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[Linus]

I recently found a couple flats of these cool calcites from Charcas. I recall that there was some controversy about what the pink drusy matrix of these specimens was - datolite, kutnohorite, chalcedony? any thoughts? By the way the calcite on the back of these is intensely fluorescent orange!

[Tobi]

Hi Linus,

as long as I remember, these specimens from Aurora Mine with calcite on pale pink matrix have always been referred to as quartz. I know them as rather microcrystalline so I would say chalcedony could also be correct instead of quartz. But I'm neither a chemist nor a mineralogist, this is just what I always thought of as a collector.

Tobi

[Bob Harman]

A very close up of the photo shows the calcite itself to be on clear colorless druzy quartz, but near and along the rim of the specimen, there is also botryoidal pale pink ??? Maybe chalcedony. Bob

[Peter Megaw]

We have analyzed the pink material....it is datolite.

[Peter Megaw]

Here are two more examples. One is a stalactite of essentially pure datolite with only a few tiny calcites...the other is a miniature of the stalactitic datolite with small calcites sprinkled over the top.

This material came out in fair quantity about 10 years ago...especially the stuff Linus posted with the large blocky calcites...most of which were damaged. The stalactitic material was much scarcer, I have only seen a few tens of examples.

I am not sure these actually came from the Aurora...if anyone has solid provenance for them I would be delighted to hear

[Linus]

Hi thank you everyone! Basing my observations on Peter's comment, on very close examination, the "drusy" effect is caused by a second overgrowth of nearly clear datolite over the pink datolite base.
There were also a couple stalactitic datolites in these flats, one shown attached.

[Josele]

How misleading can be a mineral photo, last one could be confused with globular chalcedony from Mamuju, Indonesia!

[Tobi]

[Linus]

Hi Tobias, I can understand your doubts. For a number of years, I worked as a Geologist in the town of Charcas and purchased these direct from a miner, along with several nice danburites and nifontovites. Also, these stalactites have calcite cores which Im sure the grape chalcedony does not, and the pink globes are soft, hardness less than 6 (by scratch-point test).

[Linus]

I thought maybe a picture of the base of the stalactite would be helpful. base looks to be a cross-section of a vertical core of calcite, entirely surrounded by pink datolite globules... bands of this interior calcite are weakly SW fluorescent orange, the outer pink mineral is not at all fluorescent under SW or LW.

[Tobi]

Thank you for the explanation, Linus :-)

Regards
Tobi

[Jean Sendero]

Hello,
I am a little late joining this discussion but Peter left a question unanswered.

First, I will simply mention that the material was analyzed in 2011 by Virgil Lueth at New Mexico Tech. and it confirmed the Datolite.

Secondly, Peter's question on the locality, I have Mina San Bartolo instead of Aurora. This locality was provided by Dennis Beals at the time.

Cheers