# What salts are/are not birefringent?

Is there a quick way to determine if a salt (or ionic compound in general) crystal should be birefringent from its formula without having to know/look up the crystal structure? When I'm looking for protein crystals, if I know the condition has a lot of salt in it, but it shouldn't be birefringent (e.g. NaCl), it's a hint that birefringent crystals I do see might be protein.

Is the only non-birefringent crystal type (e.g. not glasses) cubic? Does it matter if it's primitive, face-centered, or body-centered?

Back to looking for a generalization, are most (all?) salts with polyatomic ions birefringent? All +1/-1 (NaCl, KBr, etc...) aren't?

• Why didn't You read the wikipedia article on birefringerence? en.wikipedia.org/wiki/Birefringence – Georg Jul 19 '12 at 19:22
• @Georg Is there something in the article that you'd like to point out in a more constructive fashion without being accusatory? – jonsca Jul 19 '12 at 23:05
• @Georg I have absolutely no problem with you mentioning Wikipedia. It's when you say why didn't you read. Instead of addressing a specific issue with the OP, you are accusing them of ignorance without any basis. We have asked you repeatedly to treat other users with respect, so I again echo that sentiment. – jonsca Jul 20 '12 at 13:51

## 1 Answer

Yes, the only crystal system that is guaranteed not to be birefringent is cubic, because all of the other crystal systems have at least one axis distinct from the others. What matters is the symmetry not the centring, so this is equally true for face-centred or body-centred as for primitive crystals.

Sadly, however, there is no way to determine the crystal structure, or even the crystal system, simply from consideration of the formula - indeed, crystal structure prediction and prediction of polymorphism are active research fields. Counterexamples to your first generalisation include ammonium chloride, bromide, and iodide; to your second include ammonium fluoride and silver iodide.

• I probably should have said alkali halides instead of "+1/-1". Do you have a good resource that lists all the systems for the common and somewhat common compounds? WP is somewhat spotty. – Nick T Jul 20 '12 at 15:21
• With alkali halides you are probably on safer ground! For really common compounds try a book like Greenwood and Earnshaw Chemistry of the Elements. For less common crystal structures the standard resources are the Cambridge Crystallographic Database (mostly organic) and Inorganic Crystal Structure Database, both of which are subscription-based services, but the Crystallography Open Database (crystallography.net) and American Mineralogist database (rruff.geo.arizona.edu/AMS/amcsd.php) are both free. – Aant Jul 20 '12 at 16:44