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: [log in to unmask]
> [mailto:[log in to unmask]] On Behalf Of Jacob Keller
> Sent: 16 November 2007 19:49
> To: [log in to unmask]
> 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: [log in to unmask]
> *******************************************
>
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