List The Properties Of Minerals That We Can Test

Here's how we identify the silent storytellers of our Earth, the minerals, using properties we can test with relative ease.

Decoding Earth's Secrets: Properties of Minerals You Can Test

Minerals, the fundamental building blocks of our planet, each possess a unique set of physical and chemical characteristics. Identifying these properties is akin to deciphering a hidden language, allowing us to understand the mineral's origin, formation, and potential uses. Fortunately, many of these properties can be tested with simple tools and observation, turning anyone into a mineral detective.

Why Mineral Identification Matters

Before diving into the properties, let's understand why identifying minerals is important.

• Resource Exploration: Knowing which minerals are present in an area helps locate valuable resources like metals, gemstones, and industrial materials.
• Geological Understanding: Mineral identification is crucial for understanding the geological history of a region. The presence of specific minerals can indicate past environmental conditions, such as temperature, pressure, and the presence of water.
• Industrial Applications: Many industries rely on specific minerals for their unique properties. For example, quartz is used in electronics, talc in cosmetics, and diamonds in cutting tools.
• Gemology: Identifying gemstones is essential for determining their value and authenticity.
• Scientific Research: Mineral identification is fundamental to various scientific disciplines, including geology, geochemistry, and materials science.

Properties We Can Test: Your Mineral Identification Toolkit

Here’s a list of properties you can test to identify minerals:

1. Color: The most obvious property, but often the least reliable.
2. Streak: The color of a mineral in powdered form.
3. Luster: How light reflects off the mineral's surface.
4. Hardness: A mineral's resistance to being scratched.
5. Cleavage and Fracture: How a mineral breaks.
6. Crystal Form: The geometric shape of a mineral crystal.
7. Specific Gravity: The density of a mineral.
8. Diaphaneity: How transparent or opaque a mineral is.
9. Tenacity: A mineral's resistance to breaking, bending, or deformation.
10. Magnetism: Whether a mineral is attracted to a magnet.
11. Taste, Odor, and Feel: Some minerals have distinctive tastes, odors, or feels.
12. Reaction to Acid: Some minerals react with acid, producing bubbles.

Let's explore each of these in detail.

1. Color: A Colorful Clue, But Often Misleading

Color is the most readily observable property of a mineral. However, it's often the least reliable for identification.

• Why Color is Unreliable: Many minerals can occur in a variety of colors due to the presence of trace elements or impurities within their crystal structure. For example, quartz can be clear (rock crystal), purple (amethyst), pink (rose quartz), milky white (milky quartz), or black (smoky quartz), depending on the impurities present.
• Idiochromatic vs. Allochromatic: Minerals that are colored due to their essential chemical composition are called idiochromatic. These minerals tend to have a consistent color. Examples include malachite (green) and azurite (blue), where the color is due to the presence of copper. Minerals that are colored due to impurities are called allochromatic.
• Using Color as a Starting Point: While not definitive, color can be a useful starting point in mineral identification. It can help narrow down the possibilities and guide further testing.

2. Streak: The True Color Revealed

Streak is the color of a mineral in its powdered form. It is a more reliable property than color because the streak color is less affected by impurities.

• How to Test Streak: To test streak, rub the mineral across a streak plate, which is a piece of unglazed porcelain. The streak plate has a hardness of about 6.5 on the Mohs scale. If the mineral is harder than the streak plate, it will scratch the plate instead of leaving a streak.
• Interpreting Streak: The color of the streak can be very different from the color of the mineral itself. For example, hematite (Fe2O3) can be black, gray, or reddish-brown, but it always has a reddish-brown streak. Pyrite (FeS2), also known as "fool's gold," is brassy yellow, but it has a black streak.
• Limitations of Streak: Streak is only useful for minerals softer than the streak plate. Minerals harder than 6.5 will not leave a streak.

3. Luster: How Light Dances on the Surface

Luster describes how light reflects off the surface of a mineral. It's a qualitative property, meaning it's based on observation and description rather than precise measurement.

• Types of Luster:
  • Metallic: The mineral looks like a metal, with a shiny, reflective surface (e.g., pyrite, galena).
  • Submetallic: Similar to metallic, but with a duller, less reflective surface.
  • Nonmetallic: Minerals that do not look like metals. Nonmetallic lusters are further divided into:
    • Adamantine: Brilliant, like a diamond (e.g., diamond, zircon).
    • Vitreous: Glassy (e.g., quartz, tourmaline).
    • Resinous: Like resin or plastic (e.g., sulfur, sphalerite).
    • Pearly: Iridescent, like a pearl (e.g., talc, muscovite).
    • Greasy: Appears to be coated with oil (e.g., serpentine, talc).
    • Silky: Fibrous, with a sheen like silk (e.g., asbestos, gypsum).
    • Dull/Earthy: Lacking any noticeable luster (e.g., clay minerals, bauxite).
• Observing Luster: Examine the mineral under a good light source. Rotate the mineral to observe how light reflects off its surface from different angles.

4. Hardness: Scratching the Surface of Identification

Hardness is a mineral's resistance to being scratched. It is measured using the Mohs Hardness Scale, a relative scale ranging from 1 (talc, the softest) to 10 (diamond, the hardest).

