magnetite - teaching student specimens of this important iron ore - unit of 10 smaller specimens
Magnetite chemical formula: Fe3O4
Magnetite is a common ore of iron. It can form as magmatic segregations in igneous bodies. At Iron Mountain, Utah, an intrusion penetrated the Homestake Limestone of the Carmel Formation roughly 20-22 million years ago, doming up the rocks above it and eventually solidifying to form a laccolith. As magnetite crystallized it dropped to the bottom of the magma chamber.
A magmatic segregation of this type resulted in the largest magnetite deposits in the world, at Kiruna and Gellivare, Sweden.
At Iron Mountain, subsequent reheating melted the magnetite and pushed it up into fractures and joints above.
Magnetite also forms the black sands of beaches, is a constituent in many igneous rocks, and is commonly associated with rocks rich in iron and magnesium such as diorite, gabbro and peridotite. It is the only mineral strongly attracted to a magnet. According to Pliny, its name comes from the shepherd, Magnes, who noticed that the nails in his shoes and the iron ferrule of his staff adhered to the ground on Mount Ida, a neat fable.
But why is magnetite a magnet? Electricity causes magnetic fields just as magnetism causes electric currents. Magnetite is a member of the spinel group which has a standard formula A(B)2O4 The A and B usually represent different metal ions that occupy specific sites in the crystal structure. In the case of magnetite, Fe3O4, the A metal is Fe+2 and the B metal is Fe+3; two different metal ions in two specific sites. This arrangement causes a transfer of electrons between the different ions in a structured path or vector. This vectored electric transfer generates the magnetic field.
Since both the A metal and B metal are Fe, A(B)2 gives Fe3 in the formula, since the atoms are added.
These are smaller than our normal student specimens, but definitely not the tiny fingernail size we don't like. When a mineral is the same all the way through and doesn't show a texture like an igneous rock, smaller specimens are an economical alternative. Often, when we are trimming in the field, we get decent smaller specimens like these.
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