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[LDMarks]

I am looking for information about the conditions under which pyrite forms pyritohedral or icosahedral single crystals. The first is when {210} facets are favored; the second is when both {210} and {111} occur. Based upon some calculations it looks to me like the relevant conditions are either H₂S rich or S rich.

Is it known which is relevant, i.e. bacteria versus lava?

[Sante Celiberti]

Hello, LDMarks.

I'll answer your question with another question.
How is it possible that the same geological environment produces pyrite crystals with so many different habits and combinations of them?
I found these crystals, immersed in greenish clay, in a space of a few meters (let's say a radius of 2,5 meters).
If the greater or lesser richness of S is responsible for the habit, why in such a small space it is absorbed so differently?
I'm a layman in mineralogy, but I'm tempted to hypothesize a different explanation of the phenomenon such as, for example, a disturbance in electrical balance of the crystal nucleus.
But, most likely, my guess is nonsense and I'm completely off track.

Warm greetings from Tuscany.
Sante

[R Saunders]

I have a specimen with pyrite tubes. Do you know what causes that?

[Carles Millan]

Dear Sante

By far I am not an expert on that subject. There are other forum members who could write great responses.

There may be different habits of a species in the same deposit because the conditions of each single crystal were not exactly the same: the precise location, the moment in which the crystal grew, the temperature, the pressure, the chemical composition of the environment, among many others. Also, some crystals could have grown in a matter of hours, and others in years, even in thousands of them. So many variations can give rise to a lot of habits.

I hope someone else will provide more details.

[Sante Celiberti]

Dear Carles,

I fully agree with each of your words.
My question was motivated by the very limited area of discovery, which allows us to reasonably suppose that the many variants that concur to determine a habit were almost uniform for all my crystals.
The reason for my intervention was precisely to solicit the answers of the expert members of the Forum from whom, in my elementary mineralogical knowledge, I always learn with pleasure.

Receive my warmest greetings.
Sante

[Sante Celiberti]

[Bob Carnein]

Some people at Penn State did experimental work on environment and pyrite habit. I don't know that it helps with the varied habits in a small area, but here's a reference:

Murowchick, J., and H.L. Barnes, 1987, Effects of temperature and degree of supersaturation on pyrite morphology: Am. Mineralogist, v. 72, p. 1214-1250.

[Sante Celiberti]

Thank you, Bob, for the tip.

It sounds very interesting.
I will look for this article of the prestigious Journal at the geology faculty of the university of Pisa.

Best regards from Tuscany.
Sante

[Bob Morgan]

Sante,
Pyrite is my first love and I've had much the same questions about morphology as you raise.
It's a complicated subject with many variables to consider. The American Mineralogist article by Murowchick and Barnes has a thorough review of previous studies and works on two variables - temperature and degree of supersaturation. It appears that the pyritohedral form results from higher degree of supersaturation and higher temperature than cubes.
A study of Spanish pyrites from the Comeros Basin (Navajun cubes, Ambas-Aguas pyritohedra, etc.) found temperature not to be a factor, but probably greater availability of sedimentary sulphur and particularly S34 that may have made for "higher degrees of pyrite saturation in fluids" resulting in pyritohedra.
But I'm not a scientist, particularly in chemistry. I look at pyrite crystals to see what can be observed.
I pulled out my pyrites from Elba and found much of what I can see in your photos.
The striations on the pyritohedral faces run perpendicular to the the way striations tend to run from other localities. That is the 'diploid effect' on the pyritohedral faces vs the cube -pyritohedral striations on most pyritohedrons.
In both cases the main growth of those crystals is from layers on pyritohedral faces - not cube layers. Where the cube form would be is where the pyritohedral layers grew out rapidly leaving no accumulated edges where cube faces would be.
On the Rio Marina pyritohedra the growth layers have elongated edges in the zone of the diploid. The layers grew rapidly in the cube-pyritohedral direction but spread slowly to the sides in the diploid zone.
There are pyritohedral faces on dominant cubes that are smooth, with the cube areas completely striated. Those areas are the edges of pyritohedral layers. Usually those layers do not show diploid edges on the pyritohedral faces. Those layers grew completely out to the sides. Indeed, the pyritohedral faces tend to have disturbed blocky areas in their centers as if layers developed on two sides of those faces and did not align where they met. Having growth development out toward the edges of these faces indicates a tendency toward skeletal growth.
There are some cubes from Elbe that grew cube layers. They too show the dipliod effect. they have elongated sides - edges toward the pyritohedral form, and they have two faces making an acute point on each end that are in the dipliod directions.
Then there's the other main form - the octahedral. Many of the octahedral faces are flat and seem to have no relationship to the growth of pyritohedral faces. Some use the term of them being 'poisoned', that is, stopped from developing layers. That appears to be the case with most of the octahedral faces in your photos.
But there is one with striated diploid faces. My specimens from there show this as layer stacking of octahedral layers - clearly not poisoned. The diploid striations are edges of octahedral faces that did not spread completely. they are stair stepping octahedral layers.
On some more signature pyrites from Elbe there are smooth diploid bevels around octahedral faces. I take them to be well aligned edges of octahedral layers into organized faces.
This is a long way of saying that there does not have to be too different crystalizing environments to produce the variety you found in such close proximity. The question I have is what causes the diploid effect so prominent at Elba, when it's the trisoctahedral effect at Huanzala, Peru.

