Magnetite is the most magnetic natural mineral. Magnetite was not so named because it was magnetic, rather, it was found in the area Magnesia, and magnets were so named because they were like magnetite! People made the first compasses of magnetite, then called lodestone. Magnetite is the most iron-rich ore used to make steel, so its use in industry today cannot be overstated! Magnetite is one of the most influential minerals no one talks about, so here you can learn about the unique magnetite crystal properties.
Magnetite is really not rare at all. Geologists encounter it in many different circumstances, as an inclusion on several common rock types. Small grains are pretty common in igneous (from magma) and metamorphic (transformed over time) rock. Magnetite is super important in paleomagnetism, working out the history of the Earth’s magnetic field. Since magnetite is an inclusion on common rock types, its magnetization states serves as a type of barometer for what magnetic fields it may have experienced in the distant past. There are a few other names that the same magnetite stone could go by:
- black iron oxide
- magnetic iron ore
- lodestone
- ferrous ferrite
- Hercules stone
Early Technology used Magnetite Crystal Properties
Since magnetite comes naturally magnetized, it attracts pieces of iron. This is how ancient people first learned about magnetism. People discovered that hitting magnetite ‘lodestone’ against pieces of iron would magnetize the iron, making it suitable for use in compasses or wands, or other rudimentary technology.
See, ancient people used magnetite not only for navigation, but also divination: geomancy and fortune-telling. For example, a lightweight magnetite pendulum hung from a thread would move about “randomly,” and the soothsayers attach meanings to its movements. There is also evidence in Central America of the Olmec using lodestone (magnetite) around 1000 BC, but only for divination. These uses of magnetite actually far predate the recorded instances of compasses.
But, the use of magnetite crystal properties of magnetism as COMPASSES had reasonably the most impact on society in pre-scientific and industrial eras. When seamen could use compasses to reliable navigate and chart waters, trade routes and economic exchange improved drastically. However, those ships containing cargo of iron, magnetite, or other magnetic goods would find the compasses to be unreliable as the loads interfered with the magnetic fields.
Eventually by the 16th and 17th centuries, early works on magnetism such as William Gilbert’s De Magnete and Athansius Kircher’s Magnes sive de Arte Magnetica started scientific minds in the direction of inventive advancements. Today magnetite is used not only for iron ore but to engineer electronics and many sensitive devices.
Chemical Makeup of Magnetite Crystal Properties
The chemical formula for magnetite, Fe3O4, is a bit of a misnomer. Each oxygen ion has a charge 2-, which would imply the Iron is somehow charged +8/3 in order to be a neutral atom. (2- charge from each of four oxygen ion giving a net charge of -8 from oxygen, would need a charge of +8 distributed amongst the three iron atoms.) In reality, there are both Iron (II) and Iron (III) ions present in distinct ratios, having charges of 2+ and 3+. There are two Fe3+ ions for every one Fe2+ ion. The two 3+ ions make a 6+ charge and the one 2+ ion a 2+, so altogether the iron contributes the needed +8 charge to balance the oxygen.
The oxygen ions make an optimal space filling grid called a face centered cubic model. This is where at each corner there is an eighth of an oxygen and at each face of the cube there is half of an oxygen. These oxygen ions lay out a grid of tetrahedral and octahedral vacancies in space. In those vacancies lie one of two types of iron cation. In order to demarcate the full picture with both ions the “unit cell” or repeating unit needs to be bigger than one cube.
Magnetite Crystal Structure
Firstly, magnetite has a cubic crystal system, so we can describe it as repeating cubes. The oxygens pack in close, in a face-centered cubic (also known as a close-packed cubic) configuration.
Visualizing the Iron in Magnetite
To see where the irons come in to magnetite’s crystal structure, look close at the face-centered cubic. Focus in on the empty spaces between the oxygen ions. Here is a picture of multiple layers stacked.
If you are looking at the top view of layers of close-packed spheres, tetrahedral and octahedral vacancies alternate in a regular way. The tetrahedral vacancies lie under the center of each sphere showing. The octahedral vacancies lie in the nook between three spheres.
Everywhere three ions have one ion in the crux directly between them creates a tetrahedral vacancy. You have to imagine flat planes tangent to the spheres to form the sides of the tetrahedron. A tetrahedron has four corners. As the spheres are stacked, like a stack of oranges in a crate, there are spots where one sphere sits right on top of three spheres.
Then when there is three upon three spheres, an octahedral vacancy is formed. A octahedron have six corners. Each of the corners is formed at the (invisible) intersection of each plane upon those six spheres.
Spinels
Magnetite is almost like two overlapping crystal structures in a single matrix. Fe2O is iron (III) oxide, and FeO is iron (II) oxide. These added together make the ratios of Fe3O4. Now here’s exactly how they arrange and pack.
