Ferric and Ferrous Oxides, Oxides mineral group
|Fe304 – Iron oxide
|5.5 – 6.5 (harder than glass)
|5.2 (feels heavy compared to most rock-forming minerals, but about the same as other metallic minerals)
|Metallic to dull, opaque
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One of only a few naturally occurring magnetic materials, magnetite fascinated early civilizations and set many on a path toward the modern compass. Consequently open sea navigation, commerce and warfare are to a great degree part of magnetite’s legacy. A relatively common, black, metallic mineral, magnetite is also one of modern society’s most important iron ores, occurring in a variety of igneous rocks, pegmatites, contact metamorphic rocks and hydrothermal veins. In our modern Earth, magnetite seldom forms in sedimentary environments. Back in the Early Proterozoic (2.5 to 1.6 billion years ago), however, large deposits of magnetite precipitated directly from seawater, as it was a time when the world’s oceans and atmosphere had not yet become as oxygen-rich as they now are.
Description and Identifying Characteristics
Magnetite is a black, opaque, magnetic mineral that leaves a black streak when rubbed across a hard rough surface. It is one of the most abundant metal oxides, and its crystal structure contains both the ferrous (Fe+2) and ferric (Fe+3) forms of iron ions. A complex pattern of electrons between the two forms of iron is the source of its magnetic nature. Although other metallic minerals may mimic magnetite’s color, luster, hardness and specific gravity, magnetite is the only common mineral that is magnetic. So a magnet is usually all that is needed to identify its presence.
Magnetite crystals usually only occur when magma cools slowly enough for crystals to form and settle out of the magma. These crystals are typically octahedrons to dodecahedrons (eight-sided to twelve-sided shapes) that may exhibit fine lines (known as ‘striations’) on some surfaces. More often, magnetite occurs in its massive form, commonly called ‘lodestone’. This massive variety has a much more pronounced magnetic character, and large deposits of massive magnetite can play havoc with compasses. Lodestone’s magnetic attraction, however, is never strong enough to pull nails from ships as early sailing tales claimed!
In Our Earth: The Geologic Importance of Magnetite
Magnetite is one of the most widespread iron oxide minerals and occurs in a variety of geologic environments. It is a common accessory mineral in igneous rocks, but seldom forms crystals large enough to be seen in hand samples. More often, magnetite is dispersed throughout a rock as microscopic crystals that form along the edges of iron-bearing minerals such as biotite, amphiboles and pyroxenes. In this dispersed form, it seldom constitutes enough of the rock’s volume to be detected by a hand held magnet, although the rock may exhibit a paleomagnetic signature that can be detected by sensitive instruments. If a mafic magma cools slowly enough, dense magnetite crystals may settle as they crystallize, to form large magnetite ore bodies with a strong magnetic character. Magnetite can also form during contact metamorphism of impure iron-rich limestone, and in high temperature hydrothermal sulfide vein deposits.
In Early Proterozoic (2.5 to 1.6 billion years ago) sedimentary rocks, magnetite occurs in large deposits formed as shallow marine sediments precipitated in Early Proterozoic oceans, during a time when the world’s atmosphere and oceans were undergoing a significant chemical transition. During the preceding Archean Eon (3.9 to 2.5 billion years ago), little free oxygen (O2) existed and immense amounts of dissolved iron built up the early oceans. As oxygen levels increased due to photosynthetic organisms, the oxygen began to react with the dissolved iron to precipitate iron oxides, which settled across the shallow seafloor. Since the end of the Lower Proterozoic, 1.6 billion years ago, oxygen levels have been too high for dissolved iron to accumulate in seawater, so this type of marine iron ore precipitation no longer occurs. In Late Proterozoic, and more recent sedimentary rocks, magnetite only occurs as eroded sedimentary grains. These dense ‘heavy mineral’ sand grains may be concentrated by wave and current action into economically important heavy mineral sands and sandstones.
As magnetite-bearing igneous and sedimentary rocks form, the magnetite within them is aligned with the Earth’s magnetic field. Since this rock magnetism, called paleomagnetism, does not change after the rock forms, it provides a record of what the Earth’s magnetic field was like at the time the rock formed. Magnetic records preserved in igneous rocks, lava flows, ash beds and fine-grained sedimentary deposits as a result of their magnetite component allow geologists to interpret the history of the Earth’s magnetic field, changes in its polarity, and even the past motions of the Earth’s plates.
In magmatic deposits, magnetite occurs with apatite and pyroxenes, while in contact metamorphic rocks it is more commonly found with garnet, pyroxene, olivine and metallic sulfides such as pyrite and chalcopyrite. In high temperature hydrothermal veins it often associated with sphalerite and galena.
