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Magnetite
Magnetite is a type of iron ore that attracts certain metals, such as iron, nickel and cobalt. It is the most common magnetic mineral. The amount and distribution of magnetic minerals within a rock plus the size and shape of the magnetic particles determines the power of the rock's magnetic force. Tiny crystals of magnetite, an iron oxide mineral, can be found in igneous, sedimentary and metamorphic rocks. Lodestone is a form of magnetite that is a natural magnet.
Igneous Rocks
Igneous rocks generally contain at least small amounts of magnetite. Igneous rocks form after volcanic eruptions, when lava cools and becomes solid. During the crystallization of igneous rocks, magnetite forms in the melted lava. These magnetic particles align with the Earth's magnetic field. Basalt and obsidian are two types of igneous rocks.
Sedimentary Rocks
Sedimentary rocks are generally nonmagnetic unless they contain a high proportion of magnetite. Sedimentary rocks form when loose particles called sediment become compacted and harden into layers over time. Sandstone is an example of a sedimentary rock. Sometimes sediment contains magnetite crystals. The magnetite retains its magnetic properties when the sedimentary rock forms and gives it magnetic properties.
Metamorphic Rocks
Metamorphic rocks form when igneous, sedimentary or even other metamorphic rocks undergo physical or chemical alterations. The changes are spurred by intense heat or pressure, or the chemical action of hot liquids or gases. Quartzite and marble are examples of metamorphic rocks. Magnetite can occur in metamorphic rocks formed from sedimentary rocks that contain iron. Magnetite can form during chemical reactions that take place in the formation of metamorphic rocks. The presence of magnetite in metamorphic rocks gives them magnetic properties.