It is well known that pyrite shows striations along . Although cubic but according to the point group symmetry m-3 the striation is commonly only visible along one of the on {100} or {210}. Invisible on {100} but on {210} is a change of striation which is called positive or negative. The positive is assumed to be || , the negative is perpendicular to , i.e., ||.
Is it possible that in the last case {210} is actually {120} and the striation is always || ? Is there any experimental confirmation known which shows that the formed dodecahedron plane is definitely the {210} or not perhaps in rare cases {120}?
Not exactly an answer but depending on the formation environment there are some indications of the existence of pseudo-cubic pyrite. http://rruff.info/doclib/am/vol62/AM62_1168.pdf
Regards
Dominique
I find that both forms can occur even if {210} is the commonest. As reference I have only this article in Italian http://www.treccani.it/enciclopedia/pirite_(Enciclopedia-Italiana)/
Dear G. Ferraris, how did they discriminated {210} from {120}? Is there something useful mentioned?
Gert, in morphology of cubic crystals the so called direct and reverse forms can be interchanged, depending on the choice of the reference system. In pyrite the {210} and {120} forms are differently developed; in presence of the two forms the largest one is conventionally indexed {210}; the same if only one form occurs (usually). I have not the book at home, but likely you can find more in F.C. Phillips An Introduction to Crystallography.
Dear Gert, under my opinion, fistly you should take a look at the classic " Short Course Notes on Sulfide Mineralogy, pp. 1-18, which I am attaching. Secondly a method you can use is to take oriented samples with coarse-grained pyrite from the field. You can determine whatever the pyrite is 210 or 120 by comparing the real tectonic trends with the striations over pyrite faces. Please see the attacted.
Hope I have helped.
Take care
Stellios
Gert, it might be useful taking into account that sometimes the two forms coexist and for structural reasons (see e.g. Donnay-Harker theory) the most developed one is always {210}. In case of coexistence it should be easy to compare the striation directions.
Dear Giovanni, this would be perfect however I havn't seen any pyrite which shows both at the same time AND striations. However, if nobody else has any idea I will try to get both pentagondodecahedra and simply measure them. I only want to prevent to do something again which is perhaps obvious. Edo et al only compared the different topography and as far as i understood it, they did not take into account that the visibly pentagondodecahedra could be of different indexing. This must not be the case, but I would like to test whether the striations really change their directions or the habit surface is simply another one. If I compare different pyrite habit planes and their visible striation it seems to be that the latter is more probable.
Dear Gert,
it is plain that {210} and {120} forms in pyrite cannot be symmetry equivalent, since the diagonal symmetry planes do no exist. This implies that the corresponding surface structures are surely very different: in turn, the related surface energies, too.
As a consequence, {210} can belong to the equilibrium shape (E.S.) of the crystal, while the surface energy of {120} is problably to high in order to enter the E.S. Finally, if a given {hkl} crystal form does not belong to the E.S., only adsorption phenomena should reduce its rate during growth and then it hardly should appear in the Growth Shape (G.S.) of the crystal.
Donnay- Harker rule cannot be effective in order to distinguish the morphological importance of {210} and {120} forms in pyrite, since this rule is only grounded on the lattice geometry. As a matter of fact, the thicknesses of the elementary layers parallel to these forms, are the same, I suppose...
I investigated the growth morphology of pyrite in 1973... but I published the paper in an Italian Journal. I'll translate the text in English and send you as soon as possible.
All the best !
Dino
Thanks, Dino. This would be a great help. I will try to get {210} with different striations so that we can compare them.
Dear Gert, two days ago I attached to my answer a huge article by J, Struever reporting the morphological analysis of 5603 crystal of pyrite. For some reasons I do not find here: I send it to you by email [email protected]
Dear Gert,
The {210} and {120} forms are seemingly twin related by the iron-cross law. This twinned crystals are abundant in special conditions of formation.
A nice paper on this issue appears soon in the Min.Mag.
see at http://www.ingentaconnect.com/content/minsoc/mag/pre-prints/content-minmag-1255
regards, Istvan dodony
Dear Istvan, I've seen this already but as far as I understand it, it is related to penetration-twin formation only and does not discuss a possible growth of {120} instead of {210} which would end in a "negative" striation (hypothetic). Nevertheless, thanks for giving this link which underlines that many things are not yet finally clear, even for minerals like pyrite.
Dear Gert,
I already tried to give an answer to your interesting question, but didn't found my arguments on Research gate. So I'll try again.
Let's consider a projection of Pyrite structure along the [001] axis. One can find the surface profiles of {210} and {120} forms and, at the same time, the corresponding slices of thickness d210 and d120. It is plane that the surface profiles are very different while the slice thicknesses are obviously the same.
Two consequences:
- The Donnay-Harker-Bravais-Friedel rule cannot explain the growth difference between these two non-equivalent forms, since this rule is based on the thickness of the slices (pure geometry). On the contrary, Hartman-Perdok theory explain very well this interesting behaviour, because it is based on the character of the forms (and then on their attachment energy= kinetics of growth)
- The surface energy of {210} and {120} forms is necessarily different, since they are not symmetry related. Surely, one is high and the other one is low; as a consequence they cannot belong (together) to the equilibrium shape of pyrite. Lastly, if a crystal form does not belong to the equilibrium shape, hardly it will appear on the growth shape...
All the best !
Dino
Dear Dino,
I totally agree with you. I've checked the same at the beginning and investigated the differences of both planes.
As far as I know both forms do not appear together. They are different! However, you know much better know than me that with a little uric acid NaCl forms octahedra and no cubes anymore, i.e., the conditions around finally decide about the equilibrium shape. In fact this is from my point of view the reason why different forms appear in nature. The negative striation is very rare so that it might be that only under very rare consitions {120} is formed and not {210}. I have to admit that I am also not very "hopeful" since the {210} and the {120} are really different, but i will see what the experiment will show. I am expecting two samples with different striation and then it needs only a short time until I know it.
Many thanks for your interest.
Gert
Dear Gert,
thanks for your appreciation. I would like to send you a historical paper on the striations of Pyrite, hoping this will be new for you.
All the best !
Dino
Dear Dino,
I already had this paper, Nevertheless, many thanks.
We investigated the negative and positive striations and it turned out that the planes are definitively {210} and not {120}. Unfortunately the two pentagondodecahedra did not have the same size nor the same amount of striation, but it seems that both striations have a different nature. The negative striations are less "perfect" and their profile looks more rounded whereas the positive striations showed nice cristallographic planes. Since we had to polish the surface gently using silica only we cannot exclude a rounding during the polishing.
Thanks for your support!
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