Specific gravity and density are fundamental physical properties that describe how much matter is contained within a given volume of a mineral. These properties are particularly useful for mineral identification because they are intensive properties—meaning they are independent of the size or shape of the sample.
Understanding Density
What is Density?
Density is defined as mass per unit volume and is typically measured in grams per cubic centimeter (g/cm³) for minerals. The formula for density is:
Density = Mass / Volume
Factors Affecting Density
A mineral's density is influenced by several key factors:
- Atomic weight: Minerals composed of elements with higher atomic weights tend to be denser
- Atomic packing: The efficiency with which atoms are arranged in the crystal structure affects density
- Impurities: The presence of other elements can either increase or decrease a mineral's density
- Pressure and temperature: These conditions during mineral formation can impact density
Specific Gravity Explained
What is Specific Gravity?
Specific gravity (SG) is a dimensionless quantity that represents the ratio of a mineral's density to the density of water at 4°C (which is 1 g/cm³). This makes specific gravity numerically equal to the density in g/cm³, but without the units. The formula for specific gravity is:
Specific Gravity = Mineral Density / Water Density
Why Use Specific Gravity?
Specific gravity is particularly useful in mineral identification for several reasons:
- It's easy to measure using simple equipment
- Many minerals have characteristic specific gravity values
- It helps distinguish between minerals that may appear similar otherwise
- It provides clues about a mineral's chemical composition
Common Specific Gravity Ranges
| Specific Gravity Range | Description | Examples |
|---|---|---|
| 1.0-2.0 | Very light | Graphite (2.09-2.23), Sulfur (2.07) |
| 2.0-3.0 | Light | Quartz (2.65), Feldspar (2.55-2.76), Gypsum (2.32) |
| 3.0-4.0 | Medium | Calcite (2.71), Fluorite (3.18), Apatite (3.15-3.20) |
| 4.0-5.0 | Heavy | Pyrite (4.95-5.10), Hematite (5.26-5.30), Magnetite (5.17-5.18) |
| 5.0-7.0 | Very heavy | Galena (7.4-7.6), Sphalerite (3.9-4.1), Chalcopyrite (4.1-4.3) |
| >7.0 | Extremely heavy | Gold (15.5-19.3), Silver (10.1-11.1), Platinum (14.0-19.0) |
Measuring Specific Gravity
Field Methods
In the field, geologists often use simple methods to estimate specific gravity:
- Heft test: Estimating weight by lifting the sample (experienced geologists can often make surprisingly accurate estimates)
- Float test: Observing whether a mineral floats or sinks in liquids of known density
Laboratory Methods
For more precise measurements, laboratory methods include:
- Water displacement: Measuring the volume of water displaced by a mineral sample and calculating density
- Jolly balance: Using a specialized balance to measure weight in air and water
- Pycnometer: A specialized glass container for measuring the specific gravity of powders and small samples
- Digital density meters: Modern instruments that provide quick and accurate density measurements
Practical Applications
Understanding specific gravity and density has numerous practical applications:
- Mineral identification: These properties are diagnostic for many mineral species
- Ore processing: Density differences are used to separate valuable minerals from waste material
- Gemstone evaluation: Specific gravity helps authenticate gemstones and detect synthetics
- Geological interpretation: Density variations in rocks provide insights into Earth's interior structure
- Construction materials: Density is an important property for determining the suitability of rocks and minerals for various construction purposes