[meteorite-list] Correlation of Fa & Fs for ordinary chondrites
Hi Jeff, > The relationship that you found was documented in meteorites > shortly after the first electron microprobes became available >to meteorite researchers. The landmark paper was: > KEIL K. and FREDRIKSSON K. (1964) The iron, magnesium, and calcium > distribution in coexisting olivines and rhombic pyroxenes of > chondrites. J. Geophys. Res. 69, 3487-3515. Thanks for the link and the abstract excerpt -- I had hoped a petrologist would chime in on the subject, and I was not disappointed. :-) You wrote: "This relationship is due to equilibrium partitioning of Fe and Mg between olivine and pyroxene." An empirical relationship, while useful, is certainly more valuable when there is a chemical reaction explanation for the ratio. If anything useful came of the exercise (for me, anyway), it was the clear indication from the plotted points that the Fs/Fa plot does not have an origin at 0,0: that Fs will still remain if Fa goes to zero. Of course, this is outside the range of ordinary chondrites, but it does show that better results can be obtained using an Fa = A*Fs + B formula than a simpler Fa = A*Fs. Cheers, Rob
Re: [meteorite-list] Correlation of Fa & Fs for ordinary chondrites
The relationship that you found was documented in meteorites shortly after the first electron microprobes became available to meteorite researchers. The landmark paper was: KEIL K. and FREDRIKSSON K. (1964) The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites. J. Geophys. Res. 69, 3487-3515. Here is part of the abstract: "Iron, magnesium, and calcium in olivines and pyroxenes of 95 chondritic meteorites were analyzed with an electron microprobe. Of these, 86 were 'ordinary' chondrites, each having constant iron-magnesium ratios in olivine and pyroxene. Between different meteorites these ratios vary within three narrow ranges which reflect the H- and L-group chondrites, as well as a third group, designated the LL group. The ratio of iron in olivine to that in coexisting pyroxene is nearly constant (~1.1), particularly within the groups mentioned. A classification of stony meteorites based on the amount of iron in silicates and metal is proposed..." This relationship is due to equilibrium partitioning of Fe and Mg between olivine and pyroxene. In partially equilibrated chondrites (e.g., high type 3's), you will find departures from this relationship because olivine equilibrates much more rapidly than pyroxene. In unequilibrated chondrites, there is no such relationship, but it may be followed within individual chondrules. jeff At 11:02 PM 9/7/2003, Matson, Robert wrote: Hi All, I decided it was still too hot to go to the desert this weekend, so instead I spent the better part of today on a problem I've been meaning to tackle for a few months. Hopefully my results will be useful to someone here. I don't know if the work is quite worthy of a paper -- perhaps. My interest in the problem primarily has to do with pairing of equilibrated ordinary chondrites. While some labs measure both olivine fayalite (Fa) and pyroxene ferrosilite (Fs) mol %, others measure only one or the other (usually Fa). Occasionally, you'll have two specimens that are potentially paired, but Fa was measured on one, and Fs on the other. As it turns out, Fa and Fs are somewhat correlated, and thus it is possible to derive expressions for converting one value to the other, within certain error bars. So I created a database containing only Antarctic equilibrated ordinary chondrites that had measured values for both Fa and Fs, subdivided by type and petrologic grade. As you might imagine, this took a while! I excluded meteorites that had ranges (rather than single values) listed for either Fa or Fs, and I tossed out five outliers that would have unduly skewed the statistics. That still left me with 3449 meteorites! I plotted these in Excel, with separate symbols and colors for H4, H5, H6, L4, L5, L6, LL4, LL5, LL6 and LL7. It turns out that there were no major correlation differences between petrologic grades within each type, so I lumped all the grades together and did linear regression fits for H, L and LL. Here are the results for converting a ferrosilite value into a fayalite value: H: Fa = .932*Fs + 3.20 rms residual = +/- 0.38% (2448 points) L: Fa = .958*Fs + 4.60 rms residual = +/- 0.57% (801 points) LL: Fa = 1.057*Fs + 3.76 rms residual = +/- 0.66% (200 points) (I have corresponding equations for going in the reverse direction. I also computed the linear coefficients by petrologic grade if that interests anyone). When you consider that most of the Antarctic Fa and Fs values were given to the nearest whole number percentage, I'd say the fit is quite good. Cheers, Rob __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list Dr. Jeffrey N. Grossman phone: (703) 648-6184 US Geological Survey fax: (703) 648-6383 954 National Center Reston, VA 20192, USA