[Sante Celiberti]

Hello, Bob. Thank you very much for your kind answer.

I know your passion for pyrite and I envy your ability to interpret apparently insignificant signs in a crystal.

I read in your answer that studies seem to confirm that a greater availability of sulfur tends to favor the pyritohedral habit.
Therefore, the thesis of MDMarks, who started this thread, has a foundation.

As for your last question, about what causes a greater diploid tendency in the Elban pyrite versus a greater trisoctahedral tendency in Huanzala pyrite, I don't know.
The most evident distinctive element of Elban pyrite is that it is normally associated with hematite, but I completely ignore whether the latter can have any influence on the determination of the pyrite morphology.

And now, a question more relevant to the specific theme.
In the conclusion of your very interesting answer you rightly state that even a quite uniform crystalizing environment can give rise to a great variety of habits in a close proximity.
Now, I come to understand how, even in the same specimen, there can be different modifications of a single habit.
What, on the other hand, I cannot understand is how, in the light of the previous considerations, two different habits belonging to a single generation can appear in the same sample.
How do the variables that contribute to determining the crystallographic morphology act in this case?

Thank you for your help.
My warmest greetings from Tuscany.

Sante

[alfredo]

Not about pyrite, but I have a very similar old study about galena crystal habits. The authors concluded that octahedral galenas formed at higher temperatures and that lower temperatures resulted in more cubic crystals. They also found that higher temperature crystals incorporated more extraneous elements (like Sb) than lower temp crystals did.

[LDMarks]

Some useful information in these responses, thanks everyone.

To clarify a couple of points [1] as someone who has worked on this a bit, it is very possible to have a population of different crystal shapes involving twins. This would occur because they nucleate in a similar environment, and the energy difference between them is small. They then grow on these frozen twin-nuclei. Sometimes twinning occurs via two crystals growing in to each other, then the two will rotate until the boundary between them is small.

However, this only works for different twinned or polycrystalline particles. If one has a cube versus, for instance, a pyritrahedron then this means that the conditions when either they grew or in which they sat, slowly changing shape (over eons) were different. There is definite evidence from ab-initio calculations that in some fashion the pyritrahedron and pseudo-icosahedron are linked to the presence of Sulphur during growth or equilibrium.

In an ideal world it would be known that one gets pyritehedra grown in XYZ, cubes grown in ABC. I have not seen clean papers definitively proving this, I think few people want to grow with H₂S atmospheres (not surprisingly). To my knowledge nobody has grown large pyritehedra/icosahedra.

Is there any evidence in terms of where crystals come from that would be informative?

[1] See DOI: 10.1088/0953-8984/28/5/053001 and references therein for a lot of technical details, although it does not have everything related to growth of multiple shapes/twins.

