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This short paper has specific microprobe procedures and result criteria that most petrologists will find agreeable: Quinn, R.J., Valley, J.W., Page, F.Z., and Fournelle, J.H., 2016. Accurate determination of ferric iron in garnets. American Mineralogist, Volume 101, pages 1704–1707, 2016. Best, Steve On Wed, Mar 28, 2018 at 8:21 PM, Lincoln S. Hollister <[log in to unmask]> wrote: > Back in the day, pre and post Harold as a student, we followed a routine > as he describes. However, in general, I did not assume stoichiometric Al, > I did assume nothing else in Si site. So if the analysis had a bit of > Fe+3 for Al site, that was fine. However, total Fe was generally off. I > preferred an almandine garnet for Fe rather than fayalite or hedenbergite. > My suspicion was that the the high CN for Fe in garnet (viii) affected the > Fe based on Fe (vi) in olivine etc. > > A running rule was that a good garnet analysis totaled slightly over 101 > (or was it less than 99?). Sorry , senior moment here... > > Basically, black magic. > > Lincoln > > > On Mar 28, 2018, at 7:18 PM, Stowell, Harold <[log in to unmask]> wrote: > > Hi all > > My experience is also perplexing. > A few ideas and observations: > > 1 Most garnet in pelites, diorite gneiss, and eclogite is stoichiometric > and has likely has minimal amounts of Fe3+ > Therefore, criteria based on 8 cations, with 3 Si and 2 Al are generally > OK. I strive for +/-0.02 cations for each of these numbers. > > 2 Analyses can be difficult if the stds are significantly different > compositions than the unknowns. Remember B-A corrections etc…. > > 3 Si, Al, Fe are generally where the difficulties are most obvious. > > 4 For pelitic amphibolite facies garnet with lots of Fe, I use spessartine > garnet [fairly pure] for Si and Al std. I generally use fayalite for Fe > because it is hi Fe - similar to many grt. > > 5 In general, #4 has been adequate for the Ca and Mg-rich grt in Fiordland > eclogite and mafic granulite. > > cheers > H > > On Mar 28, 2018, at 5:49 PM, Wintsch, Robert P. <[log in to unmask]> > wrote: > > HI Howard, > > wishing I could help, but I cant. > I used very well characterized olivines and kyanite and wollastonite as > standards, > and that seemed to help. > > but really I wanted to say that I am also FINALLY retiring from IU, and > moving east to Conn. > Maybe we can cross paths in either Conn or Maine some time? > > bob > > Robert P. Wintsch > Dept. of Geological Sciences > Indiana University > 1005 E. 10th Street > <https://maps.google.com/?q=1005+E.+10th+Street+%0D%0ABloomington,+IN+47405&entry=gmail&source=g> > Bloomington, IN 47405 > Tele. off: 812-855-4018 <(812)%20855-4018>; Tele. dept: 812-855-5582 > <(812)%20855-5582> > Fax: 812-855-7899 <(812)%20855-7899> > [log in to unmask] > ________________________________________ > From: Metamorphic Studies Group <[log in to unmask]> on > behalf of day <[log in to unmask]> > Sent: Wednesday, March 28, 2018 6:23 PM > To: [log in to unmask] > Subject: [geo-metamorphism] What is a good garnet analysis? > > Colleagues, > > Some thirty years ago, when we acquired a new microprobe, I spent a > frustrating year attempting to produce good analyses of garnet. Although > we had some successes, we were unable to produce good analyses in a > systematic way. Every suite seemed to be a new problem. Then, I became > chair of department and that effort came to a halt. Now, I am back to the > problem again in the context of eclogite petrology, and it occurs to me > first to ask “What is a good garnet analysis?” > > Because the quality of analyses in the literature is quite variable, I ask > for your thoughts on the criteria by which we should judge the quality of > routine garnet analyses as sufficient to be published. Obviously, the > criteria may differ depending on the proposed application of the data, but > is there a consensus on what constitutes a good routine analysis? > > Thanks > > Howard > > > Howard W. Day, Professor Emeritus > The Dept. Formerly Known as Geology > University of California Davis > One Shields Ave. > Davis CA 95616 > > > Harold Stowell > Geological Sciences, University of Alabama > 201 7th Ave Tuscaloosa > <https://maps.google.com/?q=201+7th+Ave+Tuscaloosa+%0D%0A+%0D%0AAL+35487&entry=gmail&source=g> > AL 35487-0338 > > > > >