Talk:Crystal system

Overhaul needed

This article is rather chaotic and contains a lot of information that in my opinion belongs elsewhere. This article should in my opinion concentrate on describing the 7 crystal systems (cubic, tetragonal etc) and explaining the restrictions on the axial system (cubic: a=b=c, α=β=γ=90°, etc). A short explanation on the relationship with Bravais lattice should be given, but there should be a separate article for that subject (the two should not be merged as they are now). Info on crystallographic point groups should be covered in that article, and not here. This will result in a much shorter article, so I want to hear any comments first. If there are none, I'll implement the changes within a couple of days. O. Prytz 18:44, 8 January 2006 (UTC)

Some time ago I have combined several pages into Crystal system, because of the strong relationships, and the convenience of having all this related information in one place. In the two tables the first column is crystal system: these tables provide details about each crystal system (overview of all point groups possible for the crystal system, etc.), so they are not at all off-topic.--Patrick 03:07, 9 January 2006 (UTC)

I see. I'm not sure what the Wikipedia policy on merging is, but it seems to me that articles should not be merged just because they are related. Some mention of the relationship is of course good, but I still believe there should be separate articles, with this one being quite a bit shorter and more to the point. To be specific: the first table (of point groups by crystal system) would be better off in the crystallographic point group. The second table could perhaps be kept here, but the relationship between crystal system and Bravais lattice should be spelled out more clearly. This would, perhaps, cause a significant overlap with a future article on Bravais lattice, but I think it would be good for clarity. O. Prytz 08:12, 9 January 2006 (UTC)

Earlier I made crystallographic point group a redirect, because with Crystal system and Point groups in three dimensions it seems superfluous. That was reverted, and now we have the odd situation that other pages provide better info about crystallographic point groups than that page itself. If that page is kept we can perhaps best put the first table in a template, and call it from there also. For people familiar with point groups the table provides info about the crystal systems, so I like to keep it here also.--Patrick 11:20, 9 January 2006 (UTC)

I agree that crystallographic point group may be somewhat lacking, but I don't agree that for example Point groups in three dimensions gives 'better info'. I understand that you have contributed heavily to that article, and I don't want to offend you, but I'm afraid that article is almost unreadable to anyone who doesn't already know the topic.

My concern is the following: I believe the average reader of the 'point group', 'space group', 'crystal system', 'crystal class' etc articles will (at best) be undergraduates in physics or chemistry or perhaps graduates without previous experience with the subject. I'm sorry to say that any such people getting started in solid state physics/chemistry or crystallography, and wondering what the heck a point group is, won't get any help from the Point groups in three dimensions article. Sorry. For them, the crystallographic point group article would be a better starting point, although it is a bit short.

On the other hand: I think what we've now done with screw axis and glide plane is pretty good. They have a short intro (which perhaps should be expanded) giving the general idea, and then a more formal treatment in a seperate part. The same is the case for space group, although here I'm a bit worried now about the length of the group theory part.

At some point I'll start doing edits here, so we'll have to take disagreements as they come along. O. Prytz 19:31, 9 January 2006 (UTC)

I've started editing the Bravais lattice article. O. Prytz 22:18, 9 January 2006 (UTC)

Being a complete amateur scientist, I am looking for clarity and entry points into a complex topic. To me this whole article is a conversation between specialists, rather than an explanation for the general public. The use of the Greek alphabet, alone, should merit a short explanation. Why are alpha, beta and gamma used for some aspects and a, b, c for others! At least there should be links to these basic concepts and nomenclature. But the biggest complaint that I have is that this is an article about crystals, but there is only a single picture of a crystal! There should be photos of actual three dimensional crystals to go with each of the molecular diagrams. Without these there is no connection to reality. There should also be links to some of the pioneers of crystallography and how and why they were able to unravel these inner structures from the outer appearance! Wikipedia is for all mankind - not just a few mathematics students and physicists! — Preceding unsigned comment added by Dave Lea (talk • contribs) 04:42, 12 July 2020 (UTC)

Overhaul needed

I agree. There are a lot of related pages, and they are somewhat chaotic too. We should try to consolidate all the different pieces of information into a set of concise pages. Also we need to implement the different focus of crystallographers, protein crystallographers and mathematicians. This is why I added the 'enantiomorphic' column to the table of point groups, as protein crystallographers are not interested in centrosymmetric point groups. --Dan|^(talk) 09:59, 31 January 2006 (UTC)

For example, we don't appear to have consistent names for the seven crystal systems. The two different tables in this article use different names. --Dan|^(talk) 10:43, 31 January 2006 (UTC)

Yes, the whole area of crystallography is in shambles... although I think space group is ok now. Would you be interested in starting work on for example point group or crystallographic point group along with me ane any others who are interested? Lots of other articles need work as well... O. Prytz 16:06, 6 February 2006 (UTC)

I just wish I had the time! I am hapy to add things here and there as I can, but we prolly need a 'plan of action' so we can all work to the same tune... --Dan|^(talk) 23:25, 22 February 2006 (UTC)

