Determining whether a substance qualifies as a mineral requires adhering to a strict definition. The generally accepted definition requires that a mineral be naturally occurring, inorganic, solid, possess a definite chemical composition, and have a crystalline structure. A substance failing to meet even one of these criteria is, by definition, not a mineral. This article will delve into common substances that might be mistaken for minerals and explain why they fail to meet the established criteria.
The Defining Characteristics of a Mineral
Before identifying non-minerals, a clear understanding of the mineral definition is essential. Each of the five criteria – naturally occurring, inorganic, solid, definite chemical composition, and crystalline structure – plays a crucial role in classification.
Naturally Occurring
A mineral must be formed by natural geological processes without human intervention. Synthetic substances created in a laboratory, even if they possess all other mineral characteristics, are not classified as minerals. Gemstones like cubic zirconia or synthetic diamonds, while visually similar to their natural counterparts, are excluded based on their origin.
Inorganic
A mineral cannot be derived from living organisms or contain organic carbon. Substances like coal, formed from compressed plant matter, and pearls, secreted by mollusks, are therefore excluded. This criterion distinguishes minerals from biologically-derived materials.
Solid
A mineral must exist in a solid state at standard temperature and pressure. Liquids like water or gases like air, regardless of their chemical composition, cannot be considered minerals. Although water can freeze into ice, which exhibits a crystalline structure, the liquid state disqualifies it as a mineral in its common form.
Definite Chemical Composition
A mineral should have a relatively consistent chemical formula, although some minor variations are allowed. This implies that the elements composing the mineral are present in specific proportions. Minerals like quartz (SiO2) or pyrite (FeS2) have well-defined chemical compositions. Solid solutions, where one element can substitute for another within the crystal lattice, are acceptable within defined limits, but a completely variable composition excludes a substance from mineral classification. For example, opal (SiO2·nH2O) contains variable amounts of water, which makes it a mineraloid rather than a true mineral.
Crystalline Structure
A mineral's atoms must be arranged in a highly ordered, repeating three-dimensional pattern known as a crystal lattice. This internal structure is responsible for a mineral’s characteristic physical properties, such as cleavage, hardness, and optical behavior. Amorphous substances, lacking this ordered structure, are not minerals. Glass, whether naturally occurring obsidian or manufactured glass, is an example of an amorphous solid that doesn't qualify as a mineral.
Common Substances Mistaken for Minerals
Several naturally occurring and human-made substances are frequently confused with minerals. Understanding why they fail to meet the mineral definition is crucial for accurate classification.
Coal
Coal, a combustible sedimentary rock, is formed from accumulated plant matter that has been compressed and altered over millions of years. Its organic origin immediately disqualifies it from being a mineral. Although coal contains carbon, an element found in many minerals, its biological formation process overrides any potential mineral classification.
Obsidian
Obsidian is a volcanic glass formed from rapidly cooled lava. The rapid cooling process prevents the formation of a crystalline structure, resulting in an amorphous solid. While obsidian is naturally occurring, inorganic, and solid, its lack of a defined crystal lattice excludes it from the mineral definition.
Pearls
Pearls are formed within the soft tissue of living mollusks, primarily oysters and mussels. These gemstones consist of layers of nacre, a substance composed of calcium carbonate (aragonite) and organic conchiolin. The organic component and biological origin preclude pearls from being classified as minerals. Although aragonite, the mineral component of nacre, meets the mineral criteria, the overall pearl composition doesn't qualify.
Petroleum
Petroleum, commonly known as crude oil, is a naturally occurring, flammable liquid found beneath the Earth's surface. It's a complex mixture of hydrocarbons and other organic compounds. As a liquid, it fails to meet the solid criterion of the mineral definition. Furthermore, its organic composition further excludes it from consideration as a mineral.
Amber
Amber is fossilized tree resin, appreciated for its color and natural beauty. It's organic in origin, being derived from the resin of ancient trees. As an organic substance, amber does not meet the inorganic requirement of the mineral definition.
Opal
Opal, specifically common opal, is often classified as a mineraloid. While it is composed of hydrated silica (SiO2·nH2O) and can be naturally occurring and solid, the variable water content violates the definite chemical composition criterion. Precious opal does exhibit some degree of internal structure that causes iridescence, but the variability in its composition remains a defining characteristic. Because of this, it's typically termed a mineraloid, meaning it's mineral-like but doesn't fully meet all the criteria.
Water (Ice)
While frozen water, ice, is indeed a mineral, with a specific chemical composition (H2O) and a crystalline structure, liquid water is not. Because the definition of a mineral requires it to be a solid, the liquid state disqualifies water. Ice, therefore, represents a specific phase of water that meets the mineral criteria, but liquid water itself does not.
Distinguishing Minerals from Rocks
It's crucial to distinguish between minerals and rocks. A mineral is a naturally occurring, inorganic solid with a definite chemical composition and crystalline structure. A rock, on the other hand, is an aggregate of one or more minerals. Rocks can also include organic material, such as in sedimentary rocks. Therefore, a rock is a composite substance, while a mineral is a chemically and structurally defined entity. For example, granite is a rock composed primarily of the minerals quartz, feldspar, and mica.
Another example: limestone is a sedimentary rock that's primarily composed of the mineral calcite (calcium carbonate - CaCO3). Here, limestone (the rock) is composed of calcite (the mineral).
Key Takeaways
Understanding what constitutes a mineral requires careful consideration of its defining characteristics. Key points to remember include:
- A mineral must be naturally occurring, not synthetic.
- It must be inorganic, excluding substances of biological origin.
- A mineral must be a solid at standard temperature and pressure.
- It requires a definite, albeit sometimes variable within limits, chemical composition.
- A mineral must possess a crystalline structure with a repeating atomic arrangement.
Substances like coal, obsidian, pearls, petroleum, amber, and common opal fail to meet one or more of these criteria, disqualifying them from being classified as minerals. Recognizing these distinctions is essential for accurate geological and material science understanding.
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