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PhotosChalcocite - Steward Mine, Butte, Butte District, Silver Bow Co., Montana, USA

10th Jul 2017 22:46 UTCRichard Gunter Expert

I have assumed the sulphide phase associated with the Chalcocite sample is Pyrite based on its octahedral shape. It is often altered to a bronzy colour but all of the crystals are octahedrons with minor cubic faces. Is it possible these might be Colusite instead? All of the Pyrite associated with Chalcocite I have seen in photographs is either cubic or pyritohedral with no octahedral forms. The massive material in the center of the sample has a bronzy tint.

12th Jul 2017 06:17 UTCRob Woodside 🌟 Manager

They should be tetrahedra and not octahedra. Pyrite and colusite are in different xl classes.That's why killer colusites look like very strange pyrites.

12th Jul 2017 15:39 UTCRichard Gunter Expert

Hi Rob:

These look more like octahedrons but are unlike any of the photographed Pyrite from the Butte area. The staining of Pyrite with secondary Cu sulphides in the Butte Chalcocite-Bornite-Digenite samples makes colour determination difficult, even on broken surfaces. They certainly look like very strange Pyrites.

13th Jul 2017 16:08 UTCRob Woodside 🌟 Manager

Sounds like it is worth a $10. EDS.

13th Jul 2017 16:14 UTCRichard Gunter Expert

Hi Rob:

I think so as well. I will have a look to see what I want to sample from it.

13th Jul 2017 22:43 UTCRichard Gunter Expert

Hi Rob:

I was investigating the pyrite crystals and found that several of the octahedral faces have isosceles triangular growth figures centered on the octahedral face rather than striations. The terminations of the triangles point to the center of the adjacent large edge. I did not think pyrite's crystal system allowed for this, does it? I have seen something similar on carrolite, where the triangular growth figures point to the junction of the large faces and on tetrahedrite but never on pyrite.

17th Jul 2017 19:12 UTCRob Woodside 🌟 Manager

Carrolite is hexoctahedral, pyrite is pyritohedral and colusite is hextetrahedral. I suspect pyrite could show triagular growth features on octahedra.

17th Jul 2017 19:46 UTCRichard Gunter Expert

Hi Rob:

I have checked the crystallography and the only one that has this kind of growth feature is the Tetrahedrite Group, including Colusite. I have found identical growth features on my Tetrahedrite sample from Pachapaqui, Peru where triangular growth features occur with apacies intersecting the crystal faces. Interestingly the Tetrahedrite also contains triangular growth features that intersect the crystal edges but these triangles are 180 degrees to the first set.

I have not seen any report on these triangular growth features on Colusite. I will try to post a photo but it will be challenging.

17th Jul 2017 22:13 UTCRichard Gunter Expert

Rob:

A diagram of the triangular growth features on an octahedral pyrite is from a Travesella pyrite in Dana's Elements of Mineralogy. The diagram indicates that the apex of the triangles will subtend a 33 degree or 66 degree angle to the octahedral crystal edge. Sometimes you need the old data as modern mineralogy often does not look at these things. I checked my Musen Siegenite and its triangular growth features are identical to the Carrolite, as they should be.

18th Jul 2017 00:12 UTCRichard Gunter Expert

Sorry that should be 30 degrees and 60 degrees.

18th Jul 2017 00:29 UTCRobert Rothenberg

I just uploaded a photo of an octahedral pyrite with triangular growth features on a face. Photo #833006.

Bob

18th Jul 2017 09:30 UTCRob Woodside 🌟 Manager

Pyrite

Edgewater, Bergen County, New Jersey, USA

Thanks Robert. Here is his photo;

18th Jul 2017 15:27 UTCRichard Gunter Expert

That is different than the Dana diagram and is close to what I am seeing. Could it be the spinel twinning?

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