Walter,
Thank you for the question. You are familiar with a lot of this but let me
go over it once quickly.
A thin section is a slice of rock attached to a glass slide. The sample is
ground and polished flat and to a uniform thickness. The standard thickness
is 0.03mm. Various optical and other tests may be done on it in this form.
Today's Picture of the Day is of a thin section of Andi Gren's The Needle
chondrite, named for the long stringers of metal in it. The slide was
photographed under a microscope in cross polarized transmitted light. That
is, light from below passed through a linear polarizing filter (these things
have orientation) then through the thin section then through another
polarizing filter set ninety degrees to the other, up through the microscope
and into the camera.
The picture is of a portion of a barred olivine chondrule. Chondrules are
generally spherical meteorite components of debated origin. When they were
formed they were partially or wholly molten. Some show evidence of having
gone through multiple stages of accretion, melting, breaking, joining and
thermal and aqueous alteration.
Barred olivine chondrules are believed to have been fully molten and rapidly
cooled. On cooling the olivine in simple BO chondrules, like this one,
formed a single large skeletal crystal inside the solidified spherical
droplet and included the shell of the chondrule. The internal skeletal
crystal is a set of parallel plates, shaped rather like the flat tubing in
radiators that carry steam or water. When we slice through one of these
spheres the cut plates appear as bars, the vertical pieces Walter mentions.
The material between the bars is material sequestered while the olivine
organized itself. It is feldspathic in composition and begins in a glassy
state. With heat it becomes cloudy and even crystalline as its atoms become
organized.
The color gradation from left to right is probably due to a very slight
change in thickness of the sample as Bernd and Marc say. It wouldn't take
much. Would anyone out there consider that it could be from a slight change
in the orientation of portions of the crystal across its width? This is a
big ol' thing. Just the portion pictured is probably over three millimeters
across.
All the best,
- John
John Kashuba
Ontario, California
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Walter
Branch
Sent: Tuesday, October 02, 2007 5:44 AM
To: meteorite-list@meteoritecentral.com
Subject: Re: [meteorite-list] Rocks From Space Picture of the Day - October
2, 2007
Thanks Michael,
And thank-you Andi and John.
Okay, I admit I know nothing about thin sections. Someone educate me.
What are the vertical pieces that sort of remind me of mitochondria in a
cell?
What does the horizontal color gradient indicate?
-Walter Branch
- Original Message -
From: [EMAIL PROTECTED]
To: meteorite-list@meteoritecentral.com
Sent: Tuesday, October 02, 2007 7:46 AM
Subject: [meteorite-list] Rocks From Space Picture of the Day - October
2,2007
http://www.spacerocksinc.com/October_2_2007.html
Michael Johnson
www.spacerocksinc.com
www.sikhote-alin.org
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