FMF - Friends of Minerals Forum, discussion and message board

[Antonio Alcaide]

I would like to know what rock is the matrix of bolivian danburites from Alto Chapare. I have read something of dolomite-smectite, talc... It does not look like talc and smectite is a group of minerals. Both of them appear in the locality on Mindat.

I also want to confirm if the grey color of these crystals is due to magnesioriebeckite inclusions (I know Alto Chapare is the type locality for this species).

Regards

[alfredo]

Hello, Antonio!

The matrix for the Alto Chapare danburites is an altered saltdome caprock, now composed mainly of anhydrite (more or less altered to gypsum), magnesite, dolomite, smectites (species undefined), talc, more rarely pyrite, boracite... all in varying proportions. In contrast to saltdome caprocks in other countries, there is no calcite or native sulfur, probably because of absence of hydrocarbons which are necessary for the bacterial reduction of gypsum.

Very very rarely (a few tens of specimens among the tens of thousands of known danburite crystals) the danburites are coloured blue by inclusions of magnesioriebeckite. This only happens at the contact between the metaevaporite and the magnesioriebeckite asbestos ores. The rest of the danburites, 99.9%+, are grey, not blue, and do not contain magnesioriebeckite. The grey colour can sometimes be seen under high magnification to be due to included pyrite dust, but perhaps there are other materials causing the colour too. Some people have suggested carbon, but there are no analyses.

Nevertheless, some dealers have "optimistically" labelled all the grey danburites as "blue" danburites and said the "blue" colour (which I lack the imagination to see) is caused by magnesioriebeckite. Caveat emptor.

If you ever have a chance to visit Bolivia, I'll take you to the sites of both the grey and blue danburites. I did several months of research in that area for a government agency in the early 1990s. It's a very wet tropical jungle - quite different from the "normal" bolivian mineral localities!

Cheers,
Alfredo

[Antonio Alcaide]

Hi Alfredo, thank you very much for such a complete explanation. I see the situation regarding "optimistic" dealers :-)

Magnesioriebeckite reminded me of La Juanona Quarry, close to my home, as a possible inclusion for the deep blue color of its quartzs. But I understand that it is responsible for blue color, not for this grey color. However, nice danburites.

And I have to thank you for your invitation, this time for Bolivia. It sounds marvellous. I also remember you inviting me to Franklin... but I am afraid it is more likely you come here to collect some Andalusian minerals...

Cheers

[vhairap]

[lluis]

Nevertheless, I own at least one danburite from Alto Chapare, Bolivia, that looks blue to my eyes.
What causes color, no idea (magnesioriebeckite, well, I never thought in...), But still, blue(ish)... Not grey.

With best wishes

Lluís

[vhairap]

[vhairap]

And this nice cluster

[alfredo]

Vachik, Federico Ahlfeld (after whom the mineral ahlfeldite is named) published a detailed study of Bolivian danburite at the University of Córdoba, Argentina, back in 1946. I have a copy of his paper somewhere. It's a rather obscure publication and hard to find, but I managed to get a photocopy of it in Argentina in the early 1990s. Ahlfeld however did not recognize the host rock as a saltdome caprock.

I have some danburites from Hormuz island, Iran, which look very much like the Bolivian ones in habit, although the Hormuz danburites are more transparent than the Bolivian ones. I suspect that the geological origin is the same - saltdome caprocks.

[vhairap]