Mineral properties help students identify unknown minerals using careful observations and simple tests. This cheat sheet summarizes the most useful properties geologists use in the lab and field. It is helpful because many minerals can look similar, but their hardness, streak, cleavage, and density can separate them.
Students can use it as a quick reference during mineral identification activities.
The most important mineral properties include color, luster, streak, hardness, cleavage, fracture, crystal form, and density. Hardness is tested with the Mohs scale, while density is found using density = mass / volume. Cleavage means a mineral breaks along flat planes, while fracture means it breaks unevenly or in curved surfaces.
Special properties such as magnetism, acid reaction, and fluorescence can give strong clues for certain minerals.
Key Facts
- A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
- Luster describes how a mineral reflects light, such as metallic, glassy, pearly, dull, or silky.
- Streak is the color of a mineral's powder, and it is often more reliable than surface color.
- Hardness is a mineral's resistance to scratching and is measured on the Mohs hardness scale from 1 to 10.
- A mineral can scratch another material only if its hardness is greater than the hardness of that material.
- Cleavage occurs when a mineral breaks along smooth, flat planes because of weak bonds in its crystal structure.
- Fracture occurs when a mineral breaks in irregular, uneven, splintery, or curved surfaces instead of flat planes.
- Density is calculated with the formula density = mass / volume, usually in grams per cubic centimeter.
Vocabulary
- Mineral
- A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
- Luster
- Luster is the way a mineral reflects light from its surface.
- Streak
- Streak is the color of the powder a mineral leaves when rubbed on an unglazed porcelain plate.
- Mohs Hardness Scale
- The Mohs hardness scale ranks minerals from 1 to 10 based on their resistance to scratching.
- Cleavage
- Cleavage is the tendency of a mineral to break along smooth, flat surfaces.
- Fracture
- Fracture is the way a mineral breaks when it does not split along cleavage planes.
Common Mistakes to Avoid
- Using color alone for identification is wrong because many minerals have different colors due to impurities or weathering.
- Confusing streak with surface color is wrong because streak is the powder color, which can be different from the mineral's visible color.
- Saying a mineral has cleavage just because it broke is wrong because cleavage requires smooth, flat, repeating break surfaces.
- Reversing the hardness test is wrong because the harder material scratches the softer material, not the other way around.
- Forgetting units in density is wrong because density needs both mass and volume units, such as grams per cubic centimeter.
Practice Questions
- 1 A mineral has a mass of 36 g and a volume of 12 cm3. What is its density?
- 2 A mineral scratches glass but is scratched by quartz. If glass has hardness 5.5 and quartz has hardness 7, what hardness range is most likely for the mineral?
- 3 A mineral leaves a reddish-brown streak on a streak plate even though its surface looks black. Which property should be recorded as reddish-brown?
- 4 Two minerals are both green and shiny, but one has perfect cleavage and the other breaks with curved surfaces. Explain why color and luster are not enough to identify them.