have a look at the sample and see if Bismuthinite matches the properties; it would certainly make sense chemically. I did not know Bismuthinite tarnished blue but I guess anything is possible. Thanks for the information.

5th Sep 2014 23:25 UTCRichard Gunter Expert

I checked the Bismuth sample and the prismatic crystals are abundant once you find them. They occur at random angles to the enclosing Bismuth crystals and are always euhedral, as if enveloped by the later-stage Bismuth. One of the crystals has been intersected by a fracture edge and has two cleavages and a strongly striated, blue-tarnished tin-white cleavage surface. It looks a bit like your posted Bismuth-Bismuthinite aggregate but with Bismuth predominant.

Since you have seen these aggregates in both Cobalt and Silver Center I was wondering if they may be characteristic of the "secondary" vein Bismuth and the "primary" Bismuth crystals associates with the arsenides from your sample and my Langis Mine sample do not have them. This would imply that Bismuth acts in these veins in the same manner as the Silver, which has different Sb contents in its different parageneses.

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