• Mohs Hardness Scale:
  1. Talc
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase Feldspar
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond
• Testing Hardness: To test hardness, try to scratch the mineral with materials of known hardness. Common items used for hardness testing include:
  • Fingernail (hardness ~2.5)
  • Copper penny (hardness ~3)
  • Steel nail or pocketknife (hardness ~5.5)
  • Glass plate (hardness ~5.5)
  • Streak plate (hardness ~6.5)
• Interpreting Hardness: If the mineral is scratched by the test material, it is softer than that material. If the mineral scratches the test material, it is harder. For example, if a mineral is scratched by a steel nail but scratches a copper penny, its hardness is between 3 and 5.5.

5. Cleavage and Fracture: Breaking Points

Cleavage and fracture describe how a mineral breaks. Cleavage is the tendency of a mineral to break along specific planes of weakness, creating smooth, flat surfaces. Fracture is the way a mineral breaks when it does not cleave.

• Cleavage:
  • Description: Cleavage occurs along planes of weak chemical bonds in the crystal structure.
  • Types of Cleavage: Cleavage is described by the number of cleavage planes and the angles between them. Examples include:
    • Basal Cleavage: One direction of cleavage, resulting in sheet-like fragments (e.g., mica).
    • Cubic Cleavage: Three directions of cleavage at 90° angles, resulting in cube-shaped fragments (e.g., halite).
    • Octahedral Cleavage: Four directions of cleavage, resulting in octahedral fragments (e.g., fluorite).
    • Rhombohedral Cleavage: Three directions of cleavage not at 90° angles, resulting in rhombohedral fragments (e.g., calcite).
  • Identifying Cleavage: Examine the mineral for smooth, flat surfaces that reflect light evenly. Look for repeating steps or parallel breaks.
• Fracture:
  • Description: Fracture occurs when a mineral breaks in a direction that is not a cleavage plane.
  • Types of Fracture:
    • Conchoidal: Smooth, curved surfaces resembling the inside of a seashell (e.g., quartz, obsidian).
    • Fibrous/Splintery: Breaks into fibers or splinters (e.g., asbestos, serpentine).
    • Uneven/Irregular: Rough, irregular surfaces (e.g., pyrite, magnetite).
    • Earthy: Crumbling or powdery (e.g., limonite, clay minerals).
  • Identifying Fracture: Examine the broken surface of the mineral. Note the texture and shape of the break.

6. Crystal Form: The Shape of Things

Crystal form refers to the external shape of a mineral crystal. It reflects the internal arrangement of atoms in the crystal structure.

• Crystal Systems: Minerals crystallize in one of seven crystal systems, each defined by its symmetry and axial relationships:
  • Isometric (Cubic): Three axes of equal length at 90° angles (e.g., halite, pyrite).
  • Tetragonal: Two axes of equal length at 90° angles, and one axis of different length at 90° angles (e.g., zircon, rutile).
  • Orthorhombic: Three axes of different lengths at 90° angles (e.g., sulfur, barite).
  • Hexagonal: Three axes of equal length at 120° angles in a plane, and one axis perpendicular to that plane (e.g., quartz, beryl).
  • Trigonal (Rhombohedral): Similar to hexagonal, but with only three-fold symmetry (e.g., calcite, tourmaline).
  • Monoclinic: Three axes of different lengths, with one angle not at 90° (e.g., gypsum, orthoclase feldspar).
  • Triclinic: Three axes of different lengths, with all angles not at 90° (e.g., plagioclase feldspar, kyanite).
• Identifying Crystal Form: Examine the mineral for well-defined crystal faces and geometric shapes. Note the symmetry and axial relationships. Keep in mind that perfect crystal forms are rare; most minerals occur as irregular aggregates or grains.

7. Specific Gravity: Weighing In on Identification

Specific gravity is the ratio of a mineral's density to the density of water. It is a measure of how heavy a mineral is relative to its size.

• Measuring Specific Gravity:
  1. Weigh the mineral in air (Wair).
  2. Weigh the mineral submerged in water (Wwater).
  3. Calculate specific gravity using the formula: Specific Gravity = Wair / (Wair - Wwater)
• Interpreting Specific Gravity: Most minerals have specific gravities between 2 and 8. Minerals with high specific gravities feel noticeably heavier than minerals of the same size with lower specific gravities. For example, galena (PbS) has a specific gravity of 7.5, making it feel very heavy for its size.
• Estimating Specific Gravity: With practice, you can estimate specific gravity by simply holding the mineral and judging its weight relative to its size.

8. Diaphaneity: Shedding Light on Transparency

Diaphaneity describes how transparent or opaque a mineral is.

• Types of Diaphaneity:
  • Transparent: Light passes through the mineral clearly, and objects can be seen through it (e.g., clear quartz, diamond).
  • Translucent: Light passes through the mineral, but objects cannot be seen clearly (e.g., milky quartz, alabaster).
  • Opaque: Light does not pass through the mineral (e.g., pyrite, magnetite).
• Testing Diaphaneity: Hold the mineral up to a light source and observe how much light passes through it.

