Final answer:
The rocks granite and gneiss both contain the mineral quartz. Granite is an igneous rock with grains of quartz visible among other minerals, while gneiss is a metamorphic rock with layers or bands that include quartz.
Step-by-step explanation:
The two rocks that contain the mineral quartz are granite and gneiss.
Granite is a common type of igneous rock that is visibly granular in texture. It is composed of several minerals, including quartz, which can be identified by its clear to smoky, glassy grains with no cleavage. The quartz in granite occurs as irregular grains alongside other minerals such as K-feldspar, plagioclase feldspar, and biotite, as shown in Figure 2.1, where quartz is represented as clear minerals.
Gneiss, on the other hand, is a metamorphic rock that has undergone higher grades of metamorphism. It is characterized by its banded or foliated appearance and can also include significant amounts of quartz. In gneiss, quartz is present together with other metamorphic minerals such as feldspar and mica. The original sand or silt grains from the precursor rock, which contribute to the formation of gneiss, may have been composed largely of quartz, as shown in figure 2.1, where gneiss contains clear minerals resembling quartz.
Furthermore, quartzite, another type of metamorphic rock that evolves from the metamorphism of quartz-rich sandstone, is predominantly composed of interlocking quartz crystals (Figure 4). While not directly mentioned in the question, it serves as another example of a rock with a high quartz content.
Quartz is a mineral consisting of SiO2 and silicon dioxide, which is found in all rock types and is the stable polymorph at the Earth's surface. It is recognized by its non-metallic luster and six-sided crystal form, as illustrated in Figure 2.4b.