Geology Q: Aragonite or Calcite

[jfinch]

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 )


"Aragonite" rock:

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

 

[Randy Holmes-Farley]

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.

 

[jfinch]

I thought it was unstable with respect to calcite

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

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]

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.

 

[StephenS]

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]

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.

 

[Boomer]

Jon

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

Type in, Lake Boneville Aragonite

http://www.google.com/search?q=Lake+Bonneville+aragonite&sourceid=opera&num=0&ie=utf-8&oe=utf-8

Ok, now you can read or hours


You too Randy :lol:


Jon & Randy

Todays lesson in mineral chemistry

MEASURE ITS SPECIFIC GRAVITY

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]

And you have bromoform in the fridge? :lol:

 

[Hobster]

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?

 

[jfinch]

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]

The weighing would only work if it was completely nonporous.

 

[jfinch]

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

 

[Boomer]

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)?

Yes

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?

You mean you don't. Just don't get any on you or sniff the stuff 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]

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.

 

[Boomer]

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

 

[Boomer]

Marl of Lake Bonneville

http://www.google.com/search?hl=en&ie=UTF-8&q=marl+of+Lake+Bonneville&btnG=Google+Search

 

[jfinch]

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 . 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

Thanks.

 

[Boomer]

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

Aragonite

 

[jfinch]

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?).

 

[Boomer]

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

http://www.google.com/search?hl=en&ie=UTF-8&q=Travertine+of+lake+bonneville&spell=1


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