Magnetite

Mineral name: Magnetite:

Chemical Composition: Fe2+Fe3+2O4

Color: Black

Streak: Black

Hardness: 6

Cleavage: There is no cleavage. Magnetite may show some parting, which is a break along structural planes and is parallel to a possible face, just like cleavage.

Fracture: Magnetite has a Sub-Conchoidal Fracture, meaning it’s smooth with irregular rounded corners. There can also be breakage that resembles concentric ripples. These fractures do not follow any natural planes of separation.

Crystal Form: Magnetite has an isometric crystal form, meaning that the axes of the crystals are equal in all 3 dimensions and intersect at 90⁰. The most common shapes in magnetite are octahedral (eight sided polyhedron) and the least common is dodecahedral (twelve sided polyhedron). Crystals may have a combination of octahedral and dodecahedral faces and striations on the crystal faces (7).

Luster: Magnetite has a metallic luster.

Special Features: It has a strong attraction to magnets.

Varieties: Lodestone, Magnetite Ore, Iron-rich Limestone are some varieties of magnetite.

Mineral Group: Magnetite is in the Oxides group. In that group magnetite is separated into the spinel group. The spinel group shows a crystalline structure that contains oxide anions arranged in a cubic structure and the cations, in this case the two iron ions, occupy some or all of the octahedral and tetrahedral portions of the lattice (6).

Environment: Magnetite is an iron oxide mineral that occurs in many geologic environments. It’s also known for being in igneous rocks, but it doesn’t usually form large crystals that can be seen. Since its crystals are microscopic in igneous rocks, it’s known for forming on the edges of minerals that contain iron, such minerals include Biotite, Amphiboles, and Pyroxenes. In this form, it would be hard to detect with a hand held magnet. Magnetite crystals begin to occur when mafic magma is cool enough for dense crystals to form and settle out of the magma as they crystallize (6).

Associated Rock types: Magnetite can be found in a rock called diabase (6). Sedimentary rocks that have components of black sand most likely contain magnetite. Large deposits of magnetite have been found in the banded iron formations of the world, like those that can be found in the state of Michigan.

Occurrence in North America: The presence of magnetite in large quantities can create an economically important ore rock. Large quantities of magnetite have been found in the U.S, mainly in Michigan, New Jersey, Pennsylvania, Virginia, and North Carolina. In Oregon state, Magnetite was mined for in the site of what is now Lake Oswego.

Economic and Industrial uses: Human history would have been completely different without magnetite! Magnetite is one of the most economically important iron ores. Because it has such a high iron content, magnetic iron ore deposits are highly prized. The iron from magnetite and hematite deposits is the source ore used to create steel used in modern infrastructure (3). Without magnetite and hematite, humans wouldn’t have been able to create the Iron Age, which was a very important time in our history. A very important discovery by humans was that striking magnetite lodestone against pieces of iron would magnetize the iron. This newly magnetized iron could now create compasses! The discovery of the compass was a huge economic and industrial discovery giving humans the ability to navigate and explore the world’s oceans.

Magnetite ore also allows scientists to better understand the history of Earth’s magnetic field. As igneous and sedimentary rocks that contain magnetite form, the iron within them is aligned with the Earth’s magnetic field at the time of formation (6). The recorded magnetism will not change after the rock forms. This magnetic signature has been recorded in much of the ocean’s and continent’s iron-bearing rocks and scientists have been able to make a record of how Earth’s magnetic field has changed over time. By studying these preserved magnetic signatures, geologist are able to understand the history of Earth’s magnetic field, changes in its polarity, and even the past motions of Earth’s plates.

First Notable Identification: A greek shepherd named Magnes was wondering through part of Asia Minor, now known as Southern Turkey. Magnes stumbled across some mysterious black stones and found that the tip of his shepherd’s crook, which was iron, was attracted to these rocks by an unseen force (4). While experimenting with the stones he noticed that the stone was attracted to lots of metal things, but not things, like straw or grass.

How we identified it: We identified this mineral from the others due to the shining of the crystals on the rocks surface. The mineral is magnetic.

Don’t confuse it with: Magnetite can be mistaken for many metallic varieties of Hematite. Samples of the two can be distinguished because Magnetite is very magnetic, not like pure Hematite. Hematite, however, does contain some Magnetite, but it’s weakly magnetic, so a good way to differentiate them is by the streak. Hematite has a red to red-brown streak, while magnetite's streak is black.