Basic Identification Keywords: Understanding Physical Properties
To identify minerals, mineralogists and enthusiasts rely on a set of descriptive keywords based on a mineral’s physical properties. These characteristics are observable and testable, providing a reliable method for telling different minerals apart. The most crucial include:
Hardness: The Mohs Scale
Mineral hardness is its resistance to being scratched. The Mohs Hardness Scale, a ten-point scale ranging from 1 (softest) to 10 (hardest), is the key keyword system for this property. A simple scratch test using common objects helps determine a mineral's relative hardness, a cornerstone of mineral identification.
Luster
Luster describes how a mineral reflects light from its surface and is a key descriptive keyword. Terms include 'metallic' (like polished metal), 'vitreous' (glassy), 'pearly' (iridescent), 'greasy' (appears coated in oil), 'silky' (fibrous), and 'dull' or 'earthy' (non-reflective).
Streak
While a mineral's outward appearance can be deceiving due to impurities, its streak—the color of its powder—is a more reliable keyword. The test involves dragging the mineral across an unglazed porcelain streak plate. For example, the brass-colored pyrite often leaves a greenish-black streak.
Cleavage and Fracture
These keywords describe how a mineral breaks. Cleavage is the tendency of a mineral to break along flat, parallel surfaces, a property determined by its crystal structure. Calcite, for instance, has three planes of cleavage, causing it to break into distinct rhomboid shapes. Fracture, on the other hand, describes an irregular or uneven break when no cleavage is present. Conchoidal fracture, a characteristic of quartz, produces smooth, curved surfaces.
Crystal Habit and Specific Gravity
Crystal habit refers to the characteristic shape a mineral grows into, from cubic and prismatic to bladed or dendritic. Specific gravity, a measure of a mineral's density, is another important identifier.
Chemical Classification Keywords
Beyond physical traits, minerals are also classified by their chemical composition. This leads to key classification keywords that group minerals into families.
- Native Elements: Composed of a single element, such as gold (Au) or copper (Cu).
- Silicates: The largest mineral group, formed by silicon and oxygen atoms. This group includes common rock-forming minerals like quartz, feldspar, and mica.
- Oxides: Feature oxygen combined with other elements, such as hematite (iron oxide).
- Sulfides: Characterized by sulfur combined with a metal, such as pyrite (iron sulfide) and galena (lead sulfide).
- Carbonates: Contain the carbonate (CO3) anion, with calcite (calcium carbonate) as a prime example.
- Halides: Minerals containing a halogen element (like chlorine or fluorine), such as halite (rock salt) and fluorite.
Economic and Specialty Keywords
Mineral keywords extend to their economic importance and use. These keywords are vital for mining, industry, and technology.
Economic Minerals
This category includes minerals that can be profitably mined. Keywords here include 'ore,' which is a compound containing a high proportion of a valuable metal, and names of specific minerals like 'bauxite' (for aluminum) and 'chalcopyrite' (for copper).
Gemstones
Gemstone keywords like 'diamond,' 'sapphire,' 'ruby,' and 'emerald' describe minerals valued for their beauty and durability in jewelry. Many are a form of a common mineral with impurities that give them their vibrant colors.
Rare Earth Minerals
These keywords refer to a group of seventeen elements used in advanced electronics and green technology. Examples include 'bastnasite' and 'monazite'.
Identifying Common Minerals: A Comparison
To illustrate the use of these keywords, consider a comparison of three common minerals based on their physical properties:
| Property | Quartz | Calcite | Pyrite |
|---|---|---|---|
| Chemical Class | Silicate | Carbonate | Sulfide |
| Hardness (Mohs) | 7 | 3 | 6–6.5 |
| Luster | Vitreous (glassy) | Vitreous to dull | Metallic |
| Streak | White | White | Greenish-black |
| Cleavage | None (fractures) | 3 directions (perfect) | None (fractures) |
| Fracture | Conchoidal | Conchoidal to uneven | Uneven |
Conclusion
Understanding what are the keywords for minerals is the foundation of mineralogy. By learning and applying terms related to physical properties (hardness, luster, streak), chemical classifications (silicates, sulfides), and specific applications (economic minerals, gemstones), one can confidently identify and appreciate the vast diversity of minerals found on Earth. This comprehensive keyword set is essential for anyone interested in the geosciences, from the amateur collector to the professional geologist. For more in-depth resources on mineral identification and classification, refer to the US Geological Survey website.
