In a granitic rock composed of amphibole, biotite, albite, andesine, and quartz, which mineral would weather first?

• A. Amphibole
• B. Quartz
• C. Andesine
• D. Biotite

Answer: (C)

Weathering rates depend on how chemically stable a mineral is under surface conditions. Quartz is exceptionally resistant because of its strong silicon-oxygen framework, so it tends to weather last. Feldspars, on the other hand, are more susceptible to chemical attack and break down to clay minerals and dissolved ions.

Within the feldspars, calcium-rich plagioclase components weather more quickly than sodium-rich ones because the larger Ca2+ ions distort the crystal structure and make hydrolysis easier. Andesine is a calcium-rich member of the plagioclase series, so it would start to weather before albite and before the hydrous minerals in the mix.

Biotite and amphibole are hydrous silicates that weather, but their weathering typically lags behind the initial breakdown of feldspars in many environments. Therefore andesine would weather first in this granite.