The Defining Criteria of a Mineral
To be classified as a true mineral, a substance must meet five specific criteria established by geologists:
- Naturally Occurring: A mineral must form through natural geological processes, not be manufactured by humans in a lab. This excludes substances like synthetic diamonds or lab-grown crystals.
- Inorganic: With some exceptions for biogenic minerals like aragonite, a mineral is not derived from living organisms. Organic substances like coal, which forms from decayed plant matter, are therefore not considered minerals.
- Solid: A mineral must be a solid under normal conditions. The exception is native mercury, which is a liquid but was grandfathered in before modern rules. Water ice, however, is considered a mineral.
- Definite Chemical Composition: Each mineral has a specific chemical formula that dictates its elemental makeup. For example, the mineral quartz is always composed of silicon dioxide (SiO2). While some minerals, like olivine, may have a range of compositions, the chemical formula remains defined.
- Crystalline Structure: The atoms within a mineral are arranged in an ordered, geometric, and repeating pattern. This internal atomic structure is the single most defining characteristic that distinguishes minerals from amorphous solids like glass, which lack this internal order.
The Role of Crystalline Structure
The ordered atomic arrangement, or crystalline structure, is arguably the most crucial characteristic of a mineral. This internal framework is what determines many of a mineral's observable physical properties, such as its shape, hardness, and cleavage. When conditions allow for unimpeded growth, this internal order can manifest as a distinct external crystal form. Solids without this atomic arrangement, such as obsidian (volcanic glass), are more accurately termed mineraloids. This fundamental distinction highlights why internal structure is a superior identifier compared to more variable properties like color.
Examples of Crystal Systems
Based on their crystalline structure, minerals are classified into seven crystal systems with varying degrees of symmetry. Examples include:
- Isometric System: Characterized by three equal axes at right angles, forming cubic shapes. Examples include pyrite and garnet.
- Hexagonal System: Possesses a unique six-fold rotational symmetry. Quartz is a well-known example that often forms hexagonal prisms.
- Tetragonal System: Features two equal axes and a third unequal axis, all at right angles. Rutile and zircon belong to this system.
- Orthorhombic System: All three axes are unequal in length and at right angles to one another. Minerals like olivine crystallize in this system.
Other Physical Properties for Mineral Identification
While crystalline structure is the theoretical basis, geologists use several physical properties to identify minerals in the field. These properties are directly influenced by the mineral's chemical composition and internal atomic arrangement.
Identifying Characteristics at a Glance
- Hardness: A mineral's resistance to being scratched, measured using the Mohs hardness scale. Diamond, the hardest mineral, is a 10, while talc is a 1.
- Luster: The way a mineral surface reflects light, described as metallic, vitreous (glassy), pearly, or earthy.
- Streak: The color of a mineral's powder, revealed by scraping it across an unglazed porcelain plate. This is often more reliable than the mineral's external color.
- Cleavage and Fracture: Cleavage is the tendency of a mineral to break along flat, parallel planes of weakness in its atomic structure. Fracture, by contrast, is an uneven or irregular break.
- Specific Gravity: The ratio of a mineral's density to the density of water.
Mineral Property Comparison
| Mineral | Mohs Hardness | Streak | Cleavage | Luster | Distinctive Feature |
|---|---|---|---|---|---|
| Quartz | 7 | White/None | None (Fracture) | Vitreous | Conchoidal fracture |
| Calcite | 3 | White | 3 perfect directions (rhombic) | Vitreous | Reacts with acid |
| Talc | 1 | White | 1 perfect direction | Pearly/Soapy | Very soft, feels soapy |
| Hematite | 5.5-6.5 | Reddish-brown | None | Metallic or Earthy | Consistent reddish-brown streak |
| Pyrite | 6-6.5 | Greenish-black | None (Fracture) | Metallic | "Fool's gold" color |
| Galena | 2.5 | Gray to Black | 3 perfect directions (cubic) | Metallic | High density, cubic shape |
The Uniqueness of Crystalline Structure
While properties like hardness, color, and luster are useful for field identification, they can sometimes be inconsistent. For example, quartz can come in many colors, from clear to purple (amethyst). However, its underlying crystalline structure and chemical composition (SiO2) remain constant. This is why the crystalline structure is the most fundamental and reliable criterion for determining what constitutes a mineral. It represents the immutable, ordered arrangement of atoms that gives the mineral its unique identity, regardless of external impurities or weathering effects. As such, a highly ordered atomic arrangement is the most definitive and characteristic trait of minerals.
Conclusion: The Answer to What Defines a Mineral
In summary, while minerals possess a range of observable physical properties, the single most definitive characteristic is their crystalline structure. This internal, ordered atomic arrangement is the bedrock upon which all other properties are built. It is this fundamental, underlying geometry that sets a mineral apart as a naturally occurring, inorganic solid with a definite chemical composition. The external expression of this internal order manifests in a predictable crystal form under ideal conditions. Therefore, when faced with the question of which characteristic most accurately defines a mineral, the crystalline structure is the scientifically correct answer. For more on the thousands of known mineral species, visit Mindat.org.
Understanding Mineral Properties and Classification
The Defining Feature: Crystalline Structure
Highly Ordered Atomic Arrangement: The most fundamental characteristic of a mineral is its crystalline structure, an internal geometric order of atoms that differentiates it from amorphous solids like glass.
The Building Blocks: Chemical Composition
Definite Chemical Composition: Every mineral has a specific chemical formula, though some can have a range of compositions, like olivine.
The Natural Source: Inorganic Origin
Inorganic and Naturally Occurring: A mineral must be formed through natural, inorganic geological processes, not created by living organisms or in a lab.
The Observable Trait: Physical Properties
Observable Physical Properties: External features like hardness, streak, luster, and cleavage are reliable indicators used for field identification, all of which are determined by the internal structure.
The Differentiator: Mineral vs. Rock
Mineral vs. Rock: A mineral is a pure, single crystalline substance, whereas a rock is typically an aggregate of one or more minerals.
Frequently Asked Questions
What is the primary difference between a mineral and a rock?
A mineral is a single, naturally occurring, inorganic substance with a definite chemical composition and a crystalline structure. A rock, on the other hand, is an aggregate or collection of one or more minerals.
Why is color not a reliable characteristic for identifying minerals?
Color can be a misleading property because it can be altered by trace impurities within the mineral's crystal structure. For example, the mineral quartz can appear in many different colors, including clear, purple (amethyst), and yellow (citrine).
What is the Mohs hardness scale?
The Mohs hardness scale is a 10-point scale that measures a mineral's resistance to scratching. It ranks minerals from 1 (softest, talc) to 10 (hardest, diamond).
What does the term “streak” mean in mineral identification?
Streak is the color of a mineral in its powdered form. It is a more reliable property than body color because it is more consistent and less affected by impurities.
What is the difference between cleavage and fracture?
Cleavage is the tendency of a mineral to break along smooth, flat surfaces of weakness in its atomic structure. Fracture describes an irregular or uneven break when there are no planes of weakness.
Is glass considered a mineral?
No, glass is not a mineral because it lacks a crystalline structure, which is a required characteristic. Glass has an amorphous, or non-crystalline, atomic arrangement.
Can something be a mineral if it was created in a lab?
No, by definition, a mineral must be naturally occurring. Synthetic substances, even if chemically identical to a natural mineral, are not considered true minerals. For example, cubic zirconia is not a mineral.
