Hi Jacob, birefringence, the variation in refractive index depending on the direction of the plane of polarisation of the incident light has nothing to do with chirality, it's a consequence of anisotropy of the refractive index tensor, thus a crystal in any crystal system other than cubic may be birefringent, whereas a crystal in any system may be chiral but only if the space group is enantiomorphic. Chirality produces the effect observed in a polarimeter, i.e. rotation of the plane of polarised light, and also circular dichroism, the differential absorption of left- & right-handed circularly polarised light. In all trigonal, rhombohedral, tetragonal & hexagonal point groups one eigenvalue of the refractive index tensor in the unique axis direction is different from the other two and you get uniaxial birefringence; in that case if you happen to be looking down the unique axis the crystal appears isotropic in projection and you don't see birefringence. In all triclinic, monoclinic & orthorhombic point groups all 3 eigenvlaues are different and you get biaxial birefringence, and you may see birefringence in any direction. Of course whether birefringence is actually observable will depend on the degree of anisotropy.
Cheers -- Ian > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Jacob Keller > Sent: 16 November 2007 19:49 > To: [email protected] > Subject: Malonate Crystals? > > Dear Crystallographers, > > I have just found some crystals in a 1 year old screen, and, > having been repeatedly trained to be very skeptical, think > that probably they are salt. So the question: has anybody > seen malonate crystals before? I was wondering whether they > could possibly be birefringent, given that the molecular > structure is not chiral. The crystals I see are very > birefringent. None of the other ingredients is chiral either. > In general, my understanding had been that the birefringence > was due to the chirality of the molecules/crystals...is this > misguided? > > Jacob > > ******************************************* > Jacob Pearson Keller > Northwestern University > Medical Scientist Training Program > Dallos Laboratory > F. Searle 1-240 > 2240 Campus Drive > Evanston IL 60208 > lab: 847.467.4049 > cel: 773.608.9185 > email: [EMAIL PROTECTED] > ******************************************* > Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [EMAIL PROTECTED] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof. Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, Cambridge CB4 0QA under number 3751674
