These stones have a natural (!) shiny glaze (see pic) which may be colored or not. I would like to know if there are other occurrences worldwide. May be a museum houses some samples somewhere in the world?
Hello Bertram. Are these buchites? If so, have a look at the paper by Heide & Heide 2011 about natural glasses:
http://www.sciencedirect.com/science/article/pii/S0009281911000729
Hello. Is that real? I've never been to Eifel annd I'd expect glass to weather quite easily. Those are extraordinarily fresh. And concerning your question: never seen anything like that elsewhere.
That looks too good to be true, I'd say. But if you have evidence that it is natural, then it's very impressive. What is the composition of the glaze? Not seen anything like it either, on any xenolith, whether mafic or felsic.
Hello Bertram. Check Rodney Grapes "Pyrometamorphism" (2006 or 2010). You'll find excellent overview for such phenomena.
Yep, Grapes is an excellent resource for this topic. Basically, buchite glasses form when siliceous xenoliths (e.g., quartz sandstone) are in contact with hot (basaltic) melts under hornfels-facies contact metamorphic conditions.
Thanks for your answers! To Martin: yes, these are "buchites" (I know the text of Heide & Heide). The glass in buchites in normal cases can only be seen under the microscope, but with the glazed xenolithes you dont need a microscope, the glaze is hard to overlook. To Bernardo and Marline: it is real and natural; when I saw these stones for the first time, I thought its a fake, but since then I found several samples by myself. To A.V. and Martin: thanks for the hint at the book!
Hi all
I have a pair of quartzite xenoliths (see picture) with the same "glazing". I picked them in the Calatrava Volcanic field (Ciudad Real, Spain), in the volcanic rocks that build the cone of the Almodovar del Campo maar-volcano. These quartzites come originally from the palaeozoic basement but were included by the magma, reacted with it, and accumulated with the rest of the pyroclasts. As far as i am aware, this kind of xenoliths are cited elsewhere, but no detailed study has been carried out...
Interesting how the colour of the glaze differs from the Eifel xenoliths. Can this be related to the composition of the magma and/or the xenolith? Never knew anything like this existed, so thanks for starting this discussion Bertram!
Enrique: Making a thin section would be a nice idea. I offer for it. If you go back there and pick some others I'll be glad to get one...
Differences in colour seem to relate with the specific trace element composition of the magma, as stated in the Heide & Heide paper but I have no criteria on that of my own...
Bernardo, I can make a thin section here, I´ll share the microphotograph when I have it. Thanks for the offer anyway. It´s not in my plans going back to Ciudad Real in the near future, but I make a note on your request.
To Enrique: Thanks for your contribution and congratulations to the glazed xenoliths, you found (they probably are rarer than diamonds)! I had the feeling that there must be other places in the world to find these stones. The colour seems to be caused from heavy metal traces in the xenolith (not the magma), as our laboratory experiments (unpublished) suggest. The Eifel-xenoliths always are sandstones (Buntsandstein - Trias or Grauwacke - Devon). Your paleozoic quartzite-xenoliths probably also are devonian? Their orange-yellow colour might be caused by iron(III)-ions. The colours can be quite different at the same locality (see attachments, especially for Marlina; the specimens are on display in the Museum of Natural History in Gerolstein/Germany). There are two modern publications concerning glazed Eifel-xenoliths:
A. Goepel, „Natürliche Gläser und Glasuren in den Vulkaniten der Westeifel“, Dissertation, Universität Jena, 2008 and: C. S. J. Shaw, „Caught in the act - The first few hours of xenolith assimilation preserved in lavas of the Rockeskyllerkopf volcano, West Eifel, Germany“, Lithos 112 (2009) 511. Both publications have different suggestions for the origin of the glaze.
Very nice, and thanks for the information on the variable colours in a single location. That does indeed show that the xenolith, not the magma, is the cause of the colour. I now know what to hunt for next time I am standing on a volcano!
Dear Dr. Schmidkonz,
yes there are a few localities mentioned throughout Germany; for example the "Rodderberg" (Syn.: "Roderberg") to the east of Wachtberg, N of Remagen (Rolandswerth). Thomae (1835) reported yellow, brownish and purple colored Grauwacke-xenoliths from the Lower Devonian as well as yellow colored quartz pebbles from the fluviatile Hauptterrasse.
I also would like to present an additional Lower Devonian sandstone xenolith from the West Eifel Volcanic Field (photo: Bernd Fickert) [width 7cm]:
never found the nice blue ones, but have several green, brown to purple ones from Emmelberg (Udersdorf, Eifel). Even found one with som fossiel shells in the sandstone (see photos). Enclosed some literature too.
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