Defining Characteristics of Minerals
For a substance to be classified as a mineral, it must meet five key criteria: it must be a naturally occurring solid, have a defined chemical composition, possess an orderly internal atomic structure (crystalline), and be inorganic. These fundamental features are the foundation for all other physical and chemical properties used for identification.
Physical Properties for Identification
Geologists rely on a variety of physical characteristics to identify and differentiate minerals in the field and laboratory. Unlike color, which can be highly variable due to impurities, these properties are often reliable indicators of a mineral's identity.
Common Physical Properties of Minerals
- Hardness: A mineral's resistance to scratching is measured on the Mohs Hardness Scale, which ranks ten minerals from 1 (talc, softest) to 10 (diamond, hardest). A mineral can scratch any mineral with a lower number and will be scratched by any mineral with a higher number.
- Luster: This describes the way a mineral reflects light from its surface. Terms include metallic (like steel), vitreous (glassy), pearly, silky, or dull (earthy).
- Streak: The color of a mineral in its powdered form. A mineral's streak is often more consistent than its external body color and is determined by rubbing the mineral on an unglazed porcelain plate.
- Cleavage and Fracture: Cleavage is the tendency of a mineral to break along flat, parallel planes of weakness within its crystal structure. Fracture describes any breakage that does not follow these planes, resulting in uneven, curved (conchoidal), or jagged surfaces.
- Specific Gravity: A mineral's density relative to the density of water. Minerals with high specific gravity feel heavier for their size compared to minerals with average specific gravity.
- Crystal Habit: The characteristic shape that an individual crystal or mineral aggregate takes when it grows. This can be influenced by environmental conditions but is often diagnostic.
- Magnetism: Some minerals, like magnetite, are magnetic and will be attracted to a magnet.
Internal Atomic Structure: The Crystalline Framework
At the microscopic level, a mineral's atoms are arranged in a specific, repeating three-dimensional pattern known as a crystal lattice. This internal structure is responsible for many of the macroscopic physical properties. For example, the strong, compact bonding in diamond's carbon lattice accounts for its exceptional hardness, while the layered structure of graphite's carbon atoms explains why it is soft and can easily break into sheets. This ordered arrangement is the key difference between true minerals and amorphous natural solids like glass, which are classified as mineraloids.
Chemical Composition and Mineral Classification
Each mineral has a fixed chemical composition, which is often represented by a chemical formula, such as SiO2 for quartz. The specific combination of elements and their proportions gives each mineral its unique identity. While some minerals can have slight variations in composition (known as solid solutions) where one element substitutes for another, the overall chemical structure remains consistent. Geologists use this chemical composition as a basis for classifying minerals into groups, such as silicates, oxides, and carbonates.
Major Mineral Groups
| Mineral Group | Description | Example Minerals | Typical Uses |
|---|---|---|---|
| Silicates | Contain silicon and oxygen; the most common rock-forming minerals. | Quartz, Feldspar, Mica, Olivine | Glass, Ceramics, Construction |
| Oxides | Composed of metal cations bonded to oxygen anions. | Hematite (Fe₂O₃), Magnetite (Fe₃O₄) | Iron Ore, Pigments, Magnets |
| Carbonates | Contain the carbonate anion (CO₃)²⁻. | Calcite (CaCO₃), Dolomite | Cement, Building Stone, Antacids |
| Sulfides | Contain sulfur and a metallic element. | Pyrite (FeS₂), Galena (PbS) | Ores for Lead and Copper |
| Native Elements | Composed of a single element. | Gold (Au), Copper (Cu), Diamond (C) | Jewelry, Electronics, Industrial |
Conclusion
In conclusion, the major features of minerals are a combination of fundamental defining criteria and a host of physical and chemical properties. From the internal, ordered atomic structure that dictates crystalline form to the external, observable properties like hardness and luster, each characteristic provides a critical clue for identification. The consistent chemical makeup and systematic classification allow for a deeper understanding of Earth's crust and the valuable resources it contains. By examining these features, mineralogists can not only identify specific minerals but also infer their formation environment and potential uses in everything from industrial applications to precious gemstones. The study of mineral features is therefore a fundamental pillar of Earth science.
