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Yellow Crystals on Volcanic Hematite?

Posted by Robert Darabos  
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Robert Darabos March 12, 2019 11:34PM
Unknown volcanic rock found near San Antonio Aguascalientes, on the outskirts of Antigua, Guatemala.
I enhanced the photos to make the colors pop the best, so they are a little brighter than what they really are.

The bright red material leaves a red streak as bright or even brighter than the actual mineral. I'm assuming this means it is Hematite?
The small white sections are an unknown carbonate mineral, as HCL eats it up vigorously.

The small black crystals and the yellow sections I am unsure about.
I thought the black could be actual Hematite crystals on the massive material, but purely speculation.

The yellow I'm more confused about. It does not appear to be any kind of clay mineral, has no reaction to HCL, does not fluoresce under LW UV light. I thought it could be sulfur, but I can not (personally) detect a "sulfur smell" to it. Any ideas what it could actually be?




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Robert Darabos March 13, 2019 05:16PM
Could the yellow be a pure form of sulfur?
it doesn't have the "smell" of sulfur that i can tell. it's very brittle and fragile though and breaks off easily.
its hardness is hard to tell, but seems very soft. i can easily scrape it off the matrix with a knife.
it seems to evaporate with water (especially warm) but mainly if it is powdered off the matrix. On the matrix it doesn't seem to do so as much, and actually turns a little off gray-yellow with warm water.
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Donald B Peck March 14, 2019 03:40PM
Hi Robert, sulfer is insoluble in water and really has no odor, although hydrogen sulfide from volcanics does. Is the gray-yellow in the warm water, possibly a fine suspension of a solid (sulfur)? If you shine a narrow beam of light through a glass container of the liquid, a colloidal suspension will scatter the light so that the beam shows.
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Michael Hatskel March 14, 2019 06:56PM
Hi Robert,
To catch the sulfur smell, you need to burn the sulfur on a reasonably hot surface.
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Wayne Corwin March 14, 2019 07:15PM
Just touch it with a red hot needle,,, and take a wiff.
You won't have to remove anything.
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Susan Robinson March 14, 2019 07:57PM
The specimen looks like a typical skarn assemblage, and not volcanic.
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Pavel Kartashov March 14, 2019 08:25PM
Elementary sulphur is soluble in hexane or usual gasoline. After evaporation of this solution, you'll obtain crystalline sulphur film. So you may to try.

But megascopically your yellow phase is more similar to mixture of some sulphates of Al and Fe.
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