Defining Characteristics of a True Mineral
To understand what is not true of minerals, one must first grasp the rigorous criteria that define them. For a substance to be classified as a mineral by geologists, it must meet five key requirements:
- Naturally Occurring: Minerals must form through natural geological processes, without human intervention. This is why a synthetic diamond, though chemically and structurally identical to a natural one, is not considered a true mineral.
- Inorganic: A mineral must not be derived from living organisms, though some form through biological activity and are still included. Coal, pearl, and obsidian, which are formed by organic processes or lack a crystalline structure, are often classified as mineraloids instead.
- Solid: At normal room temperatures and atmospheric pressure, a mineral must exist as a solid. This excludes liquids and gases, although ice is an exception because it is a solid in nature that meets all other criteria.
- Crystalline Structure: The atoms within a mineral must be arranged in a highly ordered, repeating, three-dimensional pattern. This internal arrangement is what gives a mineral its distinct crystal shape and other physical properties. Glass, which lacks this structure, is therefore not a mineral.
- Definite Chemical Composition: Every specimen of a particular mineral has a specific chemical formula that can vary within a certain range. For example, the mineral halite (table salt) is always composed of sodium and chlorine (NaCl) in a fixed ratio.
Debunking Common Mineral Misconceptions
Several common statements can be false when applied to the broad category of minerals. The most frequent misconception is that all minerals share the same properties or chemical makeup. Below, we'll address some of the most prominent falsehoods.
All Minerals Are Not Metallic
One of the most common false statements is that all minerals are non-metallic. This is incorrect because the mineral kingdom is divided into two broad categories: metallic and non-metallic. Metallic minerals, such as gold and iron ore (hematite), are crucial for industry and exhibit characteristic metallic luster, ductility, and malleability. In contrast, non-metallic minerals like quartz and mica lack these metallic properties.
All Minerals Are Not Silicates
It is also not true that all minerals are composed of oxygen and silicon. While silicate minerals, which contain the silicon-oxygen tetrahedron, make up over 90% of the Earth's crust, they are just one of many mineral groups. Many non-silicate minerals exist, including native elements like gold (Au) and copper (Cu), and halides like halite (NaCl). For example, the native element gold does not contain either oxygen or silicon.
The Formation of Minerals Is Not Limited to Hot Solutions
Another false claim is that minerals only form from hot or cool solutions. In reality, minerals can form through a wide variety of geological processes. These processes include crystallization from molten magma, precipitation from aqueous solutions, biological processes, and the alteration of pre-existing minerals under high temperature and pressure. The vast range of formation conditions demonstrates that mineral creation is not limited to a narrow set of temperature and pressure requirements.
Mineral Properties Are Not Always Consistent
While a mineral species has a characteristic set of properties, it is false to say that a mineral's properties are always consistent. For example, the color of a mineral can be highly variable due to impurities or defects in its crystal structure. The mineral quartz, for instance, can be colorless when pure, but trace impurities can make it appear purple (amethyst) or pink (rose quartz). Therefore, color is often one of the least reliable properties for identifying a mineral. Streak, the color of a mineral's powder, is a much more consistent and diagnostic property.
Comparison of True vs. False Mineral Statements
| Aspect | True Mineral Statement | False Mineral Statement |
|---|---|---|
| Composition | Minerals have a definite chemical composition, though it can vary within a specific range. | All minerals are made of oxygen and silicon. |
| Classification | Minerals can be metallic or non-metallic. | All minerals are non-metallic. |
| Formation | Minerals form through various processes, including magma crystallization and precipitation from solution. | Minerals form only from hot or cool solutions. |
| Structure | Minerals possess a highly ordered crystalline structure. | All naturally occurring solids are minerals. |
| Properties | Physical properties like color can be variable due to impurities. | A mineral's color is always a reliable identifier. |
Conclusion: The Defining Truths of Minerals
In summary, the most common falsehoods about minerals relate to their composition, classification, formation, and physical characteristics. Understanding the five core requirements for a mineral—being a naturally occurring, inorganic solid with a definite chemical composition and an ordered crystalline structure—provides the foundation for distinguishing fact from fiction. By recognizing that not all minerals are metallic, that not all are silicates, and that their properties like color can vary, one gains a more complete and accurate understanding of mineralogy. The statement "which of the following is not true of minerals they are" fundamentally challenges the learner to apply these definitions correctly, often targeting misconceptions about mineral origin or composition.
Ultimately, a true mineral is a product of specific, natural geological processes, and any substance that deviates from these defining characteristics is, by scientific definition, not a mineral, regardless of how similar it may appear. The study of these distinct properties is what allows geologists to identify and classify the vast diversity of minerals found on Earth.
