The Strength of Mineral Growth

Salt crystal growth has the power to destroy cinderblocks, roadways and building walls from the inside out.  Surprised well take a look at this:

Figure 1: Sample of Cordoba Cream limestone exposed to Sodium 

Sulfate solution for 47 days. [Image source: G. Scherer]

This cinder block was infiltrated by a sodium sulfate solution through capillary action.    Tiny grains of salt crystals grew both on the inside and outside of this block.  The ones that grew from the internal pore spaces caused all of the damage seen.  The engineers performing this experiment were determining the conditions for which crystal growth will occur within the blocks causing instability and ultimately failure.  As you can see they definitely achieved their goal.  It was determined that the presence of a supersaturated fluid alone would not allow the crystals to continue their growth beyond simply filling the pore spaces of the material.  There must be a disjoining force to prevent the crystal from coming in direct contact with the pore space walls, allowing continued replenishment of the source fluid and thus continued crystal growth.

Figure 2: Views of typical crystallization patterns [Image source: C. Noiriel et al.]

  

So how does this knowledge help a geologist?

            Well directly it doesn’t.  Beyond the immediate crystal growth occurs within pore spaces of material.  But with a deeper look into their study you can find applications for it that may have a great deal of interest for geologists; particularly in the fields of carbon sequestration and possibly even in the oil industry regarding fracking.  In both of these fields instead of allowing a fluid (oil) to rise to the surface we are artificially forcing fluids into a confined environment.  The pore spaces in this environment are not filled with air as one might think instead they are usually filled with a brine solution with a very complex chemistry.  So what happens when this brine solution gets exposed to mixtures that are foreign to the confining pressures of such an environment?  Well that is not fully understood but one scenario that may occur is the dehydration of the brine resulting in crystal growth within pore spaces….  That sounds familiar.

Figure 3: X-ray tomography of sample ADA-1 depicting

internal crystal growth[Image source: C. Noiriel et al.]

            However simply translating the engineers work to a completely different set of conditions is not practical as there are numerous variables that do not match with their study.  One such variable is the confining pressure under which these crystals may grow.  Fortunately for us we have the ability to simulate these pressures here at Maryland.  If crystal growth does occur to the point of cracking the pore space walls could this lead to instability within the reservoir?  Well with any luck we will be able to begin to better understand at least this part of the question.

Image Sources and Further Reading

Scherer, G.W., 2004, Stress from crystallization of salt, Cement and Concrete Research, 34(9): 1613-1624.

Noiriel, C., Renard, F., Doan, M-L., Gratier, J-P., 2010, Intense fracturing and fracture sealing induced 

by mineral growth in porous rocks, Chemical Geology, 269(3-4): 197-209.