This cheat sheet covers twelve common minerals that appear often in Earth science labs, rock identification activities, and mineral property tests. Students need it because many minerals look similar until their hardness, cleavage, streak, luster, and formula are compared carefully. The property card format helps students scan key details quickly and connect observations to correct mineral names.
The most important ideas are that minerals are identified by a set of properties, not by color alone. Mohs hardness, streak color, cleavage or fracture, and luster are usually more reliable than appearance. Chemical formulas show what elements make up each mineral, such as SiO2 for quartz, CaCO3 for calcite, and NaCl for halite.
Key Facts
- Quartz has formula SiO2, hardness 7, glassy luster, no cleavage, and commonly breaks with conchoidal fracture.
- Orthoclase feldspar has formula KAlSi3O8, hardness 6, two cleavage directions near 90 degrees, and is often pink, white, or cream.
- Plagioclase feldspar ranges from NaAlSi3O8 to CaAl2Si2O8, has hardness 6, two cleavage directions near 90 degrees, and may show fine striations.
- Calcite has formula CaCO3, hardness 3, three directions of rhombohedral cleavage, and fizzes with dilute hydrochloric acid.
- Halite has formula NaCl, hardness 2.5, cubic cleavage, and a salty taste when safe lab rules allow testing.
- Gypsum has formula CaSO4 . 2H2O, hardness 2, white streak, and can be scratched by a fingernail.
- Magnetite has formula Fe3O4, hardness 5.5 to 6.5, black streak, metallic luster, and is strongly magnetic.
- Pyrite has formula FeS2, hardness 6 to 6.5, greenish-black streak, metallic luster, and brassy cubic crystals.
Vocabulary
- Mineral
- A naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
- Mohs hardness
- A scale from 1 to 10 that ranks how easily one mineral can scratch another mineral.
- Cleavage
- The tendency of a mineral to break along flat, repeating planes of weakness.
- Fracture
- The way a mineral breaks when it does not break along cleavage planes.
- Streak
- The color of a mineral in powdered form, usually found by rubbing it on an unglazed porcelain plate.
- Luster
- The way a mineral reflects light, such as metallic, glassy, pearly, or dull.
Common Mistakes to Avoid
- Identifying a mineral by color only is wrong because many minerals occur in several colors and different minerals can share the same color.
- Confusing cleavage with fracture is wrong because cleavage creates flat repeating surfaces, while fracture creates uneven, curved, or splintery breaks.
- Ignoring streak is a mistake because streak can be more consistent than surface color, especially for metallic minerals like hematite and pyrite.
- Assuming all shiny minerals are metals is wrong because nonmetallic minerals such as quartz and calcite can have glassy luster.
- Using hardness tests out of order is a mistake because a mineral must be tested against known items, such as a fingernail, copper penny, glass plate, or steel nail.
Practice Questions
- 1 A mineral scratches glass but is not scratched by a steel nail, has no cleavage, and has formula SiO2. Which mineral is it?
- 2 A sample has hardness 3, fizzes with dilute hydrochloric acid, and has three directions of cleavage. Identify the mineral and write its formula.
- 3 A black metallic mineral has a black streak and is strongly attracted to a magnet. Which mineral is it, and what is its formula?
- 4 Explain why a mineral identification card should include hardness, streak, cleavage, luster, and formula instead of relying only on color.
Understanding Twelve Common Minerals Property Cards
A mineral test works best when it follows a fixed order. Start by observing a fresh surface, since weathering can dull luster or coat a sample with rust. Note crystal shape, grain size, and whether light passes through an edge.
Next test luster in good light. Metallic minerals reflect light like polished metal, while nonmetallic minerals may look glassy, pearly, silky, waxy, or dull. Then make a streak on an unglazed porcelain plate.
Streak is the color of a mineral powder, so it can stay consistent even when the outside of a specimen varies in color. A pale mineral often leaves little visible powder, while dark metallic minerals can give a distinctive streak.
Hardness testing is a comparison, not a test of how easily a specimen breaks. A mineral can be hard but still split along flat surfaces if struck. Use objects of known hardness in a careful order, such as a fingernail, a copper coin, a steel nail, and glass.
A real scratch leaves a narrow groove that remains after loose powder is wiped away. Rubbing two specimens together may leave powder and create a misleading mark. The Mohs scale is not evenly spaced.
The difference between adjacent numbers does not represent the same increase in resistance each time. This matters when students compare a mineral that barely scratches glass with one that scratches it easily.
Cleavage records weak directions in a crystal structure. Atoms are arranged in repeating patterns, yet bonds are not equally strong in every direction. When a mineral breaks repeatedly along planes where bonding is weaker, it has cleavage.
The angle and number of these planes are useful clues. Fracture happens when there are no repeated weak planes. Some minerals form curved, shell-like break surfaces because a crack travels through their structure in a smooth expanding curve.
Look at several broken edges before deciding. A single chip can be misleading, especially in a rough classroom sample.
Chemical composition helps explain why minerals have different properties. Elements join in ordered structures, and that structure affects hardness, density, cleavage, magnetism, and reaction with acid. Some mineral groups permit one element to replace another in similar positions.
This is why certain feldspars form a range of compositions rather than one fixed recipe. Minerals are not just lab specimens. They form the grains in sand, the crystals in granite, the material dissolved in groundwater, and many raw materials used in buildings and technology.
In identification work, record each observation before naming the sample. Give more weight to reliable tests, use color as a supporting clue, and note uncertainty when the evidence does not fully agree.