What kind of rock/mineral is this?

Funny to see all these comments. Honestly its darn near impossible for us to say what type of rock or mineral it is and be 100% accurate just by looking at it. But I would say it that its a pretty fair assumption to say that it is some sort of mica (most likely Muscovite)

Glad you found out what it was though. Even though I have my doubts on it being gypsum

selenite (gypsum) is my suspicion as well

johnL wrote:I googled this "cleavage" word and came up with something very different Also, I feel funny inside.

I definately see the 2 part distinct example of cleavage in your google examples.....

could be either selenite or muscovite.....drop a drop of HCl on it to determine if you can eliminate selenite (if it sizzles or bubbles it is NOT selenite/gypsum)

My vote is still muscovite....which is common in parts of UT

Based on more of your info, the mineral is Gypsum or a close relative. It is NOT mica, or any sheet silicate of that type. Surprised no one through out Phlogopite, as this is so much more fun to say than regular old 'mica'

mozeman wrote: My vote is still muscovite....which is common in parts of UT

Muscovite is pretty common all over the west but I doubt it is common in the Swell. The Swell is a bunch of sandstone and sed rocks. That leads me to believe it is either an evaporite (gypsum) or some sort of fluid dissolution deposit.

I have seen gypsum that looks very similar to the photos so thats my vote. It also seems to have 3 planes of cleavage and not 1. Mica has 1 perfect plane of cleavage. You can see the other planes in the second photo as lineations.

Gypsum/selenite for sure. Don't listen to the mica/muscovite folks. I am part geologist and have the same looking piece in front of me from a US Gypsum operation in Southern Utah.

jon jugenheimer wrote:Based on more of your info, the mineral is Gypsum or a close relative. It is NOT mica, or any sheet silicate of that type. Surprised no one through out Phlogopite, as this is so much more fun to say than regular old 'mica'

Not saying you are wrong, but what eliminates all silicates completely? Just would like to hear why people are thinking its selenite.

With that said, I'd like to change my vote to a gypsum relative due to its surrounding of sandstone. Along with the arid and dry environment.

If you want that settled, send me a bit and I'll settle it via EDX spectra from an SEM.

mozeman- it has to do with the location it was found. most all rocks near surface in that location are sedimentary and associated with the colorado plateau. micas (including muscovite) weather easily and are not commonly found in sedimentary rocks.

if you can scratch it, my best guess would aslo be gypsum.

Interesting, but what is all that tan veining?

That's a sweet dihedral system splitting the face. It might go.

Tony B wrote:If you want that settled, send me a bit and I'll settle it via EDX spectra from an SEM.

Oh snap!

Upping the ante?

It looks like gypsum.

I've seen very similar stuff in White Sands and in the tailings at an anhydrous gypsum mine.

If it's between mica and gypsum, a simple test would be to get a fingernail under a thin layer and peel it back. If the layer flexes and bends and pretty much stays there, it's probably gypsum. If it bends back then snaps flat again when released, it's some sort of mica.

Mica only has one plane of perfect cleavage.

Unfortunately, without some sort of real analysis of the rock (in person) nobody can say for sure what you have.

It could also be a serpentine or amphibole (asbestos). Not uncommon for asbestos minerals to occur in conjunction with mica.

Chris D wrote: Oh snap! Upping the ante? It looks like gypsum. I've seen very similar stuff in White Sands and in the tailings at an anhydrous gypsum mine. If it's between mica and gypsum, a simple test would be to get a fingernail under a thin layer and peel it back. If the layer flexes and bends and pretty much stays there, it's probably gypsum. If it bends back then snaps flat again when released, it's some sort of mica. Mica only has one plane of perfect cleavage. Unfortunately, without some sort of real analysis of the rock (in person) nobody can say for sure what you have. It could also be a serpentine or amphibole (asbestos). Not uncommon for asbestos minerals to occur in conjunction with mica.

Really? serpentine or amphibole, in the San Rafael Swell?

If you don't have access to a spectral analysis tool, simply put the piece in your metric oven and heat it to above 80 degrees C. If it turns crumbly you've dehydrated it, it's another clue it is gypsum/selenite.

If you can scratch it with your fingernail that rules out calcite pretty strongly. I'm going to add to the people saying its probably gypsum, the stuff is very common in the San Rafael Swell, and I've seen entire hillsides that seem to glitter in the sunlight because of the amount of pieces of it lying around.

Geologist here, Selenite (form of gypsum), I've got a piece of it sitting right here on my desk. It's a CaSO4 with a couple of H2O's, a Calcium Sulfate. It comes in all kinds of forms, crystals, massive fine grained is called alabaster, "roses". It can form in many different environments and is very common. That particular piece looks like it formed in a vein, probably in an environment where dry lake beds were abundant.

YDPL8S wrote:Geologist here, Selenite (form of gypsum), I've got a piece of it sitting right here on my desk. It's a CaSO4 with a couple of H2O's, a Calcium Sulfate. It comes in all kinds of forms, crystals, massive fine grained is called alabaster, "roses". It can form in many different environments and is very common. That particular piece looks like it formed in a vein, probably in an environment where dry lake beds were abundant.

+1 You know what your talking about. I do quite a bit of soil mineralogy and I agree. San Rafel Swell has evaporite minerals, the layering is not mica, it has to do with deposition and the chemistry of the fluid as it evaporated. If you want a precise answer (like the chemistry and crystal structure) contact me through this site and if you send a chip, I will send you an x-ray diffraction pattern back no charge...

ab527 wrote: If you want a precise answer (like the chemistry and crystal structure) contact me through this site and if you send a chip, I will send you an x-ray diffraction pattern back no charge...

Heh... so there is an abundance of nerds here.

I taught a geology section last year. Students have the hardest time with the calcite/gypsum/mica ID. If it fizzes in HCL, it is calcite. Super soft and soapy with more than 1 cleveage, probably gypsum.
People think that anything shiny and remotely flat is mica.
To find Mica in-situ, you need to be in a region of intrusive granitics, like in the rocky mountains or the Sierra Nevada.

So, I'll join the rock-nerd vote for gypsum, having never actually handled or tested the thing.

ab527 wrote: +1 You know what your talking about. I do quite a bit of soil mineralogy and I agree. San Rafel Swell has evaporite minerals, the layering is not mica, it has to do with deposition and the chemistry of the fluid as it evaporated. If you want a precise answer (like the chemistry and crystal structure) contact me through this site and if you send a chip, I will send you an x-ray diffraction pattern back no charge...

What comes first, the geologist or the rock climber within us? Bet there are alot of geology trained climbers out there. Not that we all work or use the geologic knowledge we have, but at least we have the interest to know something about the substances we climb on. Go nerds!