An One of the three basic types of rock that also include sedimentary and metamorphic. Igneous rocks are formed by the solidification of magma and comprised predominately of silicate minerals. Based on bulk chemical analysis, igneous rocks can be grouped into four major groups based on their SiO2 content: 1. Felsic: Click on Term to Read More consisting mainly of the minerals Composed of SiO2, quartz is one of the silica group minerals most common in Earth's crust, but never found in meteorites as inclusions visible to the naked eye. Quartz in meteorites has been found in very small quantities in eucrites, other calcium-rich achondrites, and in the highly reduced E chondrites1. Click on Term to Read More and Variety of feldspar containing alkali metals potassium and sodium in a solid solution. A more complete explanation can be found on the feldspar group page that also includes the plagioclase feldspars as part of the feldspar ternary diagram. Click on Term to Read More.