View Full Version : Geology Q: Aragonite or Calcite

07/12/2004, 10:46 AM
There is a rock that is found locally (near the town of Aragonite, UT no less :) ) that many local reefers use as base rock in their tanks. As you can tell from the picture, it looks almost "oceanic" which is one of the reasons it is so popular. Rumor has it that it is aragonite in nature. I have tested it for calcium content (by dissolving some in acid and titrating for calcium). According to my calculations it is about 38% Ca (as Ca++) and very little Mg.

So, how do geologists tell the difference between aragonite and calcite? The only thing I've found from a google is that aragonite is an orthorhombic crystal and calcite is trigonal. Are there good examples of each structure ('cause I'm confused, they look similar to me). Also, there doesn't appear to be an "crystal" structure to the rock. It appears amorphous to me (see close-up picture).

(tia, boomer :D)

"Aragonite" rock:

Close-up of rock (sorry, it's a bit fuzzy on the edges):

Randy Holmes-Farley
07/12/2004, 11:19 AM
Boomer is our local rock expert, so I'll defer to him if he disagrees, but I hadn't thought Utah would have any aragonite left. I thought it was unstable with respect to calcite, so that over relatively short geologic time all aragonite becomes calcite, and that Utah has been around long enough for that to have happened.

As to telling them apart, crystallography techniques will certianly work (x-ray, etc), but other methods might be more field and hobbyist friendly. Again. Boom would be the man.

07/12/2004, 11:48 AM
I thought it was unstable with respect to calcite

My understanding too. Kinda the reason I'm asking here :D

and that Utah has been around long enough...

Officially, 1896 :lol:

The rock in question, as near as I can tell, is sediment from an ancient sea (Lake Bonneville). Thousands of years ago it emptied through Idaho leaving the Great Basin of Utah and the Great Salt Lake. If the rock is from Lake Bonneville times that ages it in the thousands of years rather then millions (or even 100,000s).

Randy Holmes-Farley
07/12/2004, 01:03 PM
Oh, OK, if it is a fresh deposit then it might be aragonite. Given that it doesn't look crystalline, it may be hard to tell anything about the clevage planes (which might distinguish the two types), but maybe under a microscope you can see it.

07/12/2004, 04:54 PM
I thought that calcite bubbles if you put acid on it. I have not tried this on agronite. Does agronite bubble also? I thought calcite good clevage along three directions.

Randy Holmes-Farley
07/13/2004, 06:09 AM
Yes, they are both calcium carbonate and will make CO2 when in contact with acid. :)

The cleavage planes could be a good way to distinguish, but if you cannot see crystals becauase of their small size, then you'd need a microscope.

07/13/2004, 09:29 AM

I need to teach you more about Google's search, than rocks :D

Type in, Lake Boneville Aragonite


Ok, now you can read or hours ;)

You too Randy :lol:

Jon & Randy

Todays lesson in mineral chemistry :D


Aragonite is very high compared to Calcite or even Dolomite. It is around 2.94 and Calcite is 2.71. Aragonite will also sink in bromoform while Calcite floats.

You can also do a Meigen's reaction test, a very common test for Aragonite. It gets really violet, not so for Calcite, more colorless to maybe a light blue

Randy Holmes-Farley
07/13/2004, 09:41 AM
And you have bromoform in the fridge? :lol:

07/13/2004, 09:46 AM
Nice looking rock whatever it may be. I seem to remember reading something on the GARF site (Idaho )that they had local areas that contained Aragonite deposits. Maybe similar?

07/13/2004, 12:13 PM
Type in, Lake Boneville Aragonite

Doh! I'm embarrassed that I hadn't done that before. There's lots of info out there. Thanks. I'm still reading. Hopefully I'll come across how this rock is formed... looking at it can you tell? Is it a marl? Back to google I go...

Aragonite is very high compared to Calcite or even Dolomite. It is around 2.94 and Calcite is 2.71.

So I could just crush up some of this rock and weigh it then drop it into a graduated cylinder of water to get it's volume (via displacement)?

Randy Holmes-Farley
07/13/2004, 12:34 PM
The weighing would only work if it was completely nonporous.

07/13/2004, 12:59 PM
The weighing would only work if it was completely nonporous.

You mean the volume measurement would be off not the wieght? If it's ground fine enough I think the water might migrate throughout the particle. I don't know... I'll try it this afternoon :D

07/13/2004, 07:51 PM
So I could just crush up some of this rock and weigh it then drop it into a graduated cylinder of water to get it's volume (via displacement)?


When a body is immersed in water it is buoyed up and weighs less than it does in air. Thus, the loss of the weight is equal to the weight of the water it displaces. You first weigh the sample in air on a pan of delicate balance and then its weight while immersed in water and subtract the second weight from the first. The difference is the weight of equal volume of water. If a sample was 3.5 g in air and 2.1 g in water, the loss of the weight or weight of a volume of water of exactly equal to ;

Sg = 3.5 / (3.5- 2.1) = 2.5

One of my mineral test books says you can also use a Jolly Balance, but have played or seen one other than a pic. ;) You can also use a Beam Balance. I also forgot that Aragonite is harder than Calcite, 3.5 -4 , where Calcite is only 3.

