Mineral identification is the process of using observable and testable properties to name an unknown mineral sample. This quick card helps students compare minerals in a careful, organized way instead of relying only on color. It is useful during labs, rock and mineral units, and field observations.
Students need it because many minerals look similar but have different physical properties.
The most important identification clues are hardness, streak, luster, cleavage or fracture, crystal form, color, and density or specific gravity. Hardness is tested with the Mohs scale, while streak is the powder color left on an unglazed porcelain plate. Cleavage describes flat breakage planes, and fracture describes uneven breakage.
A mineral is identified best when several properties match the same mineral, not when only one clue matches.
Key Facts
- A mineral is naturally occurring, inorganic, solid, has a definite chemical composition, and has an orderly crystal structure.
- The Mohs hardness scale ranks minerals from 1 to 10, where talc = 1 and diamond = 10.
- A mineral can scratch another material only if its hardness is greater than the material being scratched.
- Streak is the color of a mineral's powder, and it is often more reliable than the outside color of the sample.
- Luster describes how a mineral reflects light, such as metallic, glassy, pearly, dull, or earthy.
- Cleavage means a mineral breaks along flat, repeating surfaces, while fracture means it breaks unevenly or in curved surfaces.
- Density = mass / volume, and a mineral with greater density feels heavier than another mineral of the same size.
- Specific gravity = density of mineral / density of water, so a specific gravity of 3 means the mineral is 3 times as dense as water.
Vocabulary
- Mineral
- A naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
- Hardness
- A measure of how well a mineral resists being scratched.
- Streak
- The color of the powder a mineral leaves when rubbed across an unglazed porcelain plate.
- Luster
- The way a mineral reflects light from its surface.
- Cleavage
- The tendency of a mineral to break along flat, smooth planes of weakness.
- Fracture
- The way a mineral breaks when it does not split along flat cleavage planes.
Common Mistakes to Avoid
- Using color alone to identify a mineral is wrong because impurities can make the same mineral appear in many colors.
- Confusing streak with surface color is wrong because streak is the mineral's powdered color, not the color of the outside of the sample.
- Saying a mineral has cleavage because it broke is wrong because cleavage must show flat, repeating surfaces, while rough or curved breaks are fracture.
- Scratching the streak plate too hard is wrong because some minerals are harder than the plate and may damage it instead of leaving a true streak.
- Ignoring multiple properties is wrong because accurate identification requires several matching clues, such as hardness, streak, luster, and cleavage.
Practice Questions
- 1 A mineral scratches a copper penny with hardness 3 but does not scratch glass with hardness 5.5. What is the mineral's likely hardness range?
- 2 A mineral has a mass of 48 g and a volume of 16 cm3. What is its density?
- 3 A sample leaves a green-black streak, has metallic luster, and has hardness 6. Is its streak or its outside color more useful for identification?
- 4 Two minerals are both white and glassy, but one breaks into flat sheets and the other breaks with rough uneven surfaces. Which property helps separate them, and why?