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 > > > > > ************************************** See what's new at > http://www.aol.com > ______________________________________________ > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
