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

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Tue, 02 Oct 2007 14:06:52 -0400
Message-ID: <OFCAF49421.16BA3CBE-ON85257368.006399EE_at_usgs.gov>

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
Received on Tue 02 Oct 2007 02:06:52 PM PDT