 | CaO.(Mg,Fe)O.2SiO2 |
| (Mg,Fe)O.(Al,Fe)2O3.SiO2. |
Assuming Mg and Fe to be equal in amount in the first half of the above formula, and Mg and Fe to be equal in the first part of the second half, and Al and Fe to be equal in the last part of the second half, doubling the whole and allowing it to be acted on by CO2 and H2O, we have
2CaO.2MgO.2FeO.Al2O3.Fe2O3.6SiO2 + 6CO2 + 2H2O
= 2CaCO3 + 2MgCO3 + 2H2O.Al2O3.2SiO2 + 2FeCO3 + Fe2O3 + 4SiO2.
The hydrous silicate of the last part of the equation is kaolin.
The composition of labradorite is represented by the formula
| CaO.Al2O3.2SiO2 |
| Na2O.Al2O3.6SiO2. |
Assuming the two molecules represented by this formula to be equally abundant, and allowing the whole to be acted on by H2O and CO2, we have
CaO.Na2O.2Al2O3.8SiO2 + 4H2O + 2CO2
= CaCO3 + Na2CO3 + 2(2H2O.Al2O3.2SiO2) + 4SiO2.
When waters charged with carbonates descend into the earth they are likely to precipitate a portion of their burden, forming calcite and other crystalline carbonates, and hence these are among the most common minerals found in veins and rock cavities. Carbonates are also deposited when carbonate-charged waters come to the surface and evaporate or lose a part of their carbon dioxide.
Decarbonation also takes place, but it is, at least at the surface, a much less common process, and its conditions are less well understood. Sufficiently high heat will drive off the carbon dioxide, as in the artificial process of burning lime, but this is rarely observed in nature. Even lava intrusions do not usually reduce limestone to caustic lime at any appreciable distance from the contact. It is believed, however, that in the deeper zones, where high pressure and heat prevail, carbonates are changed into silicates, thus in a way reversing the process that prevails at the surface, and setting free again a portion of the carbon dioxide that had become locked up in the formation of the carbonates. To this action some of the carbon dioxide of deep-seated thermal springs is assigned.