The oxide minerals consist of oxygen combined with another element. For the first three oxide minerals below, iron combines with oxygen to form iron oxide (hematite, magnetite, limonite). For the last two oxides, aluminum combines with oxygen to form aluminum oxide (corundum, bauxite). These minerals have different properties, because the elements involved are different and their structures are different. Hydroxide, OH, is combined with oxygen and iron to form limonite. Here are the elements involved in these oxide minerals:
Hematite
Chemical formula: Fe 2 O 3 (iron oxide)
Structure: Hematite crystalline structure is within the hexagonal crystal system.
Hammer behavior: When struck with a hammer hematite fractures unevenly, or with slight conchoidal fracture.
Color: black to steel or silver-gray, brown to reddish brown, or red
Streak: rust red
Luster: earthy
Diaphaneity: opaque
Hardness: 5.5 to 6.5
Occurrence: Generally forming iron ore deposits, hematite is found in huge concentrations within banded iron formations, which are ancient deposits dating back to when Earth's atmosphere was not yet oxygen-rich. It can also form around hot springs.
Use: As a principal ore of iron.
Magnetite
Chemical formula: Fe 3 O 4 (iron oxide)
Structure: Magnetite crystalline structure is within the isometric crystal system.
Hammer behavior: Magnetite shows uneven fracture.
Color: dark gray to black
Streak: dark gray
Luster: metallic
Diaphaneity: opaque
Hardness: 5.5 to 6.5
Occurrence: Magnetite is found as small crystals in igneous and metamorphic rocks, but is also found in banded iron formations.
Use: Magnetite is an important ore of iron. It has also been important in the study of plate tectonics, as magnetite has magnetic properties.
Special Property: Magnetism, which is sometimes weak, but you might notice slight movement with a nail even so.
Limonite
Chemical formula: FeO(OH) . H 2 , but highly variable (iron oxide)
Structure: Limonite is not a true mineral, but is a mineraloid, occuring as a mixture of water molecules and various tiny iron oxide mineral crystals in a rather amorphous structure.
Hammer behavior: Magnetite shows uneven fracture.
Color: yellow brown to dark brown
Streak: yellow brown
Luster: earthy
Diaphaneity: opaque
Hardness: 5.5 to 5
Occurrence: Commonly forms in weathering zones where iron is found in igneous and sedimentary rocks. It may also form pseudomorphs after pyrite (recrystallized after original pyrite). It may also be associated with gold deposits.
Use: Limonite has been mined as an iron ore from peat deposits. And, it serves as a source of the earth pigment ochre.
Corundum
Chemical formula: Al 2 O 3 (aluminum oxide)
Structure: Corundum is in the trigonal crystal system.
Hammer behavior: Corundum doesn't have cleavage, but is does have what is called parting. Long crystals may part (similar to cleavage) in the direction perpendicular to the long axis of the crystal.
Color: can be clear, but usually gray, red, brown, blue
Streak: white
Luster: greasy, dull
Diaphaneity: translucent to opaque
Hardness: 9
Occurrence: Corundum as found as an accessory mineral in metamorphic rocks, intrusive igneous rocks, and because it is so hard, it may be found as an accessory mineral in sedimentary rocks.
Use: Corundum is used as an industrial abrasive, but is also used on some sandpapers.
Bauxite
Chemical formula: OH . Al 2 O 3 (hydrous aluminum oxide)
Structure: Bauxite is a mixture of several aluminum oxide ore compounds, together with iron oxide minerals, and kaolinite.
Hammer behavior: Bauxite, consisting usually of a mass of spherules, fractures unevenly.
Color: white, gray, yellow, red, brown, but often orange tones with cream color dominating
Streak: white
Luster: dull, earthy
Diaphaneity: translucent to opaque
Hardness: 1 to 3
Occurrence: Bauxite forms in soil zones, especially in the tropics where reddish-colored soils called laterites form. These soils have had much leaching of mineral compounds; bauxite forms in this intensively weathering environment.
Use: Bauxite is the main ore of aluminum.
