FMF - Friends of Minerals Forum, discussion and message board

[John Betts]

I acquired an old specimen from San Pedro Corralitos. It is the typical brown gossan coated with yellow-green mimetite microcrystals to 0.3 mm and bluish-black microcrystals in botryoidal aggregates to 0.5 mm.

The bluish-black mineral was tentatively identified as goethite by the seller. But I have my doubts. The bluish color would be unusual for goethite.And the streak color is distinctly black, not the brown/ochre usually associated with goethite.

Any ideas for possible species that would be associated with mimetite from Mexico?

[John Betts]

Detail photo.

[lluis]

Maybe just any (maybe mix) manganese oxide?

With best wishes

LLuís

[bob kerr]

Descloisite and/or Mottramite are good guesses. Los Lamentos had similar only after wiulfenite mainly..

this is a Peter Megaw problem.

bob

[John Betts]

Mottramite was a good suggestion, but I just checked for reaction to HCL. No reaction which rules out mottramite. Additionally mottramite does not have a black streak.

[Pete Richards]

Asked in ignorance: what about plattnerite?

[Peter Megaw]

John...I'm going to go along with Lluis...probably a manganese oxide, given the amount of zinc around likely chalcophanite...it is very common at Santa Eulalia and Ojuela and often has that quasi bluish metallic lustre/color.

Pete's suggestion of plattnerite could be solved under the scope with strong back lighting looking for red internal reflections...but it doesn't quite look right

Cheers AND MERRY CHRISTMAS TO ALL

Peter

[John Betts]

[John Betts]

Under the scope the botryoidal formations show they are brown internally (goethite), but have an overgrowth of blue-black micro-microcrystals (too small to see any details).

I'll just stick with "Goethite" Thank you all.

Best wishes for a merry Christmas!