Rocks are composed

Rocks are composed primarily of grains of minerals, which are crystalline solids formed from atoms chemical bonded into an orderly structure.[3]:3 Some rocks also contain mineraloids, which are rigid, mineral-like substances, such as volcanic glass,[4]:55,79 that lacks crystalline structure. The types and abundance of minerals in a rock are determined by the manner in which it was formed.

Most rocks contain silicate minerals, compounds that include silica tetrahedra in their crystal lattice, and account for about one-third of all known mineral species and about 95% of the earth’s crust.[5] The proportion of silica in rocks and minerals is a major factor in determining their names and properties.[6]

Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks.[4] Over the course of time, rocks can be transformed from one type into another, as described by a geological model called the rock cycle. This transformation produces three general classes of rock: igneous, sedimentary and metamorphic.