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What kind of rock/mineral is this?

Original Post
Tristan Higbee · · Pocatello, ID · Joined Mar 2008 · Points: 2,970

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:

Mystery mineral

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

Double J · · Sandy, UT · Joined Apr 2006 · Points: 3,931

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

M LaViolette Jr · · The Past · Joined Oct 2011 · Points: 448

Looks like mica to me.

Taylor-B. · · Valdez, AK · Joined Oct 2009 · Points: 3,186

Looks like meth or crack!

Tristan Higbee · · Pocatello, ID · Joined Mar 2008 · Points: 2,970

"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:

Mystery mineral 2

Tristan Higbee · · Pocatello, ID · Joined Mar 2008 · Points: 2,970
M LaViolette Jr. wrote:Looks like mica to me.
I think you might have nailed it. It definitely has that flaky quality...
Andy Hansen · · Longmont, CO · Joined Sep 2009 · Points: 3,130

Mica. Indicated by the two planes of cleavage.

Ryan Marsters · · Golden, CO · Joined Jan 2011 · Points: 1,431

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?

proto G · · Falmouth (MA) · Joined Jan 2009 · Points: 213

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

Mark E Dixon · · Possunt, nec posse videntur · Joined Nov 2007 · Points: 974

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.

proto G · · Falmouth (MA) · Joined Jan 2009 · Points: 213

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

Greg Gavin · · SLC, UT · Joined Oct 2008 · Points: 888

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

Chris Schenhoffer · · Prescott, AZ · Joined May 2010 · Points: 5
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.
Tristan Higbee · · Pocatello, ID · Joined Mar 2008 · Points: 2,970

Yes, I can scratch it with my fingernail.

It doesn't feel particularly soapy.

It doesn't taste like anything except sand.

jack s. · · Kamloops, BC · Joined Jan 2011 · Points: 10

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.

Christian "crisco" Burrell · · PG, Utah · Joined May 2007 · Points: 1,815

Gypsum my friend.

jaypg · · New England · Joined Aug 2009 · Points: 10

That's a space peanut

knowbuddy Buddy · · Unknown Hometown · Joined Jul 2010 · Points: 225

Thats Gypsum, definitely not muscovite

JohnnyG · · Unknown Hometown · Joined Nov 2009 · Points: 10

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

Andrew Heger · · conifer, colorado · Joined Oct 2009 · Points: 15

Gypsum. No way that is "mica"

J. Amundsen · · Unknown Hometown · Joined Jul 2011 · Points: 70

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.

Guideline #1: Don't be a jerk.

General Climbing
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