7.2 Mineral Identification and Classification

Minerals are Earth's building blocks, each with unique properties that help us identify them. From color and streak to hardness and crystal form, these characteristics reveal a mineral's identity. Understanding these traits is key to unlocking Earth's geological secrets.

Classifying minerals by composition helps us organize Earth's vast mineral kingdom. Silicates, carbonates, oxides, sulfides, and native elements are just a few groups. By mastering identification techniques, we can better grasp Earth's mineral wealth and its role in shaping our planet.

Identifying Minerals by Properties

Physical Properties for Identification

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• Color: Overall hue of a mineral in visible light, can be diagnostic but may vary within a single mineral species due to impurities or chemical substitutions
• Streak: Color of a mineral when powdered or rubbed against a streak plate, often more consistent than the mineral's overall color
• Hardness: Mineral's resistance to scratching, measured on the Mohs scale from 1 (softest) to 10 (hardest)
  • Common reference minerals: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), corundum (9), and diamond (10)
• Luster: Appearance of a mineral's surface in reflected light
• Cleavage: Tendency to break along flat surfaces
• Fracture: Pattern of breakage when not along cleavage planes
• Crystal form: Geometric shape of a mineral's crystals
• Specific gravity: Ratio of a mineral's density to that of water

Chemical Properties for Identification

• Reaction with acid: Some minerals, particularly carbonates, react with dilute hydrochloric acid (HCl) to produce bubbles of carbon dioxide gas
  • Helps identify minerals such as calcite and dolomite
• Rock-forming minerals: Most common minerals found in Earth's crust and primary components of rocks
  • Examples: quartz, feldspar, mica, amphibole, pyroxene, olivine, and calcite

Classifying Minerals by Composition

Silicates

• Most abundant mineral group, contain silicon (Si) and oxygen (O) tetrahedra as basic structural units
• Further classified based on arrangement of tetrahedra:
  • Framework silicates (quartz)
  • Sheet silicates (mica)
  • Chain silicates (amphibole)

Carbonates

• Contain carbonate ion (CO3^2-) as primary structural unit
• Examples: calcite (CaCO3) and dolomite (CaMg(CO3)2)

Oxides

• Composed of metal cations bonded to oxygen anions
• Examples: hematite (Fe2O3) and magnetite (Fe3O4)

Sulfides

• Contain sulfur (S) bonded to metal cations
• Examples: pyrite (FeS2) and galena (PbS)

Native Elements

• Consist of a single element
• Examples: gold (Au), silver (Ag), copper (Cu), and graphite (C)

Techniques for Mineral Identification

Using Mineral Identification Keys

• Flowcharts or decision trees that guide users through a series of questions about a mineral's properties to narrow down possible identities
• Questions typically address properties such as hardness, streak, luster, cleavage, and reaction with acid

Conducting Basic Tests

• Scratch mineral against materials of known hardness (glass, penny, or fingernail) to estimate hardness
• Observe mineral's cleavage or fracture
• Test reaction with dilute hydrochloric acid

Identifying Unknown Mineral Samples

• Observe physical properties: color, streak, luster, cleavage, and crystal form
• Test hardness and reaction with acid if appropriate
• Use mineral identification key or reference guide to compare observed properties with those of known minerals to determine most likely identity

Considering Mineral Solid Solutions

• Chemical composition varies within a range (olivine can range from forsterite to fayalite)
• If properties do not conclusively match a single species, consider identifying the mineral as belonging to a mineral group rather than a specific species