# 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