Minerals: Mohs Hardness Scale, Streak & Luster: Fifth Grade Science
Minerals are naturally occurring, inorganic solid building blocks of rocks defined by an orderly crystalline structure and identified through diagnostic physical properties.
What is a Mineral?
Every rock on planet Earth is constructed from one or more minerals. While a rock is an aggregate mixture of diverse substances, a mineral is a pure, chemically uniform geological substance. To be scientifically classified as a mineral, a specimen must satisfy five strict scientific criteria:
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| THE FIVE CRITERIA OF A TRUE MINERAL |
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| 1. NATURALLY OCCURRING: Formed by natural geological processes; |
| synthetic lab-grown gemstones are not true minerals. |
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| 2. INORGANIC: Not formed from living organisms or organic matter; |
| coal (decayed plant matter) and pearls are not minerals. |
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| 3. SOLID SUBSTANCE: Possesses a definite shape and volume at room |
| temperatures; liquid water is not a mineral, but glacial ice IS! |
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| 4. DEFINITE CHEMICAL COMPOSITION: Expressible by a chemical formula |
| (e.g., Quartz is SiO₂, Halite table salt is NaCl). |
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| 5. ORDERLY CRYSTALLINE STRUCTURE: Atoms are arranged in a repeating, |
| three-dimensional geometric crystal lattice. |
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Physical Properties Used to Identify Minerals
Geologists do not identify unknown minerals by guesswork. Instead, they perform a battery of diagnostic physical tests.
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| DIAGNOSTIC PHYSICAL PROPERTIES OF MINERALS |
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| Physical Property | What It Tests | Testing Tool / Technique |
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| Hardness | Resistance to | Scratch testing against Mohs |
| | being scratched | scale reference minerals |
| Streak | Color of mineral's| Scraping specimen across an |
| | powdered residue | unglazed white porcelain plate |
| Luster | How light reflects| Visual observation under light |
| | off fresh surface | (Metallic vs. Non-metallic) |
| Cleavage / Fracture | How the mineral | Hammer tap: Flat planes vs. |
| | breaks apart | jagged curved surfaces |
| Specific Gravity | Relative density | Mass in air vs. volume in water|
| Special Properties | Unique reactions | Magnetism, acid fizz, UV glow |
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Mohs Hardness Scale
Formulated in 1812 by German mineralogist Friedrich Mohs, the Mohs Hardness Scale ranks minerals from 1 (the softest) to 10 (the hardest) based on their ability to scratch one another. A harder mineral will always scratch a softer mineral.
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| THE MOHS HARDNESS SCALE |
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| Rating | Reference Mineral| Common Everyday Field | Practical Field Test |
| | | Testing Tool | |
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| 1 | Talc (Softest) | | Scratched by fingernail
| 2 | Gypsum | Human Fingernail (2.5)| Scratched by fingernail
| 3 | Calcite | Copper Penny (3.5) | Scratched by copper |
| 4 | Fluorite | | Scratched by knife |
| 5 | Apatite | Steel Nail / Blade(5.5| Scratches glass dim |
| 6 | Feldspar | Glass Plate (5.5) | Scratches glass easy |
| 7 | Quartz | Steel File (6.5) | Scratches glass clean|
| 8 | Topaz | | Scratches quartz |
| 9 | Corundum (Ruby) | | Cuts all but diamond |
| 10 | Diamond (Hardest)| Masonry Saw | Scratches EVERYTHING!|
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Understanding Scratch Hardness
Hardness reflects the internal bond strength holding atoms together. Talc has a rating of 1 because its magnesium silicate layers are held together by weak van der Waals forces, allowing it to be crushed into fine baby powder with your fingertips. Diamond has a rating of 10 because every carbon atom is locked into four rigid tetrahedral covalent bonds, making diamond the hardest natural substance known on Earth.
Streak: The True Powder Color
The outward color of a mineral specimen is often the most misleading property of all! For example, quartz can appear crystal clear, deep purple (amethyst), milky white, pink (rose quartz), or jet black (smoky quartz) due to trace chemical impurities. Streak is the true color of a mineral in its powdered form, revealed by scraping the specimen across a rough, unglazed white porcelain streak plate. While outward color fluctuates wildly, a mineral's streak color is remarkably consistent.
