Talk:Trigonal crystal system
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I concure with the contributer below. the title is confusing. I looked yp this article in reference to another article (Ettringite) while researching problems with concrete. instead of a clear susinct article on Trigonal it went off and spoke at large about Rombohedral crystal systems. Importantly without really giving a clear definition of the term Trigonal. My point of view is as an engineer not someone interested in crystals so really anything beyond a definition of the term Trigonal is wasted on me.
My reference book (a Peterson field guide) lists the same name for the 3 and 3bar classes, which I think must be a typo. But I put it in anyway, rather than just take a guess based on the names of the triclinic classes, as I did at first. If anyone knows better, please fix it. Tantalate 19:46, 21 May 2004 (UTC)
I would like to rename this and other articles so that all of the articles describing the seven crystal systems are named the same way. For example, I would like to rename this article Rombohedral crystal system. That way it woun't be confued with the general geometric shape. Any objections? O. Prytz 17:39, 5 June 2006 (UTC)
The question as to merging the trigonal system in with hexagonal system is one perpetual confusing question in crystallography. Some people certainly class the trigonal system as a kind of subset of the hexagonal.
However, at the moment, there is an omission in the coverage of the crystal systems: within the trigonal system there are both primitive (P-centered) and rhombohedral (R-centered) lattices. At the moment a search for "trigonal" redirects you to "rhombohedral". This latter entry has the statement 'There exists only one rhombohedral Bravais lattice.' True, but there is a primitive trigonal Bravais lattice, from which many space groups like P3 exist. These are not mentioned at all (so far as I can see) in the current coverage on crystal systems. In reality rhombohedral lattices are one component of the trigonal system, which consists of P- and R-centered lattices. (See for example http://cst-www.nrl.navy.mil/lattice/spcgrp/trigonal.html). The French entry on crystal systems would be good to translate and merge here (http://fr.wikipedia.org/wiki/Système_cristallin#Classification_morphologique_:_les_syst.C3.A8mes_cristallins). The latter contains a note on the confusion "Trigonal versus rhomboédrique", although I am not sure it is totally correct either.Octopodes 04:50, 11 January 2007 (UTC)
- Rhombohedral is not a crystal system, it is a lattice system and is defined by the symmetry of the lattice. Trigonal is a crystal system and is related to the morphological symmetry. A trigonal crystal may have either a rhombohedral or a hexagonal lattice. The two concepts are basically different and must not be confused. --Mahlerite 15:29, 31 October 2007 (UTC)
- The picture given is false. All angles alpha, beta and gamma should be equal. If you look at the space of quadratic form allowed by the rhombohedral symmetry, you will see that it has dimension 2. Here alpha, beta and gamma are not equal and the parameter a is allowed to vary. This implies a space of allowed quadratic form of dimension 4 which is impossible. —Preceding unsigned comment added by
This article doesn't seem to be helpful because it doesn't define "Rhombohedral lattice system" or "Trigonal crystal system".
It just gives vague characteristics. The problem seems endemic on crystal system/lattice/point group/etc articles. For example, is a "rhombohedral lattice system" defined as any set of points with equal length non-orthogonal translation vectors? Or does it just happen to have that property? Is it the only lattice system with that property?
What is the difference between a "rhombohedral lattice system" and a "trigonal crystal system"? Apparently there is one because this article berates the reader for not knowing the difference and then doesn't explain it.
For that matter, what is the definition of a crystal system as opposed to a lattice system? All of the articles either don't have definitions or the definitions refer to articles that refer back to the first article.