Identifying Minerals: A Guide to Physical Properties
Minerals are the fundamental building blocks of the Earths crust. To identify a mineral specimen, geologists and hobbyists rely on a series of diagnostic physical properties. Because chemical analysis often requires expensive laboratory equipment, these observable physical traits serve as the primary method for field identification.
The Core Physical Properties
1. Color
While color is the most obvious feature, it is often the least reliable. Many minerals appear in a variety of colors due to trace impurities. For example, quartz can be clear, white, pink, purple, or yellow. Always look at other properties before relying on color alone.
2. Streak
The streak is the color of a mineral in its powdered form. You can determine this by scraping the mineral across an unglazed porcelain streak plate. Unlike the mineral's external color, the streak is usually consistent regardless of impurities, making it a much more reliable diagnostic tool.
3. Luster
Luster describes how light reflects off the surface of a mineral. It is generally categorized into two main groups: metallic and non-metallic. Non-metallic lusters include vitreous (glassy), pearly, silky, earthy, or resinous. Identifying the luster is a quick way to narrow down the potential identity of a specimen.
4. Hardness
Hardness measures a mineral's resistance to being scratched, quantified by the Mohs Hardness Scale. The scale ranges from 1 (Talc) to 10 (Diamond). In the field, you can use common objects to estimate hardness: a fingernail (2.5), a copper penny (3.5), a glass plate (5.5), and a steel file (6.5).
5. Cleavage and Fracture
These terms describe how a mineral breaks. Cleavage occurs when a mineral breaks along smooth, flat planes of weakness in its atomic structure. Fracture occurs when a mineral breaks unevenly, resulting in jagged, splintered, or curved surfaces (often called conchoidal fracture, like broken glass).
Secondary Diagnostic Features
- Specific Gravity: This is a measure of the mineral's density compared to the density of water. Simply "hefting" a specimen in your hand can sometimes give you a sense of its relative density.
- Crystal Habit: This refers to the external shape a mineral develops as it grows. While perfect crystals are rare, noting the shape (e.g., cubic, hexagonal, fibrous) can be highly distinctive.
- Magnetism: Some minerals, such as magnetite, are naturally magnetic. Placing a small magnet near the specimen is a simple test.
- Reaction to Acid: Carbonate minerals, most notably calcite, will fizz or effervesce when a drop of weak hydrochloric acid is applied to them.
- Transparency: Minerals can be transparent (light passes through clearly), translucent (light passes through but is diffused), or opaque (no light passes through).
How to Approach Identification
To identify an unknown mineral, follow a systematic process. Start by observing the luster and color. Next, perform a hardness test using your test kit or common items. Determine if the mineral has cleavage planes or if it fractures irregularly. Finally, check the streak. By documenting these observations and comparing them against a standard mineral reference chart, you can systematically eliminate possibilities until you reach a confident identification.
Remember that identification is a process of elimination. If one property doesn't match the suspected mineral, re-evaluate your observations before finalizing your conclusion. Practice with known specimens is the best way to develop the "geologist's eye" for these subtle physical differences.
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