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Identity HelpAny Idea on an ID for this Rock in Young Fan Alluvium in Clark County, Nevada?

15th Sep 2019 20:19 BSTThomas Farley

03555140015685743718173.jpg
Odd looking rock in what Macrostrat.org identifies as young fan alluvium near the junction of Highway 160 and The Old Spanish Trail in Clark County, Nevada. Collecting spot is here:

36.0527, -115.7837

Evenly spaced lines of brown material radiate from depressions or pockets in the rock which at the base of each appears to contain dark or black limestone. Acid barely reacts to that material, 10% HCI is simply absorbed by the white material, no reaction, no time to react. But everything around is sedimentary. Mohs hardness just less than two. White material moderately showy under SW UV, brown material in the lines does not fluoresce. Note the many circles and round shapes in the brown colored lines.

Sandstone pieces and lots of limestone around, a few dead springs. The closest mountain range is the Nopah which is all marine rock. Wondering if the character in these lines is old spring activity. Thanks in advance for any thoughts. Thomas




15th Sep 2019 21:44 BSTPaul Brandes Manager

Based on the appearance, I would venture to say you have gypsum from a dead spring.

16th Sep 2019 17:20 BSTThomas Farley

08230190015686505909656.jpg
Thanks, Paul, for helping. I appreciate it. 

Turns out I have reference sample of rock gypsum from Clark County, from the Weiser Quarry on Weiser Ridge. These two rocks are nearly the exact same size and the heft is the same. The rock gypsum is unweathered, or seemingly so, and exhibits a somewhat sparkly appearance. It is softer than my rock, more like hardened chalk. Comes apart a little on my black streak plate. 

This gypsum does not fluoresce, unlike my piece. I doubt that calcite is responsible for the UV since the entire rock should respond if calcite or some such were covering it. Instead, only the white material responds. In the above picture my rock is under SW UV, the "blue bleed" you see is because I am still learning to photograph glowing rocks.  

I will read up on the chemistry involved in desert springs and rocks, to see if I can puzzle this out. I will report what I find. Thanks for the help. Tom

17th Sep 2019 01:35 BSTThomas Farley

Okay, here is my working theory. The white material is calcite, a mass of which formed around several limestone rocks. At some point the mass became tumbled and rounded in the small desert wash or channel that I collected it from. Being very absorbent, the round mass picked up rainwater which would contain carbon dioxide. A weak carbonic acid would develop when the limestone reacted to that. The limestone is now eating away at the calcite, dissolving the material over the centuries. The pockets you see are all centered on where a piece of limestone is located. The residue we see in the lines is the acidic precipitate left by this slow destructive process. Thus, that residue or the limestone does not respond to UV, being limestone and limestone related, while the calcite (the white mass or host) responds to UV as most calcite does. Any one else care to guess? Thanks in advance, Thomas

 
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