Rainbow fossils

[paleoflor]

L.S.,

 

Found some plant fossils where the mineralisation takes on all colours of the rainbow. Earlier, Ken @digit suggested the blue colour may originate from chalcopyrite (a good candidate, as chalcopyrite is known to occur at the locality). All these additional colours seem to support Ken's hypothesis, since chalcopyrite develops an iridescent tarnish more often. Unfortunately the thin film-preservation makes confident mineral identification quite challenging.

 

I'll report back once I've found the associated fossil-pot of gold.

Kind regards,

 

Tim

 

Eusphenopteris sp.

 

 

Cyclopteris sp. (same specimen photographed under slightly different lighting conditions)

 

Laveineopteris rarinervis

 

Laveineopteris cf. dussartii

[Fossildude19]

Lovely specimens, Tim!

Thanks for sharing them here.   

[piranha]

      

[Darktooth]

Love the colors on those specimens! Fantastic!

[Tidgy's Dad]

Beautiful.

[FossilNerd]

 

I second what everyone else has already said. Beautiful, lovely, fantastic…

[Yoda]

Those are really nice 

[Ludwigia]

Lovely colors. @paleoflor If these samples are from Piesburg, then someone, I think it might have been Angelika Leipner, told me long ago that the mineral that causes the colors is called Gümbelite, which has been defined as an impure Pyrophyllite.

[paleoflor]

7 hours ago, Ludwigia said:

Lovely colors. @paleoflor If these samples are from Piesburg, then someone, I think it might have been Angelika Leipner, told me long ago that the mineral that causes the colors is called Gümbelite, which has been defined as an impure Pyrophyllite.

 

Hi Roger: Yes, these specimens come from the Piesberg. You're right that most of the imprints are covered by "gümbelite", which is a Mg-rich form of illite (not pyrophyllite). The rainbow colours are difficult to explain as coming from illite, though: note this is a white-grey mineral phase, which does produce the common silver-white colour of fossil imprints from the Piesberg and gives them a somewhat silky lustre. Yellow to orange (also rather common) are caused by "limonite" and similar iron(hydr)oxides. The rainbow colours are associated with an almost metallic lustre and I expect they are caused by contamination/replacement of the illite with some other mineral, similar to how iron(hydr)oxide contamination leads to yellow colouration. Ken's suggestion of chalcopyrite seems quite plausible to me. I'd like to analyse them for Cu sometime to verify.

[FranzBernhard]

Aren´t thin coatings of iron sulfides that have been superficially oxidized also a possibility?

Franz Bernhard

[zkazyak]

Man those are super cool. 

[Jaybot]

That's a score of 100/100 on the eye appeal score for me

[paleoflor]

Following up on @FranzBernhard's suggestion, I've been looking for alternative options which might explain the colouration on these fossils.

 

I came across some really interesting papers on iridescent or "rainbow" hematite --- see Lin et al. (2018), Lin (2015), Nadin (2007), and Ma and Rossman (2007). Apparently, when hematite (iron oxide) is covered by a thin film of nanocrystals containing Al and P impurities this can produce a wide array of colours. Ma et al. (2007) is another interesting article, which investigated the origin of iridescent colours in "fire" obsidian and found these are caused by nanocrystals of magnetite (another type of iron oxide).