Bat Cave Home Page

GENERAL 

How to get access to the Cave
How to be a Good Guest
Where is Bat Cave?

The AREA 

The Geologic Time Scale 
Area rock Layers
Topography 
Geomorphology
Geological History 
           The Paleozoic
           The Mesozoic
           The Cenozoic
Water
The Hydrologic Cycle
Solution
Solution chemistry
Karst Landscapes
Erosional Features 
Depositional Features
Environmental Issues 

BAT CAVE

What is a cave?
How was Bat Cave formed?
Surface Plan of the site
Map of the Cave
Life in and around Bat Cave
A Virtual Trip Through Bat Cave

TEST YOUR KNOWLEDGE

A Quiz

Bat Cave Home Page

 LET'S TALK SOME CHEMISTRY
Calcium carbonate comes in two different minerals: aragonite and calcite. These two minerals have the same kind of atoms in them, CaCO3  (meaning 1atom of Calcium, Ca; one of Carbon, C; and 3 of Oxygen, O) but the atoms that they are made of are linked together differently (internal crystalline structure). Most of the original limestone deposits were made of aragonite, which is less stable than calcite. Usually, aragonite will change to calcite over time, or this aragonite may get dissolved. Another change that happens commonly is that both calcite and aragonite will get replaced by dolomite [CaMg (CO3)2] and silica (SiO2). 
How and why solution happens
Simply stated, acidic waters dissolve carbonates. How do these natural waters become acidic?
As water comes in contact with CO2 in the atmosphere and in soils, they combine to make carbonic acid: 
CO2(gas)+H2O <---> H2CO3 (carbonic acid) or more correctly, because carbonic acid dissociates in water
CO2(gas)+ H2O <--->H3O+ (hydronium ion, the source of acidity) + HCO3- (bicarbonate ion)
Carbonic acid is a weak acid.  Given an atmospheric concentration of CO2 of 10-3.5 atmospheres, the expected acidity of rainwater is 5.5. But in realty, it is 4.8.  That's because CO2 in the atmosphere is not the only culprit. Air pollutants such as sulfur and nitrogen oxides (produced by human and natural activity such as volcanos) also combine with water to form sulfuric and nitric acids and therefore add to the acidity of precipitation, i.e. lower its pH. As people continue to increase the amount of these oxides that they produce, it is expected that the pH of precipitation will continue to lower (meaning precipitation will become more acid), significantly increasing weathering rates over those we would expect to occur "naturally". 

Additional acidity comes from organic activity in soils. As microorganisms metabolize organic materials, they form CO2 as a by product of  metabolism. CO2 can also be produced by inorganic oxidation of organic materials. Partial pressure of CO2 in those environments can be as high as 10-2 atmospheres (up to 50 times higher than in the atmosphere). As rainwater infiltrates the soil, it reacts with this soil CO2 and the pH decreases again. In addition, there are dissolved organic acids in the soil (e.g. tannic acid) which also contribute to acidity.  The end result of all of these acid producing reactions is that the pH of waters in the soil or in the surficial aquifer is commonly between 3 and 5. 

Carbonates are especially sensitive to acidic solutions.  When acidic water reaches limestone, the following reaction takes place: 
CaCO3 + H2CO3<---> Ca+2 + 2HCO3-
As the Hydronium ions are "used up" in the reaction, the carbonates dissolve away and the pH rises to between 7 and 7.5. This leaves a void where the carbonate molecules used to be and the more insoluble (non-carbonate) parts of the layer are left as a residue. This residue is often rich in phosphatic or silicate materials such as clay, sand, silt, and usually rich in aluminum and ferric iron.