Identifying minerals is a fundamental geological skill. It allows us to understand the composition and properties of different rocks and geological formations. The mineral identification process involves a systematic approach using various physical and chemical properties.
Field guides are not really useful in identifying many minerals by image, however many give good information relating to the tests below, so it’s good to have a good field guide around. Most libraries should have one.
Here are the essential steps of mineral identification, along with descriptions of each:
The first step in mineral identification is observing the color of the mineral in its natural state. While color can be a useful clue, it is not always reliable as many minerals can have a variety of colors. As such, it is essential also to determine the mineral’s streak. Streak refers to the color of the powdered mineral when scraped against an unglazed porcelain streak plate. The streak color is often more consistent and helps distinguish minerals with similar external colors.
You can get an unglazed tile from a hardware or home improvement store. It should not be extremely rough or ridged – some are made that way to help mortar stick when they are laid. and you can certainly use the back of a glazed tile.
More on color and streak.
Luster describes the way light reflects off the mineral’s surface. Mineralogists recognize several kinds of luster, but the two primary types are metallic and non-metallic. Metallic luster appears similar to metals, while non-metallic luster can be further categorized into various types, such as vitreous (glassy), pearly, silky, greasy, resinous, and earthy. Luster is a critical clue in narrowing down the mineral’s identity.
More on Luster.
Hardness is a mineral’s resistance to scratching. The Mohs scale of hardness, ranging from 1 (softest) to 10 (hardest), is commonly used to determine the relative hardness of minerals. For example, talc has a hardness of 1, while diamond, the hardest naturally occurring mineral, has a hardness of 10. Using common objects of known hardness, one can compare the scratch resistance of the unknown mineral to identify its relative hardness.
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Cleavage refers to how a mineral breaks along planes of weakness, resulting in flat, smooth surfaces. The number and angles of cleavage planes are essential in identifying minerals. For example, mica exhibits “perfect basal cleavage,” breaking into thin, flexible sheets. In contrast, fracture describes how a mineral breaks when it lacks cleavage planes. Fracture types include conchoidal (smooth, curved), uneven, splintery, and fibrous.
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Determining Crystal Form and Habit
Minerals often display characteristic crystal forms due to their internal atomic arrangements. Crystal form refers to the external geometric shape of the mineral, while crystal habit describes the overall appearance of the mineral’s crystals. Some common habits include prismatic, tabular, acicular, bladed, and dendritic. Recognizing crystal habits can provide valuable clues to the mineral’s identity.
More on Crystal Forms.
Measuring Specific Gravity
Specific gravity is the ratio of a mineral’s weight to the weight of an equal volume of water. It is a useful property to differentiate between minerals with similar appearances. For example, quartz and calcite may resemble each other, but their specific gravities are different (quartz ~2.65 and calcite ~2.72).
More on Specific Gravity.
Identifying Special Properties
Certain minerals have unique properties that aid in their identification. For example, magnetism is a characteristic of the mineral magnetite, while double refraction is a property of calcite, causing objects viewed through the mineral to appear double. Additionally, some minerals may exhibit fluorescence under ultraviolet (UV) light, providing a distinctive feature for identification.
More about Fluorescence.
Conducting Chemical Tests
Chemical tests involve using various reagents to react with specific minerals and produce characteristic results. For instance, the acid test is performed by applying dilute hydrochloric acid to carbonate minerals, which causes effervescence due to the release of carbon dioxide gas.
Mineral identification is a crucial skill in the field of mineralogy, enabling researchers to understand the geological history and composition of rocks and minerals. By systematically evaluating properties such as color, streak, luster, hardness, cleavage, fracture, crystal form, specific gravity, special properties, and chemical reactions, mineralogists can confidently identify and categorize minerals, contributing to our understanding of Earth’s diverse geological processes.