It's not entirely accurate to say that gabbro has a higher chemical weathering rate than granite. While both rocks can be susceptible to chemical weathering, the rate and dominant weathering processes vary depending on factors like:

* Mineral composition: Gabbro is primarily composed of pyroxene and plagioclase feldspar, while granite is made up of quartz, feldspar, and mica. The feldspars in both rocks are susceptible to chemical weathering, but pyroxene is generally more easily weathered than quartz.

* Climate: Warm, humid climates promote faster chemical weathering due to the presence of water and organic acids.

* Surface area: Fractured or highly porous rocks have a larger surface area exposed to weathering agents, leading to faster rates.

* Presence of other factors: Presence of vegetation, organic acids, and other chemicals can influence the rate of weathering.

Here's why gabbro might appear to weather faster than granite in some cases:

* Higher proportion of easily weathered minerals: Gabbro has a higher percentage of pyroxene compared to granite, which is more susceptible to chemical weathering.

* More susceptible to frost wedging: Gabbro is often more coarsely crystalline than granite, leading to larger crystals and more potential for frost wedging, which can accelerate weathering.

However, there are also cases where granite might weather faster than gabbro:

* Higher quartz content: Quartz is very resistant to chemical weathering. Granite's higher quartz content can make it more durable overall.

* Presence of specific minerals: Some granite varieties contain more easily weathered minerals like biotite mica, which can contribute to faster weathering.

In conclusion, it's not a simple case of one rock always weathering faster than another. The specific weathering rates of gabbro and granite depend on a combination of factors and can vary significantly in different environments.