Final answer:
1. Silica (SiO2) is the most common chemical compound in igneous rocks, forming the basis of silicate minerals which constitute about 90% of the Earth's crust.
Step-by-step explanation:
The most common chemical compound in igneous rocks is 1. silica (SiO2). Igneous rocks are primarily composed of silicate minerals, which are structures based around the silicon-oxygen tetrahedron.
Oxygen and silicon are the two most abundant elements in the Earth's crust, with silicates forming about 90% of the Earth's crust by mineral composition. Minerals like quartz, one of the most common minerals in the Earth's crust, are composed of silicon dioxide, or silica.
Additional minerals such as muscovite, orthoclase, plagioclase, biotite, amphibole, pyroxene, and olivine are also found in igneous rocks, but none is as prevalent as silicate-based materials. Given the dominance of silicon and oxygen in the composition of the Earth's crust, and the variety of minerals that include the silicon-oxygen tetrahedron, silica stands out as the most common compound in igneous rocks.