And you have bromoform in the fridge?

:confused: You mean you don't. Just don't get any on you or sniff the stuff :D It is a common material in petroleum geology for mineral separation, when using flasks and separator funnels

OOP's time to go to work, almost late, finish the other one tomorrow Jon. In short tuff is not pumice, tuff is formed from the cementing of ash and pumice is formed by the rapid cooling of a lava that has been ejected into the air. Yes, there are both rhyolitic and basaltic pumice. Pumice floats, looks like a sponge and is very light.

Randy Holmes-Farley
07/14/2004, 05:01 AM
You mean the volume measurement would be off not the wieght? If it's ground fine enough I think the water might migrate throughout the particle.

Maybe, maybe not. Let's see what you get. If it is below 2.5 g/cc, then we'll know it was too porous. :D

07/14/2004, 07:25 AM
Ok, back to pumice and tuff, some things I forgot.

Pumice is not an igneous rock type like Granite or Gabbro, it is an igneous structure, just like the term "lace" (not a true geologic term but a layman term) or porphyritic. When part of a melt is ejected, it often has many trapped gasses, which cause it to swell, producing a "foam like melt". As the gasses escape and the material becomes rapidly cooled, in air, it forms a vesicular glass structure" called pumice. Most but not all pumice floats. Pumice is quite common in the building area, where it is added to walls or chimneys as a more or less decoration.

Tuff is not an igneous rock or igneous rock structure but a sedimentary rock type. It is as I said, a cemented volcanic ash, where the pyroclastic grain size dos not exceed 2mm. Also, don't get tuff confused with tufa

07/14/2004, 08:17 AM
Marl of Lake Bonneville


07/14/2004, 08:36 AM
Maybe, maybe not. Let's see what you get.

I repeated the measurement twice using different samples and got 2.79 g/cc and 2.85 g/cc. But the smallest cylinder I could find was a 25 ml. Kinda big for measuring a volume change of less then one ml (so take my numbers with a grain of salt).

When a body is immersed in water ...

That is ingenius! But I looked everywhere in the lab for a waterproof balance with no luck :D . The lab supervisor looked at me with that crazed dog glare when I asked him if he though his Mettler-Toledo would work in my bathtub :lol:

bromoform... It is a common material in petroleum geology

You'd think with a group of oil exploration geologists at the other end of the building, we might have some that laying around, but we don't. Nobody does anything with their hands anymore. It's all about staring at a computer screen now :(


07/14/2004, 09:02 AM
That is ingenius!

Well, it should be

Dana's Minerals and How to Study Them, 4th ed, 1998 Hulburt & Sharp. I'm sure you have heard of Dana's famous book; Dana's Manual of Mineralogy. When I broguht this book (How to) to Dr. John C. Green's attenion (UMD geology prof), about 5 years ago, he about sh^it his pants and wanted to know why Wiley's never informed him. JC, as we call him, is another Harvard boy, Magna cum laud, Igeneous Petrology. Randy, at that time, was still a dream in his father's eyes :lol:

2.79 g/cc and 2.85 g/cc


07/14/2004, 01:53 PM
Ok, please bare with me for at least one more question, boomer...

Here a couple more pictures of this rock. It is found in a sedimentary deposit in the side of the hills. The upper side (see picture in my first post) is completely filled with sand and the sand layer extends about 1-1.5 ft above the "live rock" layer. Directly below the "live rock" layer is a thin (1/2 inch thick) layer of very dense "concrete". I'm pretty sure this is also CaCO3. Below this layer is a layer of "river rock" about 1 ft thick composed of smooth rocks 1 inch to 3 inches in size. I've included pictures for clarification:

Sediment layers:

Bottom view:

Boomer, from this description does it appear to be a marl or tufa?

And am I thinking correctly that a marl is a carbonate (mostly calcium) sediment formed when carbonates precipitate from a body of water and fall to the bottom while a tufa is a carbonate (again mostly calcium) deposit formed within a lake (under water) where a spring introduces water rich in either carbonates or calcium causing a precipitate that then grows on it's self (kind of like an underwater stalagmite?).

07/14/2004, 07:16 PM
Tufa, not maral, BUT.

Marl will not have these types of growth structures. Marl is a cemented ooze of CaCO3 and clay. What you have shown here is a common sequence stratigraphy of a lake evaporate shoreline deposite, which may have Tufa or Travertine. Below your thumb is Breccia. I should of looked at your rocks a little closer when you first posted them rather than rambling on about Aragonite. I don't think this is Tufa, but Travertine or somewhere between them. Looking at you first pick and the close-up it dose not look porous enough to be Tufa and bears a more liking to Travertine. The are exactly the same thing. Tufa is Travertine when it is very porous, with many open structures like a sponge or "pumice" like appearance

Travertine of Lake Bonneville


And am I thinking correctly

Yes, that is pretty much correct

You would have a field day reading Chapter 4 of my new book.

