[meteorite-list] BO - Barred Olivine Chondrule: RFS Picture of the Day

[Jeff Grossman]

For those of you who don't know what you're looking at in this 
picture, here is a little explanation.

All of the colored bars and the circular rim in this picture are the 
mineral olivine.  The black stuff in between the bars is either 
feldspathic glass (possible if this is a highly unequilibrated 
chondrite) or microcrystalline material that probably was once 
glassy.  The image is taken with two polarizers, one below the thin 
section and another above it, with the two polarizers rotated 90 
degrees to each other (petrographers call this crossed polarizers).

Some minerals are isotropic and some are anisotropic.  Isotropic 
minerals have the same optical properties in all directions.  If you 
put a thin section of such a mineral between crossed polars, it will 
look black.  The glass in chondrules is isotropic, so it looks black 
in this photo.  However olivine is anisotropic: its index of 
refraction is not the same in every direction.  When viewed between 
crossed polarizers, interference colors are seen.  This property is 
called birefringence. Olivine has a relatively high birefringence, 
which is why it appears to have gaudy colors in photos of standard 
thin sections like this, compared to minerals like pyroxene or 
feldspar, which are much less birefringent (but not isotropic) and 
would appear white to gray.

The exact color of an individual grain depends on several 
factors.  One is the thickness of the section.  A change of a few 
micrometers in thickness could give the effect seen in this 
photo.  Such a large change over the distance of a few hundred 
micrometers would indicate this a really badly made thin section, and 
it would be obvious to the owner.  I assume it is not this.  The 
birefringence of olivine is also a weak function of composition; it 
would take a large Fe-Mg gradient to give you an effect like 
this.  This is almost certainly not the case.  Zoning from 
side-to-side in chondrules is basically unknown in chondrites; it is 
almost always radial.  The other, and almost certainly the correct 
explanation for the color change is that the orientation of the 
crystal changes slightly across the chondrule.  A small amount of 
deformation, perhaps due to light shock, or perhaps due to the way 
the olivine crystallized, could easily cause this effect.  The highly 
fractured nature of the olivine (see all the little transverse 
cracks), is consistent with shock.  The deformation may also have 
taken place during production of the thin section, if the section 
buckled a tiny bit.

jeff

At 12:36 PM 10/2/2007, bernd.pauli at paulinet.de wrote:
>Hello again,
>
>I just got mail from Marc Fries. Thank you, Marc! Very much
>appreciated. Now, Marc prefers option #3 and so he writes:
>
>"I was thinking option 3), myself. It only takes a thickness variation
>  on the order of 100 nanometers or so to get that color gradient, and
>  if it were chemical I'd expect a change in the rim vs. the interior rather
>  than an uniform gradient across the chondrule."
>
>Rather convincing! Why should the chemical composition within a single
>BO chondrule change *gradually*... especially in view of the fact that the
>bars are oriented identically!
>
>Best wishes,
>
>Bernd
>
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Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA