Earth Science 2.3 • Properties of Minerals
Properties Minerals • As you can see from the illustration at right, minerals occur in many different shapes and colors. • 8 main characteristics are used to identify and differentiate mineral groups from each other • Color • Streak • Luster • Crystal form • Hardness • Cleavage • Fracture • Density
Color • While color can be an identifying factor in some types of minerals, this property is often not useful for identifying minerals. • This is because small amounts of different elements can give the same mineral different colors.
Streak • Streak is the color of a mineral in it’s powdered form • Streak is obtained by rubbing a mineral across a streak plate, a piece of unglazed porcelain. • While the color of a mineral may vary from sample to sample, the streak usually doesn’t vary. • Therefore streak can be a good indicator to differentiate between mineral groups.
Luster • Luster is used to describe how light is reflected from the surface of a mineral • Minerals that have the appearance of metals, regardless of their color, are said to have a metallicluster. • Minerals with nonmetallic lusters are described by a variety of names: vitreous or glassy, pearly, silky, earthy, brilliant. • Materials that fall in between can be said to have a sub-metallic luster Metallic lusters Glassy translucent luster
Crystal Form • Crystal Form is the visible expression of a mineral’s internal arrangement of atoms. • Every mineral has a crystal form based off one of six distinct crystal systems
Crystal Form • Usually, when a mineral forms slowly and without space restrictions, it will develop into a crystal with well-formed sides. • When the mineral is crowded in it’s growth however, it results in an intergrown mass of smaller crystals.
Hardness • Hardness is one of the most useful properties used to differentiate between mineral groups • Hardness is the measure of resistance of a mineral to being scratched. • The harder the mineral, the less likely it can be scratched.
Hardness • To test for hardness, you can rub a mineral against a mineral with a known hardness. • Scientists use a standard hardness scale called a Mohs Scale. A Mohs Scale consists of 10 minerals arranged from 10 (hardest) to 1 (softest).
Hardness • Any material of unknown hardness can be rubbed against these known materials to determine it’s hardness. • The harder material will always scratch the softer material.
Cleavage • In the atomic structure of a mineral, some bonds are weaker than others. • These weak bonds are places where a mineral will break when stress is applied.
Cleavage • Cleavage is the tendency of a mineral to break, or cleave, along flat, even surfaces.
Cleavage • Minerals called Micas show the simplest cleavages; breaking into thin flat sheets • Quartz, on the other hand, has no cleavage but fractures instead. • Some minerals have cleavage in more than one direction.
Fracture • Minerals that do not show cleavage when stressed are said to fracture • Fracture is the uneven breakage of a mineral. • Quartz shows a curved scalloped fracture called a conchoidal fracture
Fracture • Other minerals, like asbestos, break into splinters or fibers when stressed. • Many minerals have an irregular fracture.
Density • Density is a property of all matter • Density is the ratio of an object’s mass to it’s volume (it’s weight to it’s size)
Density • The density of a pure element has a constant unchanging value. • Thus density can be used to determine the purity or identity of some minerals.