Final answer:
Quartz, primarily composed of silicon dioxide, is a prevalent and incredibly hard mineral found in various rock types. Metamorphosed quartzose sandstones or siltstones become quartzite, which is notable for its crystal texture and lack of reaction to acid.
Step-by-step explanation:
Quartz is a mineral consisting of silicon dioxide (SiO2) and is known for its hardness and resistance to weathering. It is one of the most common minerals in the Earth's crust and can be found in various rock types. Quartz commonly has a non-metallic luster and presents a six-sided crystal form.
In contrast, a mix of quartz, feldspar, and rock fragments can show different characteristics, such as being dark, smooth, or having a brittle nature that causes it to break easily. Some quartz compounds, such as those in welded quartz sandstone, may react to hydrochloric acid (HCl), indicating the presence of carbonate materials alongside quartz.