What Are The Five Properties Of Minerals

The world of minerals is a fascinating realm where chemistry meets geology, and understanding their properties is key to unlocking the secrets of our planet. Minerals are the fundamental building blocks of rocks, and recognizing them requires a grasp of their defining characteristics. These natural, solid, inorganic substances possess a definite chemical composition and an ordered atomic structure, resulting in unique physical properties that allow us to identify and classify them.

What Defines a Mineral?

Before diving into the five properties, let's reiterate the five-part definition of a mineral:

1. Naturally occurring: Minerals are formed by natural geological processes, without human intervention. Synthetic substances created in a lab are not considered minerals.
2. Solid: Minerals exist in a solid state at standard temperature and pressure conditions on Earth.
3. Inorganic: Minerals are not composed of organic (carbon-based) compounds. Substances produced by living organisms, such as wood or coal, are not minerals.
4. Definite chemical composition: Minerals have a specific chemical formula or a limited range of chemical compositions. This composition can be expressed using chemical symbols. For example, quartz has the chemical formula SiO2 (silicon dioxide).
5. Ordered atomic structure: Minerals have a crystalline structure, meaning their atoms are arranged in a highly ordered and repetitive pattern. This internal structure is what gives minerals their distinct physical properties.

These properties, taken together, differentiate minerals from other substances. They provide a framework for understanding how minerals form, how they interact with their environment, and how we can use them in various applications. Now, let's explore the five key physical properties used to identify minerals:

The Five Key Properties of Minerals

The identification of minerals often relies on observing and testing their physical properties. These properties are determined by the mineral's chemical composition and internal atomic structure. The five most commonly used properties are:

1. Hardness
2. Cleavage and Fracture
3. Luster
4. Color and Streak
5. Specific Gravity

We will explore each of these in detail.

1. Hardness: Resistance to Scratching

Hardness is a mineral's resistance to being scratched. It is a relative property, meaning we compare the hardness of one mineral to another. The most widely used scale for assessing hardness is the Mohs Hardness Scale, developed by German mineralogist Friedrich Mohs in 1812. This scale ranks ten common minerals from 1 (softest) to 10 (hardest), based on their ability to scratch one another.

• The Mohs Hardness Scale:

  1. Talc
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase (Feldspar)
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond

• Using the Mohs Scale:

  To determine the hardness of an unknown mineral, you attempt to scratch it with minerals of known hardness from the Mohs scale. For example:

  • If the unknown mineral is scratched by fluorite (hardness 4) but not by calcite (hardness 3), its hardness is between 3 and 4.
  • If the unknown mineral scratches apatite (hardness 5) but is scratched by orthoclase (hardness 6), its hardness is between 5 and 6.

• Common Objects and Their Approximate Hardness:

  It's helpful to relate the Mohs scale to common objects to get a better sense of mineral hardness:

  • Fingernail: ~2.5
  • Copper Penny: ~3.5
  • Steel Knife Blade: ~5.5
  • Glass Plate: ~5.5-6.5
  • Streak Plate: ~6.5-7

• Importance of Hardness:

  Hardness is a useful property for mineral identification because it is relatively easy to test. It also provides information about the strength of the chemical bonds within the mineral's structure. Minerals with strong, tightly bonded structures, like diamond, are very hard. Minerals with weaker bonds, like talc, are very soft.

• Limitations of Hardness:

  It's important to note that the Mohs scale is a relative scale, not an absolute one. The difference in absolute hardness between minerals on the scale is not linear. For example, diamond (hardness 10) is significantly harder than corundum (hardness 9). Furthermore, hardness can vary slightly depending on the orientation of the mineral's crystal structure. Impurities can also affect a mineral's hardness.

2. Cleavage and Fracture: How Minerals Break

Cleavage and fracture describe how a mineral breaks when subjected to stress. This property is directly related to the arrangement and strength of the chemical bonds within the mineral's crystal structure.

• Cleavage:

  Cleavage is the tendency of a mineral to break along specific planes of weakness, creating smooth, flat surfaces. These planes of weakness correspond to directions in the crystal structure where the chemical bonds are weaker. When a mineral exhibits cleavage, it will consistently break along these planes, producing characteristic shapes.