Crystal system vs. lattice system

As in other wikipedias, this article badly mixes different notions, those of crystal system, lattice system and crystal family. I am mainly working on the French wikipedia, where I had a hard time trying putting a bit of order there. I don't have the time of doing the same here, but you can refer to the IUCr online dictionary of crystallography, see: Crystal system, Lattice system and Crystal family. In particular, rhombohedral and trigonal are not synonyms, the first is a lattice system, the second a crystal system. Mahlerite 17:25, 9 April 2007 (UTC)

This confusion between rhombohedral and trigonal is well established in the English wikipedia. Virtually all trigonal minerals are said to be in the "rhombohedral crystal system". It would be a big job to correct the articles on crystal systems, and all the current links.Judge Nutmeg 01:40, 23 May 2007 (UTC)

A big job, sure, but it must be done! --Mahlerite 11:48, 23 May 2007 (UTC)

Perhaps another point of confusion is that Lattice system redirects to Crystal system; the first sentence (In crystallography, a crystal system or crystal family or lattice system...) seeming to state that they are equivalent, the remainder of the paragraph attempting to explain the differences between these confused terms.Tomásdearg92 (talk) 12:21, 13 November 2010 (UTC)

WikiProject class rating

This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 09:46, 10 November 2007 (UTC)

Monoclinic Wrongly Defined

In all the pages that discuss monoclinic crystals (including Crystal system, Bravais lattice, Monoclinic crystal, etc.), the symmetry of the crystal is wrongly defined. For monoclinic crystals, all angles are 90 degrees, except for BETA (not alpha, as currently indicated). This requires editing all text and Figures depicting monoclinic crystals. Note that, for instance, this mistake does not exist in the french wikipedia http://fr.wikipedia.org/wiki/Monoclinique. Also, you can look at http://webmineral.com/crystal/Monoclinic-Prismatic.shtml to see that beta ≠ 90° is the standard. Wolf.aarons (talk) 17:18, 20 April 2010 (UTC)

Are you sure? Alpha, beta, zeta, What does it matter? The angles do not define the crystal system anyway, it is the symmetry constraints on the cell that do that. A good reference is Glazer's "Space groups for Solid State scientists". I think on about pg 15, this is covered in quite some detail. User A1 (talk) 08:57, 21 April 2010 (UTC)

This is about conventions, not science. Anyway, as I understand, alpha and beta (and thus lattice constants a, b) are interchangeable, but c is traditionally the largest lattice constant (and thus gamma is also selected angle), thus I don't see a problem with interchanged alpha and beta. Materialscientist (talk) 09:04, 21 April 2010 (UTC)

diamond

Diamond crystal structure consists of face-centered cubic lattice.

How long has the caption said that? It's not precisely accurate, I believe. —Tamfang (talk) 07:57, 17 June 2011 (UTC)

looks like it should point to Diamond cubic but clarifying the text is beyond me EdwardLane (talk) 14:37, 22 February 2012 (UTC)

I edited the text, it seems more precise now. Nicola.Manini (talk) 07:36, 29 February 2012 (UTC)

RAD52

This statement:

"For example the Rad52 DNA binding protein has an 11-fold rotational symmetry (in human), however, it must form crystals in one of the 11 enantiomorphic point groups given above."

is severely misleading

RAD52 is a 7-mer in solution (see the original publications of the structure) and therefore most likely also "(in human)" the undecameric configuration of RAD52 is considered to be an artifact of cleaving a major fraction of the c-terminal protein regions which was necessary in order to obtain crystals. RAD52 is a heptamer in analytical ultracentrifugation and shifts to undecamer when the c-terminus is cut off. The group that solved the structure mentioned that. Also, the EM structure of RAD52 is a heptamer with rotational and translational symmetry.

Kagawa W 2002, Mol.Cell, PMID:12191481 Stasiak AZ 2000, Curr, Biol., PMID:10744977

A better example would maybe be RecA which has a ~6.1 fold symmetry when bound to DNA but has to crystallize as perfect hexamer. All RecA crystal structures are in P6(1) and not in a configuration that allows for DNA binding. The RecA-DNA structure from Pavletich's lab was composed of fused protomers that crystallized as one building block in a tetragonal spacegroup. The fused protomers have an internal ~6.1 symmetry in the asymmetric unit cell, as they should.

Muckbacher (talk) 01:16, 23 August 2012 (UTC)

Polar?

After the table with Crystal Classes, an explanation is attempted of the "Point symmetry" classification.

It looks mostly fine except for the part "If rotation of the original lattice reveals an axis where the two ends are different, then the crystal is polar. H2O is a common example of a polar molecule." What does the polarity of a molecule have to to with the mathematical properties of Point symmetry of a crystal system? Also the link of polar in the table to the chemical polarity of molecules is at best misleading. In the present article "polar" should be a property of the crystal symmetry, with little or no connection with the chemical properties of whatever is composing the crystal. We need a satisfactory definition of "polar crystal symmetry" somewhere, possibly in this page or in a specific one.

