FMF - Friends of Minerals Forum, discussion and message board

[RussEpp]

This specimen has prominent rectangular-shaped pyrite crystals. What are the geological conditions that would cause this to occur? The main crystals are 9mm thick and up to 29mm long.

[James Catmur]

The conditions must favour growth in one plane. We see the same thing in some fluorite from the northern UK and pyrite from Spain.

I am not sure what the conditions are but maybe someone has thoughts on it

James

[James Catmur]

A painting of a Spanish one

[David K. Joyce]

Hello Russ,
The pyrites are usually very bizarre and there are good reasons for it. It is not a simple concept. Thousands of unusual pyrites have come from the Nanisivik Mine.

The initial iron sulphide that crystallized in very good crystals was marcasite. Mostly, cyclic twins of marcasite. For some reason, almost ALL of the marcasite crystals were subsequently pseudomorphed to pyrite. If you look at the mindat location, you'll see many examples. Most interestingly, though, pyrite continued to crystallize ON the pseudomorphs as oriented overgrowths. The various initial blades of marcasite-turned pyrite have overgrowths oriented to the particular blade. This results in unusual aggregates of pyrite crystals. as very thin to very coarsely crystallized overgrowths. On the edge of the former pseudomorphs, these elongated overgrowths can look like bar growths of pyrite, in isolation.

Have a look at the Nanisivik Mine page of Mindat and you'll see many examples of elongated pyrite overgrowths.

Yours just formed very thick overgrowths and the original pseudomorph that supports the overgrowths is buried deep under the "bars". You can see it best, though in the aggregate on the right of your photo.

There was also a Mineralogical Record article on the subject; Vol: 21, Nov-Dec, 1990. There was also an article on the pyrite, specifically, by Dr. R I. Gait in the Min Record but I'm having a hard time finding it.

I hope this helps?!

[Bob Morgan]

Bob Gait's article is in the July-August 1978 mineralogical Record. It explains well the faces of pyrite but doesn't get into overgrowths and the development of skewed crystal growth.

A lovely and interesting pyrite cluster.

Bob

[Linus]

I recall that the Buick Mine in Missouri was very famous for rectangular pyrites, some up to 12 inchs long! The "Virtual Museum of Geology" website mentions that many of these had holes down the center suggesting that they originated as stalactic growths...

[RussEpp]

Thanks David for the detailed reply and the references. I wasn't aware of the MinRec article and am fortunate to have that issue. The aggregate you were referring to on the right hand of the specimen is actually dark sphalerite crystals. I apologize for the picture, I am still learnig to take pictures of minerals. I examined the specimen very carefully including the sides and bottom and could see no sign of any pseudomorphing. I think this specimen looks more like the elongated pyrite crystals in figure 29 than the pyrite after marcasite in figure 31 in the Nanisivik article in the MinRec Vol: 21, Nov-Dec, 1990.

[lluis]

Dear All

As Linus said, the ones from USA are widely known....
Myself have one and is a really long prism, making the ones from Nanisivik seen short...

Anyway, to me, just a nice curiosity...

With best wishes

LLuís

[David K. Joyce]

I was referring to the furthest right pyrite on the right of the specimen. Figure 29, is, in fact, very thick overgrowths on a former pseudomorph. DKJ
.

[RussEpp]

Thanks all for your responses. I have looked at a number of photos of pyrite bars from the Buick Mine, Missouri and they appear to be quite different. While some of them get quite long, they have a platy look, don't have striations and and least some of them have holes inside suggesting they are stalactic.

[RussEpp]

[David K. Joyce]

I'm sure you have noted that the pyrite crystals at Nanisivik are mostly combinations of various isometric forms which vary in dominance. in figure 29, the cube dominated but you will also see dodecahedral or pyritohedral forms beveling the sides and trapezohedral faces at each corner with a tiny octahedral face. These look more coherent to us because the cube dominates. In many of the other specimens, other isometric forms have more significantly sized faces and the pyrites become less recognizeable. The cubes with smaller additional isometric faces form oriented overgrowths on the original pseudomorphs, as well as any of the other forms when cubes are not dominant.
I doubt if you would ever see the marcasite remnants. They rarely, if ever exist. I've not seen them. The marcasite has been replaced by pyrite and then more pyrite has overgrown on the pseudos in oriented fashion. There should be no trace of the original marcasite blade. For instance in figures 32, 33 or 38, in the Min Record article. If you broke those specimens up, you would find no trace of the original crystals of marcasite. The only time you'll ever see them is if the overgrowths did not completely envelope the original pseudomorph.

[Linus]

Actually there are some interesting thoughts on this at:
https://www.mineral-forum.com/message-board/viewtopic.php?t=2980 of this forum. Look down towards the bottom.

[Linus]

i.e. bottom: Mar 23, 2013 by Pete Richards

[RussEpp]

Thanks David , I understand much better now. I really like the Nanisivik pyrites for their complexity of forms and variety. By the way, I have collected many marcasite clusters from the Stony Mountain quarry just outside of Winnipeg. There are blades that look like spears and twinned crystals that look like Xmas trees. There are other forms as well. I don't know if they are pyrite pseudomorphs or not but they are certainly interesting. When I get better at photographing small items I will photograph some of them . A well-known collector was intrigued by the forms and talked about writing an article about them, but I don't think he ever did.