What kind of rock/mineral is this?

I found this chunk of some kind of mineral when hiking in the San Rafael Swell in central Utah. The sections that don't have sand stuck in there are completely transparent. It has "layers" that look like they could be flaked off, is very shiny when the light catches it, and has a "grain" to it.

Here's a crappy video of it:

youtube.com/watch?v=fSPE2xi…

And here's a closeup pic:

There have got to be some geologists on here who can identify it, and thanks in advance for satisfying my curiosity.

Crappy video, your right. pic was good though. It looks like Calcite to me.

Looks like mica to me.

Looks like meth or crack!

"It looks like Calcite to me."

Interesting. The only calcite I've seen is the white stuff that's still stuck on cliffs. But some Google-fu shows you might be right.

Here's another pic:

M LaViolette Jr. wrote:Looks like mica to me.

I think you might have nailed it. It definitely has that flaky quality...

Mica. Indicated by the two planes of cleavage.

I would be very surprised to find in situ igneous mica in a sedimentary location. Mica is not a particularly stable mineral and degrades to clay quickly, particularly in a high energy fluvial environment (time of deposition). It also appears that your sample has a third 60/120 deg cleavage.

Calcite is more likely, but I'm thinking gypsum. Does it have a "soapy" feel?

not mica (Muscovite) (one cleavage: {001} perfect )
could be calcite
what happens if you touch it with the tip of you tongue (tasty? sticky?)

I think it looks like selenite, which is a form of gypsum. Gypsum (calcium sulfate) is related to calcite (calcium carbonate).
Mica should have obvious cleavage planes parallel to the surface of the stone, which peel off kind of like the pages of a book. I don't usually find mica this big by itself in desert sand, but selenite is reasonably common.

could be gypsum indeed (hence my question re what happens if you lick it)

Another test is to see if it scratches easily with your fingernail. If it does, chances are it is gypsum.

Andy Hansen wrote:Mica. Indicated by the two planes of cleavage.

+1

That is no mystery. That is mica. Mica is found in several places. However, based on where you were I would say that chances are it peeled out of the Chinle formation. It's that maroonish grey rouned formation you'd see at Indian Creek, real soft. PS Mark I found a book sized chunk of it once. You could litterly turn the pages, rad geology.

Yes, I can scratch it with my fingernail.

It doesn't feel particularly soapy.

It doesn't taste like anything except sand.

I only have a limited knowledge of geology, but I am pretty sure that if your specimen is clear enough, you can put it over some text printed on a page. If it is calcite, the text will appear to be on top of the specimen rather than on the page. Such a good specimen of calcite would probably be cubic, I think...Although the white stuff that is stuck to the side of it could very well be calcite, so it is hard to say.

If if it mica (muscovite), you will most likely be able to peel sheets off of it.

Gypsum my friend.

That's a space peanut

Thats Gypsum, definitely not muscovite

Gypsum. Confirmed by the mineralogist down the hall from me.

Gypsum. No way that is "mica"

Mineralogy was never my favorite course... That being said, I spent time in Utah on a paleontological dig of the Morrison Fm. and there is both calcite and selenite present at varying locations near the swell if my memory serves me well. Doesn't look like Mica to me.

Since calcite has a carbonate ion, you could very easily take a weak acid (vinegar, for example) and place a drop on the sample. If it fizzes (produces carbon dioxide), then your sample is very likely calcite. Testing the hardness is also a viable method. Gypsum (Mohs hardness of 2) is softer than calcite (Mohs hardness of 3) and can be scratched by your fingernail.