Magnetite has a cubic crystal system, with an inverse spinel structure. (For nerds: The crystal class is hexoctahedral (m3m). Hermann-Maugui 4/m3-2/m, and space group Fd3m, the same as diamond, and other spinels.)
Being a spinel means that the configuration and behavior of the crystal is like two-in-one. There are geometric set-ups that overlap each other. Each regime has its own properties and rules, and then combined together the properties merge into characteristic spinel behavior.
As a spinel, magnetite can form solid solutions with ulvospinel and magnesioferrite, for example, merging with the crystal structures of these other solids.Magnetite precipitates from peridotites and dunites during serpentization when minerals from deep in Earth are piped up by large tube-like formations.
Magnetite’s Magnetic Properties
Magnetite forms buffers that moderate oxidation, or the effects of weathering from oxygen exposure. At 120K magnetite undergoes a Verwey transition from monoclinic to cubic, the cubic structure being what we see at room temperature. So when magnetite is supercooled it becomes denser, as most compounds do, and researchers like to study novel states when it’s in those unlikely configurations.
Magnetite is ferrimagnetic at room temperature (about 300 K). 850K is the Curie temperature, at which ferromagnetic and ferrimagnetic materials lose magnetization due to thermal fluctuations.
Magnetite Nanoparticles
Even though there are some other iron-containing minerals that are even better suited for technological applications, magnetite nanoparticles are grown in lab to achieve certain ends as well. Nanoparticles are sought in industry because the exact size can tune the effects. These are just particles that are on the size order of nanometers, or billionths of a meter. At this size, the non-linear and quantum effects crop up amongst the still-active bulk classical effects.
Magnetite nanoparticles exist in nature basically everywhere magnetite does. Naturally, small fragments break off and get ground smaller and smaller. But, for use in electronic devices, engineers will use trialed methods to grow magnetite nanoparticles, so they can have precise control over size distribution.
Magnetite nanoparticles and related derivatives play a role in robust memory storage. Hard drives, CPUs, and other types of disks, including the earliest forms of tape-like memory (such as reels and VHS) make use of magnetite crystal properties. Additionally, medical devices like fMRIs and anything that needs. a tuned magnetic field often benefits from magnetite nanoparticles in the design plans.
BioActive Magnetite
Magnetite is present in multiple organ systems of animals, like birds and mollusks. Humans have magnetite in the brain as well. There are bacteria called magnetospirillum magnetotacticum that grow off of magnetic field fluctuations, that is, their metabolism is directly influenced by the fields. We likely have non-trivial amounts of these bacteria in our bodies as well, and thus our biome activity is mediated at least in that way, though likely many more, in response to ambient magnetic fields.
Magnetite also lies in the brains of humans, birds, and reptiles. Biomagnetics account for effects on humans from magnetic fields in the brain, but they need to be in golilocks amounts. Otherwise we see high and low magnetite concentrations in the human brain as possibly contributing to neurological disorders like dementia and Alzheimer’s. For birds, the magnetite appears to play a role in their ability to migrate across the Earth. Here is a whole article about magnetite in the brain.
Iron Ores are Super Neat
Any industrial application, in manufacturing, chemical engineering, and even drugs and pharmaceuticals, has to involve the harvesting of iron from the earth. Hematite and maghemite are some other main sources of iron used in industry. Iron of course has a ton of uses, (and is used in TONS) in everything from structural components of buildings and everyday objects, and delicate parts of electronics. People also eat it.
Iron supplements consist of dissolving iron filings in acid and then binding them to salts. Those are the ionic supplements, like sulfate, gluconate, etc. Iron pills have been used since 1682, and more than 6 million people in the USA take prescription ferrous sulfate alone.
So stick around and you can also learn about hematite and other forms of natural magnetic rocks. Magnetite crystal properties are super important because it serves as a template for what is possible in the natural world. It is also amazing to thing that something capable of such interesting effects is an inert, inorganic common rock.
Sources & Other Cool Books
Teja, Amyn S., and Pei-Yoong Koh. “Synthesis, properties, and applications of magnetic iron oxide nanoparticles.” Progress in crystal growth and characterization of materials 55.1-2 (2009): 22-45.
Paula Findlen (2004). Athanasius Kircher: The Last Man who Knew Everything. Psychology Press. ISBN 978-0-415-94015-3.
Gvilielmi gilberti colcestrensis, medici londinensis, de magnete magneticisqve corporibvs, et de magno magnete tellure; phsiologia noua, plurimis & argumentis, & experimentis demonstrata. William Gilbert (1540-1603) Londini: excvdebat P. Short, 1600
