Hi, all, The staurolite discussion a few weeks ago got me thinking about a thin section I've had around for about 25 years. Two thin sections, actually, with grid-twinned staurolite. It's only four grains in one section, and one in another, out of maybe 50 or 100 staurolite grains. I don't have any analyses, but these are kyanite-garnet schists from Vermont, USA. I've never read anything about grid twinning, though I think I have read about orthorhombic to monoclinic transformation. Does anyone know anything about this? Is it worth looking into further? Thanks! Kurt p.s. Field widths in the images below are 3 mm for the first two, and 1.2 mm for the second two. [image: 1.jpg] [image: 2.jpg] [image: 3.jpg] [image: 4.jpg] -- Kurt Hollocher Geology Department Union College 807 Union St. Schenectady, NY 12308-3107 USA 518-388-6518 Fax: 518-388-6417 http://minerva.union.edu/hollochk/kth/index.html

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Hi Kurt, with Gilla Simon, Volker Schenk and Michael Czank we observed something similar in a few magnesiostaurolite [XMg 0.77 to 0.96] grains from UHP rocks of the Dora-Maira massif, Western Alps, also some 25 years ago. Actually the grid was not as straight as yours and was closer to the tweed texture of microcline. Only in a few sections among several others. We felt this should be somehow connected to the monoclinic to orthorhombic transition and possibly varying H content, but were unable to pin this much further down, even with TEM work. Here is what we wrote in an abstract by Simon, Chopin and Czank for the 1998 IMA in Toronto: "Some of these staurolites show domains reminiscent of 'cross-hatched' twinning. Universal-stage measurements revealed one or two 'lamellar' systems, one of which is almost perpendicular to nbeta and the other is rotated by 80-92° with respect to the first one. TEM investigations show lamellae parallel (001) which are a few unit-cells thick and irregularly distributed. The clear presence of otherwise pseudoextinct 00l (l = 2n+1) diffraction spots indicates a different cation distribution within the lamellae. Other lamellae are parallel (100) and have various thicknesses (10 nm-0.3 µm). They are in 'twin relation' with the matrix, however additional spots parallel a* indicate a new structural variant of staurolite with a superstructure having a = 2aSt. Interestingly, all these lamellae disappear under exposure to the electron beam, suggesting that proton ordering may be one of the reasons for the formation of these lamellae." May be worth digging further! With best wishes Christian Le 2021-04-06 17:03, Hollocher, Kurt via MSA-talk a écrit : > Hi, all, > > The staurolite discussion a few weeks ago got me thinking about a thin > section I've had around for about 25 years. Two thin sections, > actually, with grid-twinned staurolite. It's only four grains in one > section, and one in another, out of maybe 50 or 100 staurolite grains. > I don't have any analyses, but these are kyanite-garnet schists from > Vermont, USA. I've never read anything about grid twinning, though I > think I have read about orthorhombic to monoclinic transformation. > > Does anyone know anything about this? Is it worth looking into > further? > > Thanks! > > Kurt > > p.s. Field widths in the images below are 3 mm for the first two, and > 1.2 mm for the second two. > > -- > > Kurt Hollocher > Geology Department > Union College > 807 Union St. > Schenectady, NY 12308-3107 > USA > 518-388-6518 > Fax: 518-388-6417 > http://minerva.union.edu/hollochk/kth/index.html > _______________________________________________ > MSA-talk mailing list -- msa-talk@minlists.org > To unsubscribe send an email to msa-talk-leave@minlists.org -- Christian Chopin chopin@geologie.ens.fr Laboratoire de Géologie Ecole normale supérieure - CNRS UMR8538 24 rue Lhomond 75231 PARIS CEDEX 05 France http://www.geologie.ens.fr/ European Journal of Mineralogy website: https://www.european-journal-of-mineralogy.net/ https://pubs.geoscienceworld.org/eurjmin European Mineralogical Union website: http://eurominunion.org/ Société française de Minéralogie et de Cristallographie https://sfmc-fr.org/