A Simple Guide to Mineral Properties: Identify Minerals by Their Key Features
Minerals vs. Rocks: The Key Distinction
One of the most common points of confusion is the difference between a mineral and a rock. While minerals are pure, naturally occurring solids with a consistent chemical formula and crystal structure, rocks are aggregates of one or more minerals. A rock's composition can be variable, while a mineral's is predictable. This means that while a rock is made of minerals, not all minerals are rocks.
How to Identify Minerals
To identify a mineral, start by observing its defining properties. Check its hardness using the Mohs scale, noting how it scratches common items like a fingernail, penny, or glass. Observe its luster, describing how it reflects light (e.g., metallic, glassy, dull). Perform a streak test by rubbing it against an unglazed porcelain plate to find its powder color, a much more reliable indicator than its body color. Look for cleavage planes or fracture patterns, and if possible, assess its density or specific gravity, which determines its heaviness relative to its size. Finally, note any unique properties like magnetism.
The Importance of Crystal Structure
Crystal structure is fundamental because it dictates a mineral's physical properties. The internal arrangement of atoms determines whether a mineral will have a perfect cleavage, a specific crystal shape, or a high or low hardness. For example, the regular, repeating pattern of salt's atoms causes it to form cubic crystals, which can be observed at a larger scale. This order is the very essence of what makes a mineral a mineral.
How Minerals Get Their Colors
A mineral's color can be caused by its chemical composition or by impurities and trace elements. For instance, malachite is always green due to its copper content, making its color diagnostic. In contrast, quartz can be many different colors, from purple (amethyst) to yellow (citrine), due to the presence of trace iron. Because color can be misleading, mineralogists use the streak test, which is more reliable.
Understanding Specific Gravity
Specific gravity is the ratio of a mineral's density to the density of water. Minerals like galena and gold have very high specific gravity, making them feel much heavier than common minerals like quartz. This property is determined by the mineral's chemical makeup and how its atoms are packed together. A high specific gravity can be a diagnostic feature, especially for metallic minerals.
Other Special Mineral Properties
Beyond the primary identifying features, some minerals have special properties that can help in their identification. For example, halite (rock salt) has a salty taste. Calcite reacts with dilute acid, producing a bubbling effect. Certain minerals, like fluorite, exhibit fluorescence, glowing under ultraviolet light. Magnetite is strongly magnetic, while quartz is piezoelectric, meaning it generates an electrical charge when compressed.
What are the main characteristics that define minerals?
The main characteristics are that minerals must be naturally occurring, inorganic, solid, have a defined chemical composition, and possess a crystalline structure. These five criteria are the basis of a mineral's identity.
How does the Mohs hardness scale work?
The Mohs scale ranks minerals from 1 (softest) to 10 (hardest) based on their ability to scratch one another. If an unknown mineral can scratch gypsum (2) but is scratched by calcite (3), its hardness is between 2 and 3.
Why is a mineral's streak more reliable than its color?
A mineral's body color can be variable due to impurities, but its streak—the color of its powder—is typically consistent. For example, hematite can be black or red but always produces a reddish-brown streak, making the streak a more reliable diagnostic tool.
What is the difference between cleavage and fracture?
Cleavage is the tendency of a mineral to break along smooth, flat planes of weakness in its atomic structure. Fracture is the irregular breakage that occurs when a mineral's bonds are equally strong in all directions, such as the curved, shell-like fracture in quartz.
Why is specific gravity useful for identification?
Specific gravity measures how dense a mineral is compared to water, providing a reliable numerical value. This is particularly helpful for distinguishing between minerals of similar appearance, as high-density minerals like galena feel noticeably heavier.
How is crystal habit different from crystal structure?
Crystal structure is the internal, orderly arrangement of atoms within a mineral. Crystal habit is the external shape or form the mineral takes as it grows. For example, the internal cubic structure of halite results in its common external cubic habit.
Are all minerals magnetic?
No, only a few minerals exhibit strong magnetism, such as magnetite and pyrrhotite. Other minerals are either weakly magnetic (paramagnetic) or repelled by a magnetic field (diamagnetic). Magnetism is a diagnostic property for the specific minerals that possess it.
