Modes of Chemical Weathering

This post was "dropped in" to help clarify the terms in past and upcoming posts in regard to chemical weathering of minerals.

To revisit Physical Weathering (aka Mechanical Weathering) briefly:

Modes of Physical Weathering include:

• Ice Wedging (water held within fractures and micro-fractures expands during freezing).
• Thermal Expansion and Contraction - Diurnal and Seasonal.  [There is (or has been) some disagreement over the relative importance of Diurnal Expansion and Contraction.]
• Exfoliation Jointing and Tectonic Jointing and Fracturing (aka Brittle Deformation).
• Root Wedging.
• Abrasion by wind or water.
• Impact Fracturing (aka Percussion) in streams or landslide events.  (May include human-caused impacts.)

A common "result" of these Physical Weathering Modes is that the rock's chemistry does not change, it is just broken into smaller pieces, yielding more surface area.  Chemical Weathering results in actual changes in chemistry due to Oxidation, Dissolution, and Hydrolysis.  In some cases, depending on the water chemistry, any two or all three of the processes may occur simultaneously. 

The stability of minerals - as solid, naturally-occurring chemicals - is largely related to the strength of their internal chemical bonds, whether Ionic, Covalent or a combination of the two.  Some minerals have both.  When a compound is composed of a metallic cation and a polyatomic anion, the compound is considered to be ionic, though there might be covalent bonds within the polyatomic anion.

The Polarity of Water molecules (one side positively-charged, the other side negatively-charged) is responsible for many of Water's interesting properties, including water acting as a "carrier" of ions (positive and negative).  

Under natural conditions, water can combine with atmospheric gases producing natural acids such as Carbonic, Nitrous, and Sulfurous (some of these may be enhanced by gases within soils as rainwater percolates downward).  Human activities can add additional acids to the mix.

Figure 1.

Figure 2.

The most ubiquitous form of Oxidation that we see is Iron Oxidation, whether it is of iron-bearing Oxides and Silicates in Diabase (Basalt), of the iron-bearing steel of old cars or old beer cans, or of iron-bearing Sufides, e.g., Pyrite.

Dissolution is the "dissolving" of certain minerals, most easily seen with the halides which have weak Ionic Bonds (NaCl - halite, table salt easily dissolves in water).  In the case of calcium carbonate (Calcite in Limestone) and calcium sulfate (Gypsum in Evaporites), the ionic bond between the Calcium and the polyatomic anion is fairly-easily broken by the combination of water and naturally-occuring acids. 

 Figure 3.

Figure 4.

When we think of caverns, we often think about structures such as stalagmites, stalactites, columns, and such.  These are actually secondary depositional structures, not Dissolution Features.  As pictured above, Sculpted Ceilings are a good subsurface example of Dissolution.  

In the early stages of Karst development, the caverns begin as exposed joints, fractures, and bedding planes in limestone and marble (and rarely gypsum) - below the Water Table.  Dissolution slowly takes place as the water replenished by more-acidic percolated groundwater, following surface precipitation events.  

Where the Water Table is "tied" to exposed surface features, e.g., nearby creek and river valleys, as down-cutting occurs, the underground Water Table is "dragged down", exposing the cavern ceiling, while the cavern floor dissolution continues.  

(It is after this subterranean exposure that the stalactites begin to grow.  But that's another story.)

Figure 5.

The hand-sized sample of Potassium Feldspar above shows the effects of Hydrolysis (here termed Kaolinization), as seen by the chalky appearance.  As you can see traces of the original Feldspar Cleavage, the Kaolinization is not yet complete.

Except for Quartz, every Silicate Mineral in the Bowen Reaction Series is susceptible to Hydrolysis.  As related previously, the minerals near the "top" of the Bowen Reaction Series (as with the presumed aphanitic Olivine, Pyroxene, and Plagiolcase in the Diabase) are more susceptible to the effects of Chemical Weathering than is the K Feldspar (near the bottom). 

[Below the surface, Quartz is soluble in certain circumstances, but "that's another story", too.]