  • Describing Cleavage: Cleavage is described by:

    • The number of cleavage planes: This refers to the number of different directions along which the mineral breaks smoothly. Minerals can have one, two, three, four, or even six cleavage planes.
    • The angles between cleavage planes: The angles at which the cleavage planes intersect are also important. For example, some minerals have cleavage planes that intersect at 90 degrees (right angles), while others intersect at different angles.
    • The quality of cleavage: This describes how perfect or imperfect the cleavage is. Perfect cleavage results in smooth, flat, reflective surfaces. Good cleavage produces less perfect surfaces, and poor cleavage results in uneven or stepped surfaces.

  • Examples of Cleavage:

    • Mica (e.g., muscovite, biotite): Exhibits perfect cleavage in one direction, resulting in thin, flexible sheets. This is due to the layered structure of mica minerals.
    • Halite (NaCl): Has perfect cubic cleavage in three directions at 90 degrees, causing it to break into cubes.
    • Calcite (CaCO3): Shows perfect rhombohedral cleavage in three directions, resulting in rhombohedral-shaped fragments.
    • Fluorite (CaF2): Exhibits octahedral cleavage in four directions.

• Fracture:

  Fracture describes the way a mineral breaks when it does not cleave along specific planes. It is an irregular or uneven breakage pattern.

  • Types of Fracture:

    • Conchoidal Fracture: Produces smooth, curved surfaces resembling the interior of a seashell. This type of fracture is common in minerals like quartz and obsidian (volcanic glass).
    • Uneven or Irregular Fracture: Results in rough, irregular surfaces. Many minerals exhibit this type of fracture.
    • Hackly Fracture: Characterized by jagged, saw-toothed edges. This is often seen in metals.
    • Earthy Fracture: Produces a crumbly or powdery surface, similar to that of soil.

• Distinguishing Cleavage from Fracture:

  It's essential to distinguish cleavage from fracture. Cleavage planes are smooth, flat, and often reflective, while fracture surfaces are irregular and uneven. Use a hand lens or magnifying glass to examine the breakage surface closely. If you see a series of parallel, stepped surfaces, it's likely cleavage. If the surface is curved, rough, or jagged, it's likely fracture.

• Importance of Cleavage and Fracture:

  Cleavage and fracture are valuable properties for mineral identification because they reflect the internal atomic structure of the mineral. They can help distinguish between minerals with similar appearances. For example, both feldspar and quartz are light-colored and hard, but feldspar exhibits cleavage, while quartz exhibits conchoidal fracture.

3. Luster: How Light Reflects from a Mineral's Surface

Luster describes the way light interacts with the surface of a mineral. It refers to the quality and intensity of light reflected from a mineral's surface, and it is a subjective property based on visual observation.

• Two Main Categories of Luster:

  1. Metallic Luster: Minerals with a metallic luster have a shiny, reflective surface similar to that of polished metal. They are opaque (do not transmit light) and often have a dark or gray color. Examples include pyrite (fool's gold), galena (lead sulfide), and chalcopyrite (copper iron sulfide).

  2. Nonmetallic Luster: Minerals with a nonmetallic luster do not look like metal. They can transmit light, at least through thin edges. Nonmetallic lusters are further subdivided into several categories:

     • Vitreous (Glassy): This is the most common type of nonmetallic luster, resembling the appearance of glass. Examples include quartz, tourmaline, and olivine.
     • Resinous: Looks like resin or plastic, with a slightly greasy appearance. Examples include sphalerite and some types of sulfur.
     • Pearly: Has a iridescent, opalescent appearance similar to that of a pearl. This is often seen in minerals with layered structures, such as talc and apophyllite.
     • Greasy: Appears as if the surface is coated with a thin layer of oil or grease. This is due to the scattering of light by microscopic surface irregularities. Examples include serpentine and some forms of nephrite jade.
     • Silky: Exhibits a fibrous, silky appearance due to the presence of fine, parallel fibers. Examples include asbestos minerals and some forms of gypsum (satin spar).
     • Adamantine: Possesses a brilliant, diamond-like luster. This is characteristic of minerals with a high refractive index, such as diamond and cerussite.
     • Earthy: Has a dull, nonreflective appearance similar to that of soil. This is common in minerals with a porous or fine-grained texture, such as bauxite and some types of hematite.

