Dear Dr. Finke,

The Mohs´s hardness number, as you have already indicated, does not really reflect the vulnerability of minerals in terms of supergene and hypogene alteration. I have attached some charts which may give you an idea how the vulnerability of minerals can be put in order as to their stabilities. The resistance to weathering strongly depends upon the chemical composition, e.g., Fe/Mg ratios, solubility of minerals, e.g. phosphates and the morphology of mineral grains.

Even if you use the same sort of mineral such as pyrite you will find a close relationship between the type of crystal morphology and resistance to weathering. A complex intergrowth of pyrite or a multi-facetted habit is less resistant than a simple hexahedron.

DILL, H. G. and KEMPER, E. (1990) Crystallographic and chemical variations during pyritization in the Upper Barremian and Lower Aptian dark claystones from the Lower Saxonian Basin (NW Germany). - Sedimentology 37: 427-443.

I have not tested it by myself in terms of classification of minerals as to their vulnerability of weathering but I would suggest making some tests. It is the Vickers's Hardness Number, which is used in combination with the reflectance of minerals. I found that the VHN often strongly varies as the minerals show any defects or corrosions by whatever reasons so that their diagnostic features are totally blurred. It should work in your case.

My guess is stimulated by another device called the Schmidt Hammer which is used to test the rock strength in the field and is also very sensitive to the degree of weathering. I used it for carbonate rocks.

These methods have another edge over the Mohs´s classification scheme, because you obtain numbers which you can compare with other characters and the reproducibility of the data is also fine.

These methods are done on polished sections under reflected light and you can do a lot of counts for statistical treatments

For the moment I think it show get you on the right scent to your goal.

Best regards

Harald G. Dill

Dear Dr Towe and Dr. Dill,

Thank you both for your time and answers. I recognize that any quantification of the vulnerability of mineral grains to physical weathering is uncertain, especially because not any grain has the same crystal structure/mineralogical composition.

As I am modeling over pedogenetic timescales (millenniums) I cannot assume texture of unconsolidated material constant. So far I have modeled it as a probabilistic process whereby the probability of any grain to break into smaller grains is a function of temperature change over time. I found that such process can be calibrated fairly well for real sediments. I also think, however, that grains of quartz and of muscovite (as examples) cannot be expected to disintegrate at the same rate (as Xray analyses show when different particle size fractions are compared). This is why I would like to add mineral type as factor in the equation (the "downstream" remark by Dr Towe).

I'll do some more reading on VHN!

Thanks,

Peter Finke

May be here you can find out some answers:

https://www.researchgate.net/publication/240445318_Mechanical_abrasion_of_kimberlite_indicator_minerals_experimental_investigations

Article Mechanical abrasion of kimberlite indicator minerals: experi...

Surely, most of the minerals are silicates and their fate during weathering depends on their temperature of formation. The high-temp. minerals (like olivine) are first to disintegrate and the opposite for the low-temp. minerals (like quartz)

In natural environment the physical weathering of minerals is depend of chemical stability to organic acids. The considering only mechanical weathering (abrasion) is not be correct. The influence of organic acids on silicate minerals is more significant than other acids. For example, olivine is very resistant to mechanical abrasion but fast but quickly collapses under the influence of organic acids in the natural environment.

Dear dr Nikolenko

you are right, but I am looking for non-empirical descriptions of physical weathering right now. The "chemical" weathering is much better understood, and we want to model both mechanistically.

best,

PF