FMF - Friends of Minerals Forum, discussion and message board

[prospector]

One of the most uncommon and impressive pyrite crysals must be the octahedral form. I have seen a few very good examples from Peru and none until now from Canada. I have been working a former pyrite mine in the Province of Ontario with some spectacular results especially in relation to size. I have removed specimens as small as 1/2 cm all the way up to 20 cm. to the best of my knowledge, according to the Royal Ontario Museum, the Smithsonian and other sources, this appears to be possible one of the largest if not the largest octahedral pyrite found. Currently this specimen is on loan to the Royal Ontario Museum.

[Peter Megaw]

Al, is there much arsenic around this locality? many of the world's octahedral pyrite localities seem to have abundant enargite and related sulfosalts and I think there is some literature suggesting arsenic "poisoning" is invoilved in octahedral growth development.

[prospector]

Peter


I do not have any data that shows an arsenic content in the pyrites. That is not to say there is an absence of arsenic. It does show protochloride of cobalt at .775 cobalt oxide. From any informaton that I can find on octahedral pyrite formation and growth, high temperatures would have to be present. This location would be hydrothermal with a slow cool down allowing for large crystal growth. Crystals in the range of 3 to 4 inches are most common, with 5 to 8 inches and 1 to 3 inches less common.

[Peter Megaw]

Al, I'll buy hydrothermal, but large crystal growth is much more a function of low nucleation rate and fast crystal growth than prolonged cooling or other processes.

[prospector]

I have been attempting to find the exact reason for the formation of this form of crystals and any attributing factors without much success. One point that seems to come up somewhat consistantly along with temperature is geomagnetic influences at the time. An acquaintance of mine who is a Dr. of Geo-physics was at a loss for an answer.
I have the a map that was produced to show the geomagnetic forces in this area and the mine site is in one of the stronger areas. So, might it be, that both temperature and geo-magnetic forces were at play? It is only a theory.

I do have another thing that occationally happens to batteries when I am in the mine. But that is another story.

[alfredo]

Peter, Interesting theory about As poisoning and octahedral pyrites. Octahedral and cuboctahedral habits are perhaps the most common in Bolivia's tin and silver veins, being abundant in Llallagua, Colavi, Tazna, Animas... But there's not much arsenic there, and almost no enargite. Almost all the sulphosalts are decidedly Sb-dominant. I wonder whether Sb can have the same effect? Or whether only traces of As will do the trick?
There is an old study on Sb in octahedral galena, but I had the impression that this was more of a temperature-dependent phenomenon, with higher temperatures leading to octahedra and incidentally also allowing more extraneous elements into the structure - I'm not sure whether cause and effect are the right way around here. Pete Richards would probably be able to clarify this. (Incidentally, pyritohedrons are unknown in the tin and silver mines, despite being so common elsewhere.)

[Peter Megaw]

Alfredo...Sb might do it, I just remember running across the As argument somewhere in the literature. As for octahedral galena-higher temperature link there's a real problem given its abundance in certain Mississippi Valley Deposits which are decidely low T and I think those galenas are pretty clean

[jimB]

I want a 1 inch floater !

[prospector]

JimB

The best that I can do at the moment is a 1.5 inch floater. The majority of what I have are floaters which range up to 8 inches, Let me know about the 1.5 inch. The 8 inch is on loan and is worth bia considerable sum.

[Peter]

I think there may be the main component as Alfredo mentioned regarding the temperature influence of the habit. I have not seen the Murgul (Turkey) octahedral pyrite deposit but ther are also other forms present. We have several deposits in Sweden w frozen pyrite octahedrons and there some are Co or As carrying, but I do believe that ir not the reason for the habit. However cubic to more complex pyrites from the Malmberget mine with disturbed curved faces I believe are connected to the inclusions of other elements.

Al, please do find Jim B a killer thumbnail !!! He would really appreciate!
Peter

[prospector]

Peter

I will have a look at my data base and see what I have. I nearly had a big crystal for you but is split in two. It measured about 24cm, so there is still hope, but not until next year.

Al

[prospector]

Peter asked me to send you a thumbnail of the octahedral pyrite, so here it is.

[prospector]

Peter

One thing that I have found in regards to the crystal growth and influencing factors as to what crystal form will develop, is somewhat ambiguous. It would seem through reading what literature I can find about the growth and crystal forms of pyrite and talking to those experts in the field,is that temperature does play a major part in this. Also as I mentioned before, geo-magnetic influences may also play a role in determining what crystal for will take place. Apparently, the octahedron form indicates a high temperature environment, while cubes are of a lower temperature environment. However, it is less clear as to why they form as they do. So far it is all speculation.

[Peter]

Fluorite seem to do the same!
Wish you a great season next year! and hope to see you in Tucson!
Peter

[David Von Bargen]

The best book on crystal growth/habist is Kostov and Kostov (1999) Crystal Habits of Minerals which is a distillation of much of the eastern European research on the subject. (the largest pyrite octahedra they record is from Huanzala at 20cm on edge, and 30 to possibly 60cm cubes from Ahchatau, Kazakhstan.) Conditions other than temperature that have been considered also include amount of supersaturation, cooling rate, ratios of iron and sulfur in the hydrothermal solutions, and impurities (As, Sb, Co, Ni).