• Describing Luster:

  When describing luster, it's important to use precise terminology and compare the mineral's appearance to familiar materials. Consider the intensity of the reflected light, the quality of the surface (e.g., smooth, rough, fibrous), and any distinctive visual effects (e.g., iridescence, opalescence).

• Factors Affecting Luster:

  Luster is influenced by several factors, including:

  • Chemical Composition: The type of elements present in the mineral and their arrangement in the crystal structure affect how light is absorbed and reflected.
  • Surface Texture: A smooth, polished surface will produce a more brilliant luster than a rough, uneven surface.
  • Refractive Index: Minerals with a high refractive index bend light more strongly, resulting in a brighter, more brilliant luster.
  • Transparency: Opaque minerals have a metallic luster, while transparent or translucent minerals have a nonmetallic luster.

• Importance of Luster:

  Luster is a useful property for mineral identification, especially when combined with other physical properties. It can help distinguish between minerals with similar colors and hardness. For example, galena (metallic luster) and calcite (vitreous luster) can both be gray, but their luster is distinctly different.

4. Color and Streak: Visual Clues to a Mineral's Identity

Color and streak are visual properties that can be helpful in mineral identification, but they must be used with caution. Color can be unreliable due to impurities, while streak provides a more consistent indicator.

• Color:

  Color is the most obvious property of a mineral, but it is often the least reliable for identification purposes. Many minerals can occur in a variety of colors due to the presence of trace elements or impurities in their crystal structure. These impurities can absorb certain wavelengths of light, resulting in the mineral appearing a different color.

  • Examples of Color Variation:

    • Quartz: Can be clear (rock crystal), white (milky quartz), purple (amethyst), pink (rose quartz), gray (smoky quartz), yellow (citrine), and many other colors. These color variations are due to trace amounts of elements like iron, aluminum, or titanium.
    • Fluorite: Occurs in a wide range of colors, including purple, blue, green, yellow, and colorless. The color is caused by defects in the crystal lattice and the presence of trace elements.
    • Tourmaline: Can be black (schorl), brown (dravite), green (verdelite), pink (rubellite), blue (indicolite), and multicolored. The color depends on the mineral's chemical composition, particularly the presence of iron, magnesium, and lithium.

  • Describing Color: When describing color, it's important to be as specific as possible. Use terms like "deep blue," "pale green," "yellow-brown," or "reddish-orange" to accurately convey the mineral's hue.

• Streak:

  Streak is the color of a mineral in its powdered form. It is a more reliable property than color because it is less affected by surface alterations or impurities. To determine the streak, you rub the mineral across a streak plate, which is a piece of unglazed porcelain. The streak plate has a hardness of approximately 6.5 on the Mohs scale, so it will only work for minerals softer than that.

  • Performing a Streak Test:

    1. Hold the streak plate firmly on a flat surface.
    2. Grasp the mineral firmly and rub it across the streak plate with moderate pressure.
    3. Observe the color of the powder left on the streak plate.

  • Describing Streak: The streak can be the same color as the mineral, a different color, or colorless. Some minerals do not produce a streak because they are harder than the streak plate.

    • Examples of Streak:

      • Hematite (Fe2O3): Has a characteristic reddish-brown streak, even though the mineral itself can be black, gray, or reddish-brown.
      • Pyrite (FeS2): Has a black or greenish-black streak, despite its brass-yellow color.
      • Gold (Au): Has a golden-yellow streak.
      • Galena (PbS): Has a gray streak.
      • Quartz (SiO2): Does not produce a streak because it is harder than the streak plate.

• Why Streak is More Reliable than Color:

  Streak is more reliable than color because the powdered form of a mineral is less affected by surface alterations, such as weathering or oxidation. The streak represents the true color of the mineral's internal composition.

• Limitations of Color and Streak:

  • Color: As mentioned earlier, color can be highly variable and unreliable for mineral identification.
  • Streak: The streak test only works for minerals softer than the streak plate (hardness 6.5). Minerals harder than that will scratch the streak plate and not produce a streak. Some minerals have a very faint or powdery streak that is difficult to observe.