Nicola.Manini (talk) 16:57, 5 March 2013 (UTC)

I've tried to clarify the part about polar crystals. I'm a physicist: what is written now makes sense, and it is far better than what was there before. I'm not a crystallographer: there may be better ways to put it. The link of polar is still to be fixed, I still cannot tell where it should point to. Nicola.Manini (talk) 11:45, 14 March 2013 (UTC)

I like definition from "International Tables for Crystallography", vol A, page 804: "A direction is called polar if its two directional senses are geometrically or physically different. A polar symmetry direction of a crystal is called a polar axis." Also I substituted 'lattice' to 'structure'. Lattice always possess inversion center. Bor75 (talk) 23:01, 3 April 2013 (UTC)

Nicola.Manini, I created a new article Polar point group, which provides a geometric, rather than a chemical definition. Wikfr (talk) 02:25, 10 December 2013 (UTC)

Bor75, thank you! I added appropriate hyperlinks from the present Crystal system page. Nicola.Manini (talk) 00:57, 18 December 2013 (UTC)

Symmetries of proteins

I removed paragraph

The protein assemblies themselves may have symmetries other than those given above, because they are not intrinsically restricted by the Crystallographic restriction theorem. For example the Rad52 DNA binding protein has an 11-fold rotational symmetry (in human), however, it must form crystals in one of the 11 enantiomorphic point groups given above.

This information is not related to this article, it is better to mention about it in Crystal structure or Crystallographic point group, for example. Second, this information is related not only to "protein assemblies", but to any molecule. For example, Ferrocene possess 5-fold axis, but crystallizes in the monoclinic space group. Bor75 (talk) 00:21, 4 April 2013 (UTC)

Group structure

The section Crystal classes has a table with a column called Group structure, which I think is inconsistant or wrong. Dihedral and 2xdihedral seem to be used interchangably to mean groups of order 8, 12, and 24. I am also wondering why 2xcyclic is shown, instead of dihedral.

The notation seems non-standard. Does 2xdihedral mean the product of cycle group of order 2 and the dihedral group of unspecified order, or two copies of the dihedral group? What does 2xcyclic mean?

I would like to change the column heading from "Group structure" to "Abstract group".

Wikfr (talk) 00:32, 14 November 2013 (UTC)

As I understand (I just checking this column after your comment), 2xdihedral mean the direct product of cycle group of order 2 and the dihedral group of unspecified order. The same for 2xcyclic = direct product of cycle group of order 2 and cyclic group. In all cases this "cycle group of order 2" is perpendicular, horizontal plane. Or inversion center. Doesn't matter in these particular cases.

In case of 2×alternating and 2×symmetric this would be inversion center.

Bor75 (talk) 02:07, 14 November 2013 (UTC)

You made a big improvement to the page. You did what I had been trying to do. Thank you. Wikfr (talk) 23:45, 14 November 2013 (UTC)

Thank you!

Wikfr, I don't agree with your last correction about polar groups and directions. This first sentence, you corrected, were taken from International Tables for Crystallography. After your correction it is wrong. "A point group is called polar if two directions are geometrically or physically different." is incorrect. For example, take 2/m group. Take one direction along 2-fold axis, and the second one - in plane. Obviously, they are different, but group is not polar, and even centrosymmetric. To define polar group, first polar direction should be defined "A direction is called polar if its two directional senses are geometrically or physically different." Bor75 (talk) 00:12, 18 November 2013 (UTC)

Bor75, until I have more information, I reverted my change. Wikfr (talk) 21:45, 5 December 2013 (UTC)

I am not sure, but is this proper english?

It's below the first table: Caution: No "trigonal" lattice system. To avoid confusion of terminology, not use the term "trigonal lattice"; or use the definition that "trigonal lattice"="hexagonal lattice"≠"rhombohedral lattice". — Preceding unsigned comment added by 2001:4CA0:4FFF:1:0:0:0:123 (talk) 13:35, 8 May 2015 (UTC)

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2D confusion

In other dimensions, the first section briefly discussed 2D crystal structures:

"Two dimensional space has the same number of crystal systems, crystal families, and lattice systems. In 2D space, there are four crystal systems: oblique, rectangular, square, and hexagonal."

The first sentence originally reads as if there are the same number of crystal systems (and etc) as in the 3D space (7). The second sentence clarifying the four categories disputes this. I believe the first sentence is trying to say that there are four of each or rather systems=families=lattices all with the same four names. That makes sense given the limits of rotational symmetry in 2D, but I'm not confident in my understanding. If this is what was meant, could we rewrite the passage to make that clearer? I can make subtle adjustments like below, but I still think it needs something else.

"In 2D space, there are four crystal systems: oblique, rectangular, square, and hexagonal. Two dimensional space has an equal number of crystal systems, crystal families, and lattice systems." AniKitt (talk) 22:33, 4 May 2024 (UTC)