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: Density=Mass in air−Mass in waterMass in air×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 (SG=3.32) and bromoform (SG=2.89)
- Jolly balance (spring balance) measures weight in air and water
- Calculate specific gravity using formula: SG=Weight in air−Weight in waterWeight in air
- 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