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Identity HelpGalena alteration product

16th Jun 2010 04:45 UTCDean Allum Expert

Last weekend, in the St. Peters Dome district, Colorado, I happened upon a pegmatite mainly composed of astrophyllite, thorite and fluorite.

I was surprised to find an ugly 8cm specimen of galena on the mine dump. This is unusual for this region, and it the first galena I have found ever. It appears to have experienced some etching, with the beginnings of skeletal hoppering.

All the galena pictures on mindat are gleaming silver. This one is mostly coated with an alteration product.

What should I describe this tan/creamy coating as? Is it angelsite? cerussite?

Thanks for your help,

Dean Allum

16th Jun 2010 05:53 UTCDana Slaughter 🌟 Expert

Hi Dean,

I've had a lot of old specimens from the Tri-State district and have tested them with a bit of dilute HCl and found them all to be cerussite---I would try an acid test. As you suggested, it may well be either.

Dana

16th Jun 2010 11:29 UTCUwe Kolitsch Manager

From (XRD) experience I would guess that at least part is fine-grained/powdery anglesite.

17th Jun 2010 05:26 UTCDean Allum Expert

Thanks Dana and Uwe. After a few seconds the HCl stopped fizzing, and it was never as effervescent as the malachite that I also checked. I am calling this angelsite.

-Dean Allum

17th Jun 2010 06:12 UTCDana Slaughter 🌟 Expert

Hi Dean,

Chemistry is not my forte, but I believe that anglesite is relatively insoluble. Any reaction to HCl seems to indicate either cerussite or other carbonate contamination. I hope that someone more competent in chemistry, etc. can clear this up for me as I've been labeling my specimens as cerussite on galena! Thanks guys.

Best regards,

Dana

19th Jun 2010 17:39 UTCKnut Edvard Larsen 🌟 Manager

The creme alteration product can perhaps be a mixture of both anglesite and cerussite ? Have you tried a UV lamp ? Cerussite may give a light yellowish color under ( longwave) UV-light.

19th Jun 2010 19:47 UTCDean Allum Expert

Knut,

you are absolutely right. The tan regions have a yellow emmision under LW UV, while there is nothing from the creme regions. So this is a mixture of cerrussite and anglesite. I won't be making any white paint from it ;)

Thank you.

21st Jun 2010 02:32 UTCDwaine Edington

Care to give more information on the locality?

21st Jun 2010 03:20 UTCAlfredo Petrov Manager

HCl is not the best acid for testing cerussite; HNO3 would be better. The fact that you did get some short-lived effervescence with HCl tends to support cerussite rather than anglesite.

21st Jun 2010 03:33 UTCDean Allum Expert

Alfredo, thanks for the acid tip.

Dwaine, see my un-answered REE unknown topic on this same board. I believe it is the type locality (within 3 meters) for murataite.

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