How does the mineral composition of granite affect its resistance to chemical weathering?

The resistance of granite to chemical weathering is fundamentally determined by the stability of its primary mineral constituents. Granite is primarily composed of quartz, feldspars (both alkali and plagioclase), and micas.

Quartz is highly resistant to chemical weathering due to its stable silicate structure and lack of cleavage planes. It remains largely unaffected by weak acids and water, often persisting as sand-sized particles long after other minerals have weathered away. Consequently, granite with a higher quartz content generally exhibits greater overall durability.

In contrast, feldspar minerals are significantly more susceptible to chemical attack. They undergo hydrolysis, a reaction with weakly acidic water (often containing carbonic acid). This process breaks down feldspars into clay minerals such as kaolinite, which are soft and easily eroded. Potassium feldspar (orthoclase) weathers at a slower rate than plagioclase feldspar, which contains calcium and sodium.

Mica minerals, particularly biotite, also contribute to weathering susceptibility. Biotite undergoes oxidation and hydrolysis, forming iron oxides and clay minerals. This process can create expansion pressures within the rock, facilitating further physical breakdown.

The interlocking crystal structure of granite initially provides some protection, as it limits the surface area exposed to chemical agents. However, once weathering begins along fractures and crystal boundaries, it proceeds more rapidly in zones rich in feldspar and mica. The resulting clay products can further trap moisture and promote continued degradation. Therefore, the variable mineral composition not only dictates the initial resistance but also controls the pattern and rate of chemical weathering in granite formations.