Anglais scientifique

How does a geologist identify a mineral in the field?

It is often difficult to identify a mineral simply by looking at it, but each mineral has a set of distinctive characteristics that are easily tested in the field or laboratory. Geologists commonly use physical properties such as crystal habit, cleavage, and hardness to identify minerals.

 Hardness

Hardness is a distinctive quality of minerals that is determined by the Mohs hardness scale. Talc is the softest mineral on the scale at a value of 1, and diamond is the hardest at a value of 10 (Table 5-1). Geologists often scratch minerals with a knife blade that has a hardness of about 5. If the mineral scratches the knife, it is harder than 5; if the mineral is scratched, its hardness is less than 5. A thumbnail is about 2.5 on the Mohs scale.

Table 1 Mohs’ hardness scale of mineral hardness: A method of identifying minerals.

Color

Color is the most obvious property of a mineral, but it is commonly unreliable for identification. Color would be a reliable identification tool if all minerals were pure and had perfect crystal structures. However, both small amounts of chemical impurities in crystal structure can alter color. For example, corundum (Al₂O₃) is normally a cloudy, translucent, brown or blue mineral. Addition of a small amount of chromium can convert corundum to the beautiful, clear, red gem known as ruby. A small quantity of iron or titanium turns corundum into the striking blue gem called sapphire.

 Streak

Streak is the color of a fine powder of a mineral. It is observed by rubbing the mineral across a piece of unglazed porcelain known as a streak plate. Many minerals leave a streak of powder with a diagnostic color on the plate. Streak is commonly more reliable than the color of the mineral itself for identification.

Luster

The luster is the appearance of the light that is reflected from a mineral's surface. Luster can be metallic (shiny), glassy or vitreous, or earthy (dull, not shiny).

 Crystal form

Sometimes minerals have a distinctive crystal form that reflects a specific internal arrangement of atoms. The crystal form is best developed when the mineral can crystallize slowly from the fluid that contains its elements.

 Cleavage

Cleavage is the tendency of a mineral to break along preferred crystalline planes that are weakly bonded. The angle between various crystal faces is often distinctive for different mineral groups and can be determined with a magnifying lens in the field. 

Fracture

Fracture is the pattern in which a mineral breaks other than along planes of cleavage. Many minerals fracture into characteristic shapes. Conchoidal fracture creates smooth, curved surfaces . It is characteristic of quartz and olivine. Glass, although not a mineral because it has no crystalline structure, also typically fractures in a conchoidal pattern. Some minerals break into fibrous fragments. Most minerals fracture into irregular shapes.

Specific gravity

Specific gravity is the weight of a substance relative to that of an equal volume of water. If a mineral weighs 2.5 times as much as an equal volume of water, its specific gravity is 2.5. You can estimate a mineral’s specific gravity simply by hefting a sample in your hand. If you practice with known minerals, you can develop a feel for specific gravity. Most common minerals have specific gravities of about 2.7. Metals have much greater specific gravities; for example, gold has the highest specific gravity of all minerals, 19. Lead is 11.3, silver is 10.5, and copper is 8.9.

Other properties

Properties such as reaction to acid, magnetism, radioactivity, fluorescence, and phosphorescence can be characteristic of specific minerals. Calcite and some other carbonate minerals dissolve rapidly in acid, releasing visible bubbles of carbon dioxide gas. Minerals