The black crystals in this picture are a magnetic mineral called magnetite!
Click on image for full size
Find out how to identify minerals (...and learn what shape, luster, color, streak, hardness, cleavage and fracture are all about!)
Meet some other nonsilicate minerals!
Just like the name implies, magnetite is a natural magnet! It is part of a mineral group called the oxides. Magnetite can usually be recognized by its strong magnetism and dark color.
Magnetite is not very abundant, but it can be found in many different varieties of igneous, metamorphic, sedimentary rocks, and even some meteorites. Most intrusive (plutonic) igneous rocks contain a small smattering of magnetite crystals. The mineral also occurs in metamorphic rocks that formed from iron-rich sedimentary rocks. For instance, Banded Iron Formations that formed during the Precambrian time commonly contain magnetite.
Magnetite does not weather away as easily as many other minerals. This means that it is often leftover as black sand grains when other parts of a rock have worn away. Black sand grains at beaches and elsewhere are often made of magnetite. If you find a beach that has black sand, try running a magnet over the surface and see whether the little black sand grains stick to it. Over thousands of years, those sand grains may cement together forming a sedimentary rock.
Why is magnetite so magnetic?
Any magnetic field is caused by a flow of electrons. In magnetite, the electrons flow between ions of iron within the crystal.
There are two different types of iron within magnetite: Fe +2 and Fe +3. Because these types of iron have different changes, electrons flow between the different irons in an attempt to equalize the change. Just as electricity generates a magnetic field, the flow of electrons within magnetite also generates the magnetic field.
- Shape: Cubic (typically forming octahedron shapes)
- Luster: Metallic to sub-metallic
- Color: Gray, brown to black
- Streak: Black
- Hardness: 5.5 to 6.5 on Mohs Hardness Scale
- Cleavage: None
- Fracture: Conchoidal
Shop Windows to the Universe Science Store!
TES XXVI, 3 fall 2010
The Fall 2010 issue of The Earth Scientist
, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!
You might also be interested in:
Each type of mineral is made of a unique group of elements that are arranged in a unique pattern. However, to identify minerals you don’t need to look at the elements with sophisticated chemical tests....more
Quartz is the second most common mineral in Earth’s crust. It is a member of the quartz group, which includes less common minerals such as opal, crystobalite, and coesite. Silica (Si) and Oxygen (O) are...more
Mica minerals make some rocks sparkle! They are often found in igneous rocks such as granite and metamorphic rocks such as schist. They sparkle because light is reflected on their flat surfaces, which...more
Feldspar is the most common mineral in the Earth’s crust, so you are very likely to find it in the rocks you collect! It is found it all of the three rock types, but is most common in intrusive igneous...more
Olivine looks like little green crystals. It is typically found in some igneous and metamorphic rocks. Often the crystals are so small that you need to use your hand lens or magnifying glass to see them...more
So far, over 2000 minerals have been found, and every year new ones are discovered. This is a pretty overwhelming number of different types of minerals, however, you don't need to know them all to be...more
Minerals occur naturally on rocky planets and form the building blocks of rocks. They are non-living, solid, and, like all matter, are made of atoms of elements. There are many different types of minerals...more