[Bob Morgan]

It's taken a while to get back to you. I needed some time to look at your photo's and think.
Here are the few thoughts:
1) All the crystals show recent growth by stacking cube layers. The 'pyritohedral' crystal has such layers as steps. They are the ones flashing light. In between are what appear to be pyritohedral surfaces. If they were aligned they would make a pyritohedral face, but without alignment the overall slope of the face will not match that of any Miller Index.
The small crystal below it also has cube layers with edges that step down in the pyritohedral directions.
The other larger crystals appear to have multiple slivers of cube growth layer stacks with stepped edges in the pyritohedral zone. There also appears to have been some natural dissolution blurring the sharpness that would make analyzing the situation clearer.
2) Probably the main difference between the two habits of crystals is that the flashy one does not have multiple growth centers, so that it's layers cover the whole expanse of that portion of the crystal.
3) The edges of the layers showing flash are bounded by pyritohedral edges in the long direction, by pairs of what appear to be trapezohedral edges on both ends. If they make a right angle where they meet they are trapezohedral, If they make an acute angle they are diploid. These layers failed to spread in either direction out to adjacent crystal faces. But the cubes with multiple centers did. Also at the corners of that crystal face are small faces indicating further failure to spread. Perhaps that indicates more enriched mineralizing solution toward the cubes.
I realize these thoughts are not part of recent orthodoxy and concern among crystallographers but they do keep me interested in pyrite and other crystal morphology, and particularly striations as edges to layers that are the midpoint between structure and morphology.
Perhaps some day we'll be able to explain all this, at which point I'll no longer need to have my collection to study and enjoy.
It's a real pleasure to have this discussion with someone raising such similar questions and loves the variety of shapes of pyrite.

[Sante Celiberti]

Hello, Bob.

Thank you for your very interesting information which helps me to better understand the structure and morphology of pyrite.
Here are two samples, from the Gavorrano mine, showing interrupted growth.
I do not know how to add more, but I suppose that there are a lot of considerations about the growth of crystals.

I am sure you will never stop enjoying your collection.
Warm greetings from Gavorrano.
Sante

[Bob Morgan]

I sure am enjoying yours.
In the first of this last two the irregular striations - not so straight -indicates some natural dissolution took place.
The growth pits on the pyritohedral face of the other was probably from cube layers notching, leaving a triangular base and the aligned edges of subsequent layers making the other two sides of the pits. There is so much going on, with layers nucleating and stacking so much faster than their spread. Interesting indeed!
Thank you for sharing them.

[Sante Celiberti]

Thank you, Bob, for your prompt and thorough reply.

Warm greetings.
Sante

[Sante Celiberti]

Hello, LDMarks.

Returning to the original theme of your first post, perhaps this can be of some interest to you.

The Niccioleta pyrite mine is very rich in sulfur and has produced very interesting samples of this element, often associated with calcite and quartz, but above all with gypsum and pyrite.
I do not rule out that there may be other pyrite mines that also produce native sulfur, but I am not aware of them.
Although Niccioleta has produced pyrite samples with pentagonal-dodecahedral habit and combinations of cube and pyritohedron, it is renowned for its cubic crystals that sometimes exceed 20 cm.

Here are the reflections and doubts of a simple amateur.

I have found that here the sulfur is always associated with cubic pyrite.
The selective deposition of sulfur on cubic crystals, which at first sight seems to contradict the thesis of the influence of sulfur on the crystal morphology, could actually be a confirmation of this thesis if we assume that the cubic crystals of pyrite, less saturated with sulfur compared to the pyritohedra, attract it more easily.

There remains a basic question.
It is true that the native sulfur that accompanies these samples is of a new generation and that it has nothing to do with the morphology of the underlying pyrite.
But, if a greater availability of sulfur tends to favor a pyritohedral habit, why did the Niccioleta mine produce a much greater quantity of cubic pyrite?
The possibility that the geological environment has changed radically from the first generation of pyrite to the second generation of sulfur, although plausible, seems to me a little exhaustive explanation.
What do you think?

Warm greetings from Gavorrano.
Sante