Dear Dr. Mestre,

The brown translucent mineral is most likely rutile TiO2 with the typical extreme light-refraction. The isotropic optical property in crossed polarizers may be due to the cutting position of the grains perpendicular to the optical axis of the tetragonal optically uniaxial mineral rutile.

Best regards,

G. Grundmann

Dear Ana Isabel Mestre,

I'm trying to magnify your attached photos, but it does not work. Therefore I'll give you just an additional idea to that of Guenter. The grain in the upper right corner of the lowermost figure reminds me the most of Cr-spinel, chromite, which is (redish to orange) brown translucent in plane polarized light and isotropic (black) under crossed polarizers. Usually, very characteristic is black rim under plane polarized light, representing chromite alteration to maghemite and magnetite, even hematite.

All the best,

Mirka

Dear Dr. Mestre,

Dear Dr. Trajanova,

Chromite was my first idea to identify this brown mineral. But there are two important arguments against chromite: The image in crossed polarizers shows after enlargement still in the middle of the grains brown color. This is absolutely not the case in chromite. Furthermore, the detrital assemblage present in the thin section with biotite, quartz and feldspar derived from granitic rocks (magmatic and/or metamorphic). Chromite in contrast forms in ultramafic rocks. No further detritic ultramafic alterations products are recognizable. Therefore I plead for rutile.

Best regards,

G. Grundmann

Ah, sorry for misleading, dr.Mestre and dr. Grundmann. Now I manage to see big photos and the situation is completely different. The rock seems to represent carbonate rock, similar to intrapelsparite with mostly washed micrite. Relics of micrite in sparite can be seen on the third fig. Intraclast of quartz and biotite flake can be seen on the second fig. and a lot of brown pelets in the first figure and some other semi-opaque grains which could belong rutile, or may-be to goethite (?).

Best regards.

Mirka

I also agree it is rutile, I remember having very similar ones on my samples from the high methamorphic grade The Acatlan Complex in Mexico, granitic-migmatite formation. Even with the same associations described.

Kind regards,

German.

It seems to me that your initial guess is the most inspired variant. Rutile is an unlikely candidate, because the variable refractive index is too low in some of the grains. The isotropy cannot be explained away by the optical orientation, because it would be to strong a coincidence to have all grains with the same orientation. It looks like a gel in various stages of transformation - I strongly doubt it is about lithoclasts... so iron hydroxide gel seems ok.

Dear all: Thank you for your suggesting. Gavril you have any information about genesis of hydroxide gel???

Not really, I'm sorry, my field of incompetence is rather elsewhere. but had it been rutile.. Chemical precipitation, I would venture.

Dear,

The brown mineral which is translucent and shows light-refraction seems to be RUTILE. Please also see the attached for examples.

Lucky,

S.R