1. Mineralogical Composition:
The hardness of igneous rocks is largely influenced by the mineralogical composition. Minerals that are harder on the Mohs scale of hardness, such as quartz, feldspar, and pyroxenes, contribute to a harder igneous rock. Minerals like olivine and micas are generally softer. The relative abundance and distribution of these minerals within the rock affect the overall hardness.
2. Grain Size:
Grain size plays a role in determining the hardness of igneous rocks. Rocks with a fine-grained texture (aphanitic) tend to be harder than those with a coarse-grained texture (phaneritic). This is because finer-grained rocks have more interlocking mineral crystals, resulting in a more cohesive and denser structure.
3. Alteration:
Secondary alteration processes can affect the hardness of igneous rocks. Weathering, hydrothermal alteration, and chemical reactions with groundwater or other fluids can cause the breakdown of minerals and the formation of softer alteration products. This can lead to a decrease in the overall hardness of the rock.
Based on these factors, some common igneous rocks and their hardness ranges are:
- Granite (hard): Mohs hardness of 6-7
- Basalt (hard): Mohs hardness of 5-6
- Obsidian (hard): Mohs hardness of 5-6
- Pumice (soft): Mohs hardness of 5-6 (but can be friable and easily abraded)
- Peridotite (hard): Mohs hardness of 5-7
It's worth noting that hardness can vary within the same rock type due to variations in mineral composition and other factors. Additionally, the hardness of a rock can also be affected by long-term exposure to weathering and erosion processes.
