Quartz is a common mineral found in both igneous and sedimentary rocks due to its stability and wide range of formation conditions. Here are two scenarios explaining how quartz can be present in both rock types:

1. Igneous Rock Formation:

- Quartz is a primary mineral that can crystallize from molten magma during the formation of igneous rocks. For example, in granitic rocks such as granite or rhyolite, quartz is a major component, along with feldspar and mica.

- Quartz can also form as a secondary mineral in igneous rocks through hydrothermal alteration. Hot, water-rich fluids can react with the minerals in the rock, altering their composition and forming new minerals, including quartz.

2. Sedimentary Rock Formation:

- Quartz is a very resistant mineral and is not easily weathered or broken down by natural processes. As a result, it can be transported over long distances by wind, water, or ice.

- Quartz-rich sediments, such as sand or sandstone, can accumulate in sedimentary basins. Over time, these sediments can be compacted and cemented together to form sedimentary rocks like sandstone or quartzite.

- Quartz can also be present in sedimentary rocks as a result of the weathering and erosion of pre-existing igneous or metamorphic rocks containing quartz.

Therefore, the presence of quartz in both igneous and sedimentary rocks can be attributed to its stability, multiple modes of formation, and resistance to weathering.