Properties of Minerals

Properties of Minerals The physical properties of minerals are used by mineralogists to help determine the identity of a mineral. Some of the tests can be performed easily wherever the mineral is found; however, other tests require laboratory equipment.

Color What causes color in minerals? Minerals are colored because certain wave lengths of light are absorbed, and the color results from a combination of those wave lengths that reach the eye. How useful is color in identifying minerals? The color of a mineral is the first thing most people notice, but it can also be the least useful in identifying a mineral. Most minerals occur in more than one color. Fluorite can be clear, white, yellow, blue, purple, or green. The other properties, such as hardness, cleavage, and luster, must be used instead.

Texture The feel or appearance of a surface What is the texture of the rocks and minerals shown below?

Streak What is streak? The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it.

Streak Where do you observe streak? A mineral’s streak is determined by rubbing it on a streak plate, which is a piece of unglazed porcelain. Interesting Fact: The streak plate is essentially a type of glass, so it isn't used on minerals with hardness greater than 7.

Streak How useful is streak in identifying minerals? Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks. Hematite, for example, always gives a reddish-brown streak no matter what type of luster it displays.  

Hardness What is hardness? The hardness of a mineral is its ability to resist scratching.

Hardness Where did the hardness scale originate? Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardest mineral known, diamond, was assigned the number 10.

Hardness How does the hardness scale work? The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc. Interesting Fact: A common misunderstanding of how to identify a diamond is that it will scratch glass. While this is true, other minerals can scratch glass too as long as they have a hardness > 6.

Cleavage Interesting Fact: Some minerals show different colors along different crystallographic axes. This is known as pleochroism. What is cleavage? Cleavage is the ability of a mineral to break along preferred planes.

Cleavage How is cleavage described? Minerals that have "perfect" cleavage almost always break in a preferred direction. Minerals that have "good" cleavage will sometimes break in a particular direction, and other times, they may not.

Luster What is Luster? Luster refers to how light is reflected from the surface of a mineral. The two main types of luster are metallic and nonmetallic. What is Metallic Luster? Minerals exhibiting metallic luster look like metal, such as a silvery appearance or that of a flat piece of steel.

Luster

Luster Interesting Fact How many types of nonmetallic luster are there? Vitreous: The luster of glass Resinous: The luster of resin. Pearly: The luster of pearls. Greasy: Looks like it is covered in a thin layer of oil. Silky: The luster of silk. Adamantine: A hard, brilliant luster.   Another common nonmetallic luster is called translucent luster, where you can see into the mineral, but not completely through it. A mineral that displays a transparent luster transmits light completely through it, resembling glass.

Luster Interesting Fact Another common nonmetallic luster is called translucent luster, where you can see into the mineral, but not completely through it. A mineral that displays a transparent luster transmits light completely through it, resembling glass.  

Opaque Not able to be seen through; not transparent Schist, a metamorphic rock