Physical and Chemical Properties in Mineral Identification
Fortunately, the most common minerals are fairly easy to identify by general appearance or with simple tests for hardness, crystal form, color, magnetism, and "streak" (does it leave a colored line when scratched on a piece of tile).
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Photos from the left to right. Tourmaline
Majesty credit: Orbital Joe.|| Zoisite var. Tanzanite photo: mim
museum.|| Elbaite with albite and Lepidolite.|| Rubellite Tourmaline,
Quartz, MIca, Cleavelandite and Orthoclase.© DI (FH) Rudolf Watzl ||
Fluorite on sphalerite (Elmwood, TN, USA) || A slice of an amethyst by
Rob Lavinsky|| |
Note that some tests can be destructive to mineral samples (such as measuring hardness, streak, malleability, elasticity, and testing with acid). In addition, tasting a mineral is not recommend - some are actually poisonous! Washing your hands after handling mineral samples is always recommended.
| Observable Characteristics | |
|---|---|
| Easily Observable | Simple Tests Requiring Equipment |
| Crystal form | Hardness |
| Color | Magnetism |
| Cleavage | Streak |
| Luster (metallic, non-metallic) | Acidic reaction |
| Diaphaneity (transparent, translucent or opaque) | Specific gravity |
| Double refraction | Malleability |
| Striations | Elasticity |
| Feel | Electrical resistivity |
| Odor (smell) | Fluorescence |
| Taste | Phosphorescence |
| Radioactivity | |
| Thermoluminescence | |
Easily Observable Characteristics
These are the physical properties that can often be identified without any advanced tools, simply by examining the mineral in hand.
Crystal Form: Minerals grow into distinctive shapes (crystals) based on their internal atomic structure. Recognizing these forms (e.g., cubic, hexagonal) can be a significant aid in identification.
Color: While not always reliable due to impurities, color can sometimes be an immediate indicator (e.g., the green of malachite or the yellow of sulfur).
Cleavage: The way a mineral breaks along flat, parallel planes (like mica) helps distinguish between minerals that may look similar but break differently.
Luster: Describes how the mineral reflects light, whether metallic (shiny like a metal) or non-metallic (glassy, silky, dull), which is a quick way to categorize minerals.
Diaphaneity: Refers to how light passes through a mineral (transparent, translucent, or opaque), another key characteristic that helps narrow down mineral types.
Double Refraction: Some minerals, like calcite, split light into two rays, creating a double image when viewed through the crystal—a unique identifier for certain minerals.
Striations: Fine parallel grooves on the surface of a mineral (such as those seen on quartz or feldspar) help in distinguishing between minerals of similar appearances.
Feel: Some minerals have distinctive tactile qualities. For instance, talc feels soapy, while graphite feels greasy.
Odor: Though rare, some minerals give off a noticeable smell when scratched or struck (like sulfur’s distinct rotten-egg smell).
Taste: Halite, also known as rock salt, can be identified by its salty taste. (Note: Tasting minerals is not always advisable due to safety concerns.)
Simple Tests Requiring Equipment
These properties typically require tools or tests but offer reliable ways to identify minerals.
Hardness: The Mohs hardness scale measures a mineral's resistance to scratching. For instance, diamond is the hardest mineral, rating a 10 on the scale, while talc is the softest at 1.
Magnetism: Minerals like magnetite can be identified by their magnetic properties, a quick and easy test.
Streak: The color of the powdered mineral, obtained by rubbing it across a streak plate, often differs from the mineral’s appearance and helps in distinguishing between similar-looking minerals.
Acid Reaction: Carbonate minerals, such as calcite, will fizz and release carbon dioxide when exposed to dilute acid, a useful diagnostic test for these types of minerals.
Specific Gravity: This property measures the density of a mineral compared to water, which can be helpful when differentiating between visually similar but differently weighted minerals like gold and pyrite.
Malleability: Some minerals (like native metals) are bendable or can be flattened without breaking, helping distinguish metals from non-metals.
Elasticity: Minerals like mica that bend but return to their original shape when released demonstrate elasticity, a property that can help in identification.
Electrical Resistivity: Certain minerals, like galena, conduct electricity well, while others, like quartz, are insulators, making resistivity a useful test in mineral identification.
Fluorescence: Some minerals glow under ultraviolet light (like fluorite), a characteristic that can help identify them.
Phosphorescence: A mineral that continues to glow after the UV light is removed, such as some forms of calcite, can be identified by this trait.
Radioactivity: Minerals containing uranium or thorium may emit radiation. Geiger counters or scintillometers can measure radioactivity, useful in identifying radioactive minerals like uraninite.
Thermoluminescence: This property occurs when a mineral releases light after being heated, which can be used to help identify certain minerals.
Why These Properties Matter
Accuracy: Physical properties alone can sometimes be misleading due to impurities or environmental factors affecting a mineral’s appearance. By combining physical observations with chemical analysis, geologists achieve more accurate identifications.
Classification: Both physical and chemical properties help classify minerals into groups such as silicates, carbonates, and sulfides, based on their chemical composition and structure.
Understanding Formation: Analyzing the chemical properties of minerals can also provide insights into the geological processes that formed them. For example, the presence of certain minerals might indicate hydrothermal activity or metamorphism.
See also: How do Geologists identify minerals?
How to Identify Minerals in 10 Steps (photos)