FMF - Friends of Minerals Forum, discussion and message board

[Joan R.]

Last April 2007, a team of mineral collectors from Huelva extracted after a hard work excellent stalactitic-botrioidal goethites, in the Filón Sur Mine (closed from 2001), near Tharsis. They get out several specimens in a small cavity, that was only a 30 cm width in the goethite mass. Stalactitic goethite hungs inside.
Stalactites were embedded in a thick and fine orange mud that covered everything and everybody. They couldn’t see what they were extracting because mud not allowed to know if the stalactit was long or short, whole or broken... a good portion of luck played in this work. Some specimens, when washed, they showed intense iridiscence with colours like red, blue, green, mauve and gold. This locality hasn't produced significant samples in the last twenty years.



Stalactitic-botryoidal growth with intense, very bright, iridescence,
with metallic reflections of dominant colors between golden and blue.
Filón Sur Mine, Tharsis, Huelva Spain (05-10/2007)
Specimen size: 6.7 x 4.5 x 4 cm.

Old mining train in Tharsis (40’ film in Spanish - Video Google) :
https://video.google.es/videoplay?docid=-925978439510782157

[alfredo]

Hello Joan! Thanks for posting these beautiful pictures. I've always had a fondness for iridescent iron oxides, even though I hardly ever find any as good as yours.

Question: Given the difficulties of studying thin surface films, has anyone actually studied the composition of the iridescent films on such stalactites? A lot of collectors call such films "lepidocrocite", without any evidence I'm aware of that that's what they really are. I have no hesitation calling these specimens "goethite", as Joan does, since that is the composition of the bulk of the stalactite. (The cream on top of a cake does not change the fact that most of it is still "cake".)

A similar question passes through my mind with regard to the iridescent films on some lavas - films that some "systematic" or "species" dealers are happy to sell as the mineral "siderazot" (iron nitride), even though evidence for the existence of that mineral seems to be as thin as the films it forms.

Anyone aware of modern studies on thin mineral films, please tell us.
Regards,
Alfredo

[Jordi Fabre]

Alfredo,

Having the same doubt concerning the named "Lepidocrocite" I analyzed on the past this Goethites and them are pure Goethite, no traces of Lepidocrocite. I ignore if in some other place Lepidocrocite could appear with this appearance, but for sure, the Spanish ones are just Goethite

Jordi