Hidden Collection, In den Dellen, Germany

Mineral Matters #398

From time to time, I add to my Hidden Collection.

This specimen is a case in point. Labelled simply as zircon and biotite, it hails from the In den Dellen quarries, Mendig, Germany.

The zircon is easy to spot. Greenish-grey crystals in a typical habit for the species. “Biotite” is the black mica. I don’t know the exact species. As per Mindat, the name is most commonly used generically (in the field or petrologically) for any unanalysed or incompletely analysed dark-coloured, Fe-rich micas, including annite, fluorannite, tetra-ferri-annite and siderophyllite. Of the four, annite is confirmed from the locality. It also closely resembles phlogopite from the locality. Without analysis, it is not possible to determine exactly what it is. The matrix is also made up of lots of colourless crystals. More on that later.

Anyway, as zircon often fluoresces, I shone my longwave torch on the specimen. And I saw lots of orange. Likely to be sodalite, or something similar. But the orange fluorescence is patchy, and there were still those colourless crystals. No reaction to the longwave UV.

My thought was that these were probably sanidine. So I checked the Database of Luminescent Minerals (fluomin.org), and for sanidine, the fluorescent response, if any, is pink under shortwave. Out with my shortwave torch, and yes, almost the whole specimen fluoresces pink! I had to take the shortwave photo with my iPhone though as I couldn’t get a decent result with my camera setup.

So here are the photos…

Below: Zircon, biotite, sanidine, In den Dellen quarries, Mendig, Mendig, Mayen-Koblenz, Rhineland-Palatinate, Germany. Width of view 6mm. Click on the image below for a higher resolution version.

Below: Zircon, biotite, orange sodalite(?), sanidine, Under longwave UV. Click on the image below for a higher resolution version.

Below: Pink sanidine. The zircon crystals in the other photos are middle top in this image. Under shortwave UV. Specimen is 22mm at its widest point. Click on the image below for a higher resolution version.