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W. Richard Gunter July 10, 2017 10:46PM
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.
Rob Woodside July 12, 2017 06:17AM
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.
W. Richard Gunter July 12, 2017 03:39PM
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.
Rob Woodside July 13, 2017 04:08PM
Sounds like it is worth a $10. EDS.
W. Richard Gunter July 13, 2017 04:14PM
Hi Rob:

I think so as well. I will have a look to see what I want to sample from it.
W. Richard Gunter July 13, 2017 10:43PM
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.
Rob Woodside July 17, 2017 07:12PM
Carrolite is hexoctahedral, pyrite is pyritohedral and colusite is hextetrahedral. I suspect pyrite could show triagular growth features on octahedra.
W. Richard Gunter July 17, 2017 07:46PM
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.
W. Richard Gunter July 17, 2017 10:13PM

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.
W. Richard Gunter July 18, 2017 12:12AM
Sorry that should be 30 degrees and 60 degrees.
Robert Rothenberg July 18, 2017 12:29AM
I just uploaded a photo of an octahedral pyrite with triangular growth features on a face. Photo #833006.

Rob Woodside July 18, 2017 09:30AM
Thanks Robert. Here is his photo;

Spinel twinned pyrite octos
W. Richard Gunter July 18, 2017 03:27PM
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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