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| OUTWARD COLOR VERSUS STREAK COLOR |
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| PYRITE ("Fool's Gold"): |
| - Outward Appearance: Brilliantly shiny, metallic, brassy gold. |
| - Streak on Porcelain Plate: GREENISH-BLACK powder line! |
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| REAL GOLD: |
| - Outward Appearance: Rich metallic yellow gold. |
| - Streak on Porcelain Plate: VIBRANT GOLDEN-YELLOW powder line! |
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| HEMATITE (Iron Oxide): |
| - Outward Appearance: Can be silver, shiny metallic, or jet black. |
| - Streak on Porcelain Plate: UNMISTAKABLE RUST-RED powder line! |
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Luster: Reflection of Light
Luster describes how light reflects off the freshly broken surface of a mineral:
- Metallic Luster: Shines brilliantly like polished metal, silver, copper, or steel (e.g., Galena, Pyrite, Magnetite).
- Non-Metallic Luster: Does not look like metal, categorized into descriptive textures:
- Glassy (Vitreous): Reflects like window glass (e.g., Quartz, Calcite).
- Pearly: Iridescent sheen like the inside of a seashell or pearl (e.g., Talc, Gypsum).
- Silky: Fibrous sheen like satin fabric (e.g., Asbestos, Satin Spar Gypsum).
- Dull / Earthy: Non-reflective like dry mud, chalk, or clay (e.g., Kaolinite).
Cleavage versus Fracture
The way a mineral breaks apart reveals the internal geometric arrangement of its chemical bonds:
- Cleavage: The tendency of a mineral to break cleanly along flat, smooth, parallel planes of atomic weakness. Mica peels effortlessly into wafer-thin paper-like sheets (basal cleavage); halite breaks into perfect 90-degree cubes (cubic cleavage); calcite breaks into leaning rhomboids (rhombohedral cleavage).
- Fracture: Occurs when atomic bonds are equally strong in all directions; the mineral shatters along jagged, uneven, or curved surfaces. Quartz and volcanic obsidian display conchoidal fracture, breaking with smooth, curved surfaces resembling broken glass or clamshells.
Special Properties: Unique Identification Clues
Certain minerals display unique, unmistakable diagnostic properties:
- Magnetism: Magnetite (Lodestone) is naturally magnetic, attracting steel paperclips and deflecting compass needles.
- Acid Reaction (Effervescence): Calcite (calcium carbonate, CaCO₃) fizzes and bubbles vigorously when a drop of dilute hydrochloric acid is applied, releasing carbon dioxide gas.
- Double Refraction: Clear optical calcite splits light rays into two beams, making words on a page beneath appear doubled!
- Fluorescence: Fluorite glows with brilliant neon green or purple light when illuminated by an ultraviolet blacklight.
Exercises and Solutions
Exercise 1: Identifying "Fool's Gold" in the Field. A gold prospector discovers a sparkling, metallic brass-yellow mineral embedded in quartz rock. The prospector wants to verify whether it is genuine gold or iron pyrite ("fool's gold"). Describe three specific physical tests the prospector can perform in the field to identify the specimen definitively.
Solution 1: The prospector can perform three diagnostic tests: First, a streak test: scraping the specimen on an unglazed white porcelain tile will leave a vibrant golden-yellow streak if it is genuine gold, but will leave a telltale greenish-black powdered streak if it is pyrite. Second, a Mohs hardness test: real gold is extremely soft (hardness 2.5 to 3.0) and can easily be scratched by a copper penny or indented by a steel pocketknife; pyrite is much harder (hardness 6.0 to 6.5) and cannot be scratched by a knife (in fact, pyrite will scratch a glass plate). Third, a malleability test: striking real gold gently with a small hammer will flatten or dent the soft metal without shattering; striking brittle pyrite will shatter it into sharp, faceted crystalline shards.
Exercise 2: Determining Unknown Mineral Hardness. An unknown clear mineral specimen is subjected to scratch testing. The mineral easily scratches a copper penny (hardness 3.5) and scratches a fluorite crystal (hardness 4.0). However, the mineral cannot scratch a glass plate (hardness 5.5), and a steel knife blade (hardness 5.5) leaves a distinct scratch on the mineral's surface. Estimate the Mohs hardness rating of this unknown mineral and identify which reference mineral it most closely matches.
Solution 2: The unknown mineral scratches materials up to hardness 4.0, but is scratched by a knife and glass plate at hardness 5.5. Therefore, the hardness rating of the unknown mineral lies between 4.0 and 5.5 (specifically around hardness 5.0). On the Mohs Hardness Scale, this specimen matches the reference mineral apatite, which has a defined Mohs hardness rating of exactly 5.0.