What compound(s) make(s) up about3/4s of the earth's crust?
The mineral silica, which is silicon dioxide or SiO2, makes up 60% of the earth's crust. Alumina, which is Al2O3, makes up 15% so these two compounds combined account for about 3/4 of the earth's crust. They are found together in Feldspars, the most abundant group of mineral in the earth's crust.
The second most abundant mineral in the earth's crust, quartz, also has the chemical formula SiO2. It's a component of igneous rock such as granite, as well as metamorphic and sedimentary rocks. It's a major component of sand, from which glass is made. Quartz forms crystals of gemstone quality that are used for jewelry. There are many varieties and colors of quartz, such as amethyst and citrine.
Quartz is a piezoelectric material, meaning it facilitates conversion between vibrations and voltage. It's used in clock and watch movements because it maintains a precise frequency of vibration.