• Importance of Color and Streak:

  Despite their limitations, color and streak can be useful in mineral identification when used in conjunction with other physical properties. They can help narrow down the possibilities and provide additional clues about the mineral's identity. For example, if a mineral has a metallic luster and a reddish-brown streak, it is likely hematite.

5. Specific Gravity: Density in Disguise

Specific gravity is the ratio of the density of a mineral to the density of water. It is a measure of how much heavier a mineral is compared to an equal volume of water. Specific gravity is a unitless number, as it is a ratio of two densities.

• Density vs. Specific Gravity:

  • Density: Is defined as mass per unit volume (e.g., grams per cubic centimeter, kg/m3).
  • Specific Gravity: Is the ratio of a substance's density to the density of a reference substance, usually water at 4°C (density = 1 g/cm3).

• Determining Specific Gravity:

  Specific gravity can be determined by measuring the weight of a mineral in air and its weight when submerged in water. The following formula is used:

  Specific Gravity = Weight in Air / (Weight in Air - Weight in Water)

• Approximate Specific Gravity:

  For most purposes, an approximate specific gravity is sufficient for mineral identification. You can estimate specific gravity by simply comparing the "heft" or weight of a mineral in your hand to the size of the mineral. Minerals with high specific gravity will feel noticeably heavier than minerals with low specific gravity of the same size.

  • Examples of Specific Gravity:

    • Quartz (SiO2): Has a specific gravity of about 2.65.
    • Feldspar (e.g., orthoclase): Has a specific gravity of about 2.56-2.76.
    • Galena (PbS): Has a high specific gravity of about 7.4-7.6 due to the presence of lead.
    • Gold (Au): Has a very high specific gravity of about 19.3.

• Factors Affecting Specific Gravity:

  Specific gravity is primarily determined by the chemical composition and crystal structure of the mineral. Minerals containing heavy elements, such as lead, gold, or uranium, will have high specific gravities. The way the atoms are packed together in the crystal structure also influences the density and specific gravity.

• Importance of Specific Gravity:

  Specific gravity can be a useful property for mineral identification, especially for distinguishing between minerals with similar appearances. For example, gold and pyrite (fool's gold) can both be brass-yellow and metallic, but gold has a much higher specific gravity, making it feel significantly heavier.

• Limitations of Specific Gravity:

  Determining specific gravity accurately requires specialized equipment and careful measurements. However, even an approximate estimation of specific gravity can provide valuable information about a mineral's identity.

Other Properties Useful for Mineral Identification

While the five properties described above are the most commonly used, several other properties can aid in mineral identification:

• Magnetism: Some minerals are attracted to a magnet. This property is most pronounced in minerals containing iron, such as magnetite (Fe3O4), which is strongly magnetic.
• Taste: Some minerals have a distinctive taste. For example, halite (NaCl) has a salty taste. Note: Tasting minerals should be done with extreme caution and only with minerals known to be non-toxic.
• Odor: Some minerals have a characteristic odor when struck, heated, or moistened. For example, sulfur has a distinctive sulfurous odor.
• Feel: Some minerals have a distinctive feel. For example, talc feels soapy or greasy.
• Reaction to Acid: Some minerals react with dilute hydrochloric acid (HCl), producing effervescence (bubbling). Calcite (CaCO3) is a common example of a mineral that reacts strongly with acid.
• Tenacity: Describes a mineral's resistance to breaking, bending, or deforming. Minerals can be brittle, malleable, ductile, sectile, or flexible.
• Piezoelectricity and Pyroelectricity: Some minerals generate an electrical charge when subjected to mechanical stress (piezoelectricity) or changes in temperature (pyroelectricity). Quartz and tourmaline are examples of piezoelectric minerals.
• Double Refraction: Some transparent minerals, such as calcite, exhibit double refraction, meaning that they split a beam of light into two rays, causing objects viewed through the mineral to appear doubled.

Conclusion

Mastering the identification of minerals requires a combination of knowledge, observation, and practice. By understanding the five key properties of minerals – hardness, cleavage and fracture, luster, color and streak, and specific gravity – you can begin to unlock the secrets of the mineral world. Remember that no single property is definitive, and it is always best to use a combination of properties to identify a mineral accurately. With careful observation and systematic testing, you can develop the skills to identify minerals and appreciate the diversity and beauty of these fundamental building blocks of our planet.