Mineral Characteristics

Although there are thousands of minerals in Earth, there are only about 20 common minerals. The same mineral found in different parts of the world will always look the same and will have a characteristic crystal form and a consistent chemical formula. Minerals can be identified in rocks by a process of elimination. A variety of features can be used to identify specific minerals. Some of the most common are listed below.

Crystal Form

Minerals form crystals with specific shapes (Pic. 1) when they have been able to grow without obstruction. Common shapes are prisms, pyramids, needles, cubes, and sheets.

Picture 1. A range ofcrystal shapes including pyramid(calcite, left) and acicular (needlelike,tourmaline, right).

Cleavage

Minerals break along specific planes of weakness related to their atomic structure (Pic. 2). Amphibole’s cleavage planes intersect at 120 degrees; pyroxene has a distinctive 90 degree cleavage intersection. Quartz has no cleavage planes but fractures irregularly.

Picture 2. Cleavage planes in amphiboleintersect at an angle of 120 degrees

Moh’s Hardness Scale

Minerals are ranked from 1 to 10 based upon their relative hardness (Pic 3). Harder minerals can scratch softer minerals. Ten index minerals make up Moh's scale and other minerals are ranked relative to these. For example, a mineral that could scratch feldspar but not quartz would have a hardness of approximately 6.5.

Picture 3. Corundum ranks 9th on Moh's hardness scale

Color

Minerals come in a variety of colors (Pic. 4). Examples of common dark-colored minerals (black, dark brown, dark green) are amphibole, olivine, pyroxene, biotite mica. Light-colored minerals (white, pink, gray, translucent) are represented by quartz, feldspar (orthoclase, plagioclase), muscovite mica, gypsum, halite, calcite. However, we must be careful in using color to identify minerals because some minerals can be found in a wide range of colors.

Picture 4. Brightly colored Azurite (blue) and malachite (green).

Luster

Characteristics of light reflected from mineral is luster (Pic. 5). Types of luster include earthy, silky, glassy, and metallic.

Picture 5. Metallic luster of galena.

Streak

Minerals will leave a mark when pulled across an unglazed porcelain streak plate (Pic. 6). The streak represents the mineral in powdered form and is often constant even when mineral color changes. This property is most useful for metallic minerals that leave a dark-colored streak.

Picture 6 . Brown streak on white porcelain streak plate from metallic iron mineral hematite.

Other

Calcite reacts (bubbles) with weak acids (Pic. 7); halite (rock salt) has a salty taste.

Picture 7. Left: Calcite and dropper prior to addition of acid. Right: Reaction of calcite and weak

hydrochloric acid