9. Tenacity: Resistance to Deformation

Tenacity describes a mineral's resistance to breaking, bending, or deformation.

• Types of Tenacity:
  • Brittle: Easily broken or powdered (e.g., quartz, calcite).
  • Malleable: Can be hammered into thin sheets (e.g., gold, silver).
  • Ductile: Can be drawn into wires (e.g., copper, gold).
  • Sectile: Can be cut with a knife into thin shavings (e.g., gypsum, talc).
  • Flexible: Can be bent without breaking and remains bent (e.g., talc, chlorite).
  • Elastic: Can be bent and returns to its original shape when released (e.g., mica).
• Testing Tenacity: Try to bend, cut, or break the mineral. Observe how it responds to the applied force.

10. Magnetism: An Attractive Property

Magnetism describes whether a mineral is attracted to a magnet.

• Types of Magnetism:
  • Ferromagnetic: Strongly attracted to a magnet and can become permanently magnetized (e.g., magnetite).
  • Paramagnetic: Weakly attracted to a magnet (e.g., platinum, some iron-rich minerals).
  • Diamagnetic: Repelled by a magnet (e.g., quartz, gold).
• Testing Magnetism: Use a magnet to test whether the mineral is attracted. Magnetite is a common magnetic mineral that can be easily identified.

11. Taste, Odor, and Feel: Sensory Clues (Use with Caution!)

Some minerals have distinctive tastes, odors, or feels. However, these properties should be tested with extreme caution, as some minerals can be toxic or harmful.

• Taste:
  • Halite (NaCl): Salty taste. Only test if you are certain the mineral is halite and it is clean.
• Odor:
  • Sulfur (S): Distinctive sulfurous odor, especially when heated or rubbed.
  • Arsenopyrite (FeAsS): Garlic-like odor when struck with a hammer (due to the release of arsenic vapor – do this outdoors with caution!).
• Feel:
  • Talc (Mg3Si4O10(OH)2): Soapy or greasy feel.

12. Reaction to Acid: A Bubbly Test

Some minerals, particularly carbonates, react with dilute hydrochloric acid (HCl), producing bubbles of carbon dioxide gas.

• Testing with Acid: Place a drop of dilute HCl on the mineral. Observe whether bubbles form.
• Identifying Carbonates: Calcite (CaCO3) and dolomite (CaMg(CO3)2) are common carbonates that react with acid. Calcite reacts vigorously with cold, dilute HCl, while dolomite reacts weakly unless powdered or heated.
• Safety Precautions: Always wear safety glasses and gloves when working with acid. Work in a well-ventilated area.

Putting It All Together: A Systematic Approach to Mineral Identification

Identifying minerals is like solving a puzzle. It requires a systematic approach and the careful application of multiple tests. Here's a suggested workflow:

1. Initial Observation: Begin by observing the mineral's color, luster, and crystal form (if present).
2. Hardness Test: Determine the mineral's hardness using the Mohs scale and common testing materials.
3. Streak Test: Perform a streak test to determine the color of the mineral in powdered form.
4. Cleavage and Fracture: Examine the mineral for cleavage planes and fracture patterns.
5. Specific Gravity: Estimate or measure the mineral's specific gravity.
6. Special Properties: Test for magnetism, taste, odor, or reaction to acid, if appropriate.
7. Consult Resources: Use mineral identification guides, online databases, and expert knowledge to narrow down the possibilities and confirm your identification.

Common Mineral Misconceptions

• All that glitters is gold: Pyrite, often called "fool's gold," is a common mineral that can be mistaken for gold. However, pyrite is harder than gold and has a black streak, while gold has a gold-colored streak.
• Color is everything: As we've discussed, color can be misleading. Always use other properties in conjunction with color to identify minerals.
• One test is enough: No single test is definitive for mineral identification. Use multiple tests to confirm your identification.
• Minerals are always perfect crystals: Perfect crystal forms are rare in nature. Most minerals occur as irregular aggregates or grains.

The Tools of the Trade: Building Your Mineral Identification Kit

To become a successful mineral detective, you'll need a few basic tools:

• Streak Plate: A piece of unglazed porcelain for testing streak.
• Hardness Kit: A set of materials with known hardness, such as a fingernail, copper penny, steel nail, glass plate, and streak plate.
• Hand Lens or Magnifying Glass: For examining small details and crystal structures.
• Magnet: For testing magnetism.
• Dilute Hydrochloric Acid (HCl): For testing reaction to acid (use with caution!).
• Safety Glasses and Gloves: For protecting your eyes and skin when working with chemicals.
• Mineral Identification Guide: A comprehensive guide with descriptions and images of common minerals.
• Notebook and Pencil: For recording your observations and test results.

Final Thoughts: The Earth Speaks Through Its Minerals

Identifying minerals is a rewarding and fascinating pursuit. It allows us to connect with the Earth on a deeper level and understand the processes that have shaped our planet. By learning to recognize the properties of minerals, we can unlock the secrets hidden within these silent storytellers and appreciate the beauty and complexity of the natural world. So, grab your tools, venture into the field, and start decoding Earth's secrets, one mineral at a time.