Carbonate Sedimentology, Ch 4, Swallow-Water and Lacustrrine Carbonates

What takes place here is not an easy subject. The chemistry and what you get are very complex, depending on x, y, or z. Just this one chapter is 225 pages :eek2:

If you haven't figure this out yet, the geology and chemistry of Lake Bonneville is a very complex subject. This lake has gone dry and refilled like 28 times. Each time forming evaporite deposits

07/14/2004, 10:57 PM
Thanks boomer. I've really enjoyed my geology lessons!

To bring this topic back to reefing :D about 15 miles from this rock is dune after dune of oolitic sand GSL oolitic sand (http://ugs.utah.gov/utahgeo/rockmineral/collecting/oolitic.htm)


I don't have to travel to the tropics to scuba with full grown angel fish and 5 ft long sharks! Bonneville Seabase (http://www.seabase.net/index.html)

07/15/2004, 07:00 AM
dune of oolitic sand

Yes, while checking through things for you I saw this a couple of times. GSL has all kinds of varieties of sediments, most are evaporites. I might add this is where Morton mines their salt from :D Even some of your National Parks down there are a function of GSL. Utah is one of my favorite places and is where UMD has their Geology Filed Camp ;)

tropics to scuba

That looks like a cool place :D

07/15/2004, 09:31 AM
Boomer, is there a geology book (or books) you could suggest for someone who isn't a geologist but has more then a passing interest in the topic? Something along the lines of a good undergrad textbook.

That looks like a cool place

It's down right amazing when you think about it. Out in the desert of Utah there's a spring (warm water no less) filling a couple ponds with salt water suitable for keeping reef fish year round! Unfortunately the water chemistry (or ?) won't support corals :(

07/15/2004, 10:51 AM
Sure, I have many of them. You may need two, as you stated, as intro geology is divided up into two aeras. A physical geology (, rocks, minerals, rivers, glaciers, mountains, etc.) book and a historical geology ( the history of the earth, with fossils and dinosaurs, the great ice age etc.) book


The Earths Dynamic Systems by Hamblin. They best one I have ever seen> get the new 10th addition


The Earth Through Time by Levin. The best there is.

For years these two are the #1 selling books in their field. They are always rated as the best teaching books there are. Mine are older editions.

07/15/2004, 01:02 PM
Thanks for all the book recommendations!

If you're looking to upgrade your copy of Hamblin's book, a used 9th edition can be had from Amazon.com for under $4! One's on it's way to my house :D

07/15/2004, 07:02 PM
I have a very nasty habit Jon:mad: I must buy new if at all possible:rolleyes:

07/24/2004, 01:16 PM
you could also try sedimentary geology by Prothero. I used it in my sedimentology and stratigraphy course, and it is very detailed and useful especially if thats your field of interest. Hard rocks suck!!!

07/24/2004, 04:40 PM
Thanks jashort!

Kinda funny. My copy of Earth's Dynamic Systems came last week. I'm thumbing through it when my wife sees it. She walks down stairs, pulls out a box of her old college texts and produces her own copy :lol: I should have consulted her first...

07/24/2004, 05:27 PM
that may be good enough for you if you dont want to get into extreme detail. that book is pretty good and its what i used in physical geology. there arent enough people interested in geology, its always to see that there is another geonerd out there.

07/25/2004, 12:12 AM
sedimentary geology by Prothero.


I have that book also. I also have

Principles of Sedimentology and Stratigraphy by Boggs. But I like Prothero better.

pulls out a box of her old college texts and produces her own copy

:rollface: :rollface: :rollface: :rollface:

Dont' feel bad though, I did the same thing two years ago with a invert book and it was the same edition :lol:

07/25/2004, 06:46 AM
boomer, you sound like a prof. if so, what university do you teach at?

07/25/2004, 11:19 PM
boomer, you sound like a prof. if so, what university do you teach at

:rollface: :rollface: :rollface: :rollface: :rollface:

I work at the University of Open Pit Iron Ore Minning (OPIOM)

My old geology prof and mentor, Dr. Richard W. Ojakangas would get a chrage out of this. with a "well yes, he should be, if he would just go back and finish is degree "He still says I'm not to old :lol: and wants me to take his place at UMD. He is one of the "big guns" in the field of sedimentary petrology, mostly clastic sediments, my area also.

Here is me at work;


07/26/2004, 04:30 AM
i understand that, i have to make a decision here pretty soon about grad school. it would be cool to go to ut austin and do stratigraphy but i dont think thats going to happen

04/21/2007, 03:09 PM
Boomer's situation is exactly the reason why I'm force-feeding myself grad school at this time... When I'm done I'll never go back. :D

This has been an extremely informative thread. Thanks Randy, Jon, and Professor Boomer, sir. :p

04/21/2007, 05:44 PM
You bet six and thanks :D It is nice to see to an old thread pop-up ever now and then.