In Our Society: The Economic Importance of Magnetite
From trade routes to conquest, the pageant of human history would have been very different without the magnetic properties of this non-descript black mineral. Many people assume that magnetite’s name is derived from its magnetic properties, but the term ‘magnet’ actually comes from the mineral. ‘Magnetite’ was named after the Magnesia region of Thessaly, Greece, the home of the Magnetes and an important center of iron production. The historical importance of this region’s mineral deposits is reflected by two elements, magnesium and manganese, which were also named for this region.
By far, the two most economically important iron ores are hematite and magnetite. Although hematite is more abundant than magnetite, magnetite has the higher iron content, so magnetite iron ore deposits are highly sought after. Economic magnetite deposits primarily occur in layered igneous rocks that formed from the slow cooling of magma, heavy mineral sedimentary deposits, and Early Proterozoic (2.5 to 1.6 billion years ago) marine precipitates. The iron from magnetite and hematite deposits is the source of the steel used almost universally through our modern society’s physical infrastructure. Without these two ores, human society would literally not have made it to the Iron Age, much less to modern civilization.
The importance of iron ore to human society should be obvious, but magnetite has also played a subtler, but equally important, historic role in human civilization. Early on, people discovered that striking magnetite ‘lodestone’ against pieces of iron would magnetize the iron. This magnetized iron was used to make the first compasses, which greatly expanding our ability to navigate the world’s oceans. So magnetite not only provided the basis for much of human society’s infrastructure, but also played a significant role in exploration and trade route development, along with the myriad historical implications of increased interaction between widely spaced cultures.
Magnetite in the Upper Midwest
The iron ranges of Minnesota and Wisconsin once held enormous amounts of magnetite and hematite, along with lesser amounts of other iron ores, such as goethite and siderite. Although hematite forms the bulk of the iron ore in Minnesota and Wisconsin iron ranges, enough magnetite occurs in the deposits that many of them were originally discovered and mapped by their impact on compass readings. Explorers could measure true north by the stars and sun, and knew the expected declination of the Earth’s magnetic field. They would traverse a region and note any declinations that were due to magnetite deposits, rather than the Earth’s magnetic field. Once iron mining began though, the mineral’s magnetic character provided an unexpected risk for the mining industry. Ore ships on the Great Lakes could not rely on compasses to plot their position or course because of the magnetite they carried. Before the advent of global positioning satellites transporting iron ore was a riskier endeavor.
Some of the more notable regional iron ranges include the Mesabi (from the Ojibwa word ‘missabe’ which means ‘sleeping giant’), Vermilion and Cuyuna Ranges in Minnesota, Black River Falls and the Penokee-Gogebic Range of Wisconsin, as well as the Marquette and Menominee Ranges of Upper Michigan. Although many of the more easily accessible high grade ores have been mined out, enough secondary grade iron ore remains that the rise and fall of iron mining has largely determined the local economy of these regions. The Upper Midwest’s Iron Range deposits also played a critical role during World War II, when they became the primary source of iron for the steel tanks, planes and ships of the entire Allied war effort. Without these deposits, it is quite likely that World War II would have had a different, far more tragic, ending.
Commonly confused with...
Magnetite resembles a number of other metal ores, but it is the only common mineral that exhibits magnetism. So magnetism alone is sufficient to distinguish relatively pure magnetite samples. It is more difficult to distinguish cases where magnetite is mixed with other dark colored minerals, such as hematite and ilmenite. If enough magnetite is present to give the sample a magnetic character, the magnetism can mask the presence of these other minerals or lead to their misidentification.
Hematite is economically the most important iron ore, since it is more abundant than magnetite. Hematite typically occurs as one of two varieties; a massive red earthy form or a black specular variety that has a strong metallic luster. The latter variety may be mistaken for magnetite, but is not magnetic. When rubbed across a rough hard surface metallic hematite also leaves a distinctive red brown streak that is easily distinguished from magnetite’s black streak. Since metallic hematite sometimes has enough associated magnetite to appear weakly magnetic, it is a good idea to always test an unknown sample’s streak, rather than to identify it by magnetism alone.
Ilmenite is a dense, black metallic mineral that, when rubbed across a rough hard surface, exhibits a black streak similar to that of magnetite. As ilmenite is not magnetic, a magnet can be used to distinguish pure samples of the two. Some ilmenite samples appear weakly magnetic, however, due to inter-grown magnetite crystals. These samples can only truly be distinguished by their weaker magnetic signature or by specialized chemical tests.
Graphite has a metallic appearance and black color that may initially be confused with magnetite, but the two are easily distinguished by their other properties. Magnetite is magnetic, much harder than graphite, and has a higher specific gravity (feels heavier than a similar-sized piece of graphite). Graphite has a distinctive greasy texture, feels relative light, and is soft enough that it will leave marks on paper, as well as your fingers!