5.3 Density, Specific Gravity, and Magnetic Properties

Density, specific gravity, and magnetic properties are crucial for identifying and understanding minerals. These physical characteristics help geologists distinguish between similar-looking minerals and provide insights into their composition and formation.

Measuring density and specific gravity involves various lab techniques, from simple water displacement to advanced X-ray tomography. Magnetic properties, ranging from diamagnetism to ferromagnetism, play a key role in mineral exploration and understanding Earth's magnetic field.

Density and Specific Gravity in Mineral Identification

Defining Density and Specific Gravity

• Density measures mass per unit volume, expressed in g/cm³ for minerals
• Specific gravity represents the ratio of mineral density to water density at 4°C (dimensionless)
• Both are intensive properties, independent of sample size
• Used to distinguish minerals with similar appearances but different compositions
• Indicate elemental composition (heavier elements generally yield higher values)
• Often combined with other physical properties for mineral identification
• Variations within a mineral species suggest impurities or solid solution series

Applications in Mineralogy

• Differentiate visually similar minerals (quartz vs. calcite)
• Estimate mineral composition in solid solution series (plagioclase feldspars)
• Identify gemstones (distinguish diamond from cubic zirconia)
• Assess purity of mineral samples (pure vs. impure gold)
• Determine metal content in ore minerals (galena vs. sphalerite)
• Aid in understanding mineral formation conditions (high-pressure vs. low-pressure polymorphs)
• Support mineral classification systems (Dana classification)

Determining Density and Specific Gravity

Laboratory Methods

• Pycnometer measures volume of displaced liquid by known mineral mass
• Hydrostatic weighing employs Archimedes' principle
  • Weigh mineral in air and suspended in water
  • Calculate density using formula: $\text{Density} = \frac{\text{Mass in air}}{\text{Mass in air} - \text{Mass in water}} \times \text{Density of water}$
• Sink-float method uses heavy liquids of known density
  • Bracket specific gravity by observing if mineral sinks or floats
  • Common liquids methylene iodide ($\text{SG} = 3.32$) and bromoform ($\text{SG} = 2.89$)
• Jolly balance (spring balance) measures weight in air and water
  • Calculate specific gravity using formula: $\text{SG} = \frac{\text{Weight in air}}{\text{Weight in air} - \text{Weight in water}}$
• Water displacement in graduated cylinder for large, irregular samples
  • Combine volume measurement with mass to calculate density

Advanced Techniques and Considerations

• X-ray computed tomography for non-destructive density determination
  • Useful for valuable or rare specimens (meteorites, unique mineral assemblages)
• Factors affecting accuracy
  • Mineral purity (presence of inclusions or intergrowths)
  • Experimental conditions (temperature, pressure)
  • Sample preparation (removal of air bubbles, surface tension effects)
• Density variations in minerals
  • Caused by chemical substitutions (Fe-Mg in olivine)
  • Structural defects (radiation damage in zircon)
  • Polymorphism (diamond vs. graphite)

Magnetic Properties of Minerals

Types of Magnetism

• Diamagnetism weakly repels magnetic fields
  • Present in all materials but often overshadowed
  • Examples quartz, calcite, and feldspar
• Paramagnetism weakly attracts to magnetic fields
  • Caused by unpaired electrons
  • Attraction disappears when field removed
  • Examples biotite, pyroxene, and amphibole
• Ferromagnetism exhibits strongest magnetism
  • Magnetic moments align parallel, even without external field
  • Examples iron, nickel, and cobalt (pure metals)
• Antiferromagnetism involves opposed magnetic moments
  • Results in no net magnetic field
  • Example hematite at room temperature
• Ferrimagnetism has complex magnetic moment arrangements
  • Unequal opposing moments create net magnetization
  • Example magnetite

Magnetic Behavior and Properties

• Curie temperature marks loss of ferromagnetic/ferrimagnetic properties
  • Above this point, material becomes paramagnetic
  • Example magnetite Curie temperature 585°C
• Magnetic susceptibility measures ease of magnetization
  • Dimensionless quantity, varies widely among minerals
  • Used in mineral classification and exploration
• Remanent magnetism persists after external field removed
  • Important in paleomagnetism studies (basalt, magnetite)
• Magnetic domains form within ferromagnetic/ferrimagnetic materials
  • Regions of uniform magnetization
  • Size and arrangement affect overall magnetic properties

Magnetic Minerals in Exploration

Key Magnetic Minerals

• Magnetite (Fe₃O₄) exhibits strongest natural ferromagnetism
  • Benchmark for strong magnetic properties in mineralogy
  • Common in igneous and metamorphic rocks
• Pyrrhotite (Fe₁₋ₓS) shows variable magnetic properties
  • Depends on iron content and crystal structure
  • Important indicator in sulfide ore deposits (nickel, copper)
• Hematite (Fe₂O₃) weakly magnetic in pure form
  • Becomes strongly magnetic when heated or mixed with magnetite
  • Common in banded iron formations
• Ilmenite (FeTiO₃) and chromite (FeCr₂O₄) display ferrimagnetic properties
  • Important in igneous petrology and ore formation
  • Associated with mafic and ultramafic rocks

Exploration Techniques and Applications

• Magnetic minerals serve as pathfinders for ore deposits
  • Help locate deposits associated with magnetic anomalies
  • Example magnetite in iron ore exploration
• Airborne magnetic surveys map subsurface geology
  • Detect variations in Earth's magnetic field due to mineral concentrations
  • Used in regional-scale exploration
• Ground-based magnetic surveys provide detailed local data
  • Higher resolution for specific target areas
  • Handheld magnetometers measure magnetic susceptibility
• Presence and distribution of magnetic minerals inform about
  • Formation conditions (temperature, oxygen fugacity)
  • Metamorphic history (changes in magnetic mineralogy)
  • Tectonic setting (magnetic lineaments, crustal structures)
• Magnetic separation techniques in mineral processing
  • Concentrate valuable magnetic minerals (magnetite, ilmenite)
  • Remove magnetic impurities from industrial minerals