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 1. Color 2. Streak 3. Luster 4. Crystal form 5. Hardness 6. Cleavage 7. Fracture 8. 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 metallic luster.  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.