Guide to Rocks and Minerals of Illinois

Using Characteristics of Minerals to Identify Them

Most minerals can be characterized and classified by their unique physical properties: hardness, luster, color, streak, specific gravity, cleavage, fracture, and tenacity.

Hardness

The ability to resist being scratched—or hardness—is one of the most useful properties for identifying minerals. Hardness is determined by the ability of one mineral to scratch another. Federick Mohs, a German mineralogist, produced a hardness scale (table 5) using a set of ten standard minerals. The scale arranges the minerals in order of increasing hardness. Each higher-numbered (harder) mineral will scratch any mineral with a lower number (softer).

A rough measure of mineral hardness can be made by assembling a kit of handy objects (table 6). A fingernail has a hardness ranging from 2 to 2.5, a penny is a little harder than 3, window glass ranges from 5.5 to approximately 6 in hardness, and a knife blade is generally in the range of 5 to 6.5.

Creating a Mineral Hardness Test Kit

A mineral hardness test kit can be easily created from common household or hardware items (table 6). Parents should help children make the kit. Wear safety glasses and gloves when cutting class. All pieces in the hardness test kit should be compared to one another and specific minerals with a known hardness.

Items in a hardness test kit

Luster

Luster is how a mineral reflects light. The terms metallic and nonmetallic describe the basic types of luster. Table 7 lists the most common terms used to describe luster and an example of a corresponding mineral. Some minerals that don't exhibit luster are referred to as "earthy," "chalky," or "dull."

Color

One of the most obvious properties of a mineral is color. Color should be considered when identifying a mineral, but should never be used as the major identifying characteristic.

Streak

Streak is the color of the powdered mineral, which is usually more useful for identification than the color of the whole mineral sample. Rubbing the mineral on a streak plate will produce a streak. A streak plate can be made from the unglazed back side of a white porcelain bathroom or kitchen tile. Some minerals won't streak because they are harder than the streak plate.

Specific Gravity

Specific gravity is the ratio between the mass (weight) of a mineral and the mass (weight) of an equal volume of water. A mineral's specific gravity (SG) can be determined by dividing its weight in air by the weight of an equal volume of water. For instance, quartz with a density of 2.65 is 2.65 times as heavy as the same volume of water.

Cleavage

The way in which a mineral breaks along smooth flat planes is called cleavage. These breaks occur along planes of weakness in the mineral's structure. However, if a mineral breaks along an irregular surface, it does not have cleavage.

Fracture

When a mineral breaks irregularly, the breaks are called fractures. The breaks can be described as grainy, hackly (jagged), conchoidal (curved), or splintery.

Tenacity

How well a mineral resists breakage is known as tenacity. Tenacity is described using these terms:
      • Brittle - Mineral crushes to angular fragments (quartz).
      • Malleable - Mineral can be modified in shape without breaking and can be flattened to a thin sheet (copper, gold).
      • Sectile - Mineral can be cut with a knife into thin shavings (talc).
      • Flexible - Mineral bends but doesn't regain its shape once released (selenite, gypsum).
      • Elastic - Mineral bends and regains its original shape when released (muscovite and biotite mica).

Other Diagnostic Characteristics

Other characteristics may be useful in identifying some minerals:
      • Transparency - Objects are visible when viewed through a mineral.
      • Translucency - Light, but not an image, is transmitted through a mineral.
      • Opaqueness - No light is transmitted, even on the thinnest edges.
      • Taste - Taste can be used to help identify some minerals, such as halite (salt).
      • Acid reaction - Object reacts to hydrochloric acid. The most distinguishing characteristic of calcite is that it effervesces when hydrochloric acid is applied.
        Dolomite shows a reaction on a freshly broken or powdered surface. Testing for calcite, limestone, or dolomite calls for 10% hydrochloric acid,
        but strong white vinegar can be substituted for the acid.
      • Magnetism - Magnetism is a distinguishing characteristic of magnetite.
      • Crystal shape - Cubic, rhombohedral (tilted cube), hexagonal (six-sided), etc. Some crystal shapes are illustrated below.

 

 

 

 

 

Rocks and Minerals of Illinois Table of Contents

Updated 11/29/2011 SLD