[meteorite-list] Correlation of Fa & Fs for ordinary chondrites
Hello Rob, Thank you for sharing this "correlation" formula. Not only is this of interest to me, but others may find this of some use in pairing meteorites. Just wanted to let you know that your time and effort was appreciated. Can you remind me where we first heard that there was an actual numerical correlation? If it turns out that this is a little known or poorly understood relationship, then I really do think you should write a paper, or at least, an abstract and see how it is received. Hopefully, you will get some replies from the petrologists that are subscribed to this List. And you were right, as well, about the desert. It hit 105 degrees at some of the areas where I was this weekend. With best regards, Bob V. -- Original Message -- [meteorite-list] Correlation of Fa & Fs for ordinary chondrites Matson, Robert [EMAIL PROTECTED] Sun, 7 Sep 2003 20:02:02 -0700 Hi All, I decided it was still too hot to go to the desert this weekend, so instead I spent the better part of today on a problem I've been meaning to tackle for a few months. Hopefully my results will be useful to someone here. I don't know if the work is quite worthy of a paper -- perhaps. ++ (I have corresponding equations for going in the reverse direction. I also computed the linear coefficients by petrologic grade if that interests anyone). When you consider that most of the Antarctic Fa and Fs values were given to the nearest whole number percentage, I'd say the fit is quite good. Cheers, Rob __ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
[meteorite-list] Correlation of Fa & Fs for ordinary chondrites
Hi All, I decided it was still too hot to go to the desert this weekend, so instead I spent the better part of today on a problem I've been meaning to tackle for a few months. Hopefully my results will be useful to someone here. I don't know if the work is quite worthy of a paper -- perhaps. My interest in the problem primarily has to do with pairing of equilibrated ordinary chondrites. While some labs measure both olivine fayalite (Fa) and pyroxene ferrosilite (Fs) mol %, others measure only one or the other (usually Fa). Occasionally, you'll have two specimens that are potentially paired, but Fa was measured on one, and Fs on the other. As it turns out, Fa and Fs are somewhat correlated, and thus it is possible to derive expressions for converting one value to the other, within certain error bars. So I created a database containing only Antarctic equilibrated ordinary chondrites that had measured values for both Fa and Fs, subdivided by type and petrologic grade. As you might imagine, this took a while! I excluded meteorites that had ranges (rather than single values) listed for either Fa or Fs, and I tossed out five outliers that would have unduly skewed the statistics. That still left me with 3449 meteorites! I plotted these in Excel, with separate symbols and colors for H4, H5, H6, L4, L5, L6, LL4, LL5, LL6 and LL7. It turns out that there were no major correlation differences between petrologic grades within each type, so I lumped all the grades together and did linear regression fits for H, L and LL. Here are the results for converting a ferrosilite value into a fayalite value: H: Fa = .932*Fs + 3.20 rms residual = +/- 0.38% (2448 points) L: Fa = .958*Fs + 4.60 rms residual = +/- 0.57% (801 points) LL: Fa = 1.057*Fs + 3.76 rms residual = +/- 0.66% (200 points) (I have corresponding equations for going in the reverse direction. I also computed the linear coefficients by petrologic grade if that interests anyone). When you consider that most of the Antarctic Fa and Fs values were given to the nearest whole number percentage, I'd say the fit is quite good. Cheers, Rob __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
