Density is a crucial physical property used to identify minerals because it provides a consistent measure that is inherent to the mineral's makeup.
What is Mineral Density?
Density is defined as mass per unit volume (Density = Mass / Volume). For minerals, this property is particularly useful because it is intrinsic, meaning it remains the same regardless of the sample's size or shape. As stated in the reference, density is one of several intrinsic physical properties of minerals that relate to the composition of the mineral and to the pattern in which the mineral's atoms are arranged.
Why Density is Key to Mineral Identification
Unlike properties that can vary depending on the sample's environment or form (like shape, which changes if you break a crystal), density is constant for a specific mineral under standard conditions. This makes it a reliable characteristic for identification.
Here’s how density helps:
- Reflection of Composition: Different elements have different atomic weights. Minerals composed of heavier elements (like lead in galena or iron in pyrite) will generally have higher densities than those made of lighter elements (like silicon and oxygen in quartz or feldspar).
- Influence of Atomic Arrangement: The way atoms are packed together in a mineral's crystal structure also affects density. Even minerals with similar compositions might have slightly different densities if their atomic arrangements differ (polymorphs). For instance, graphite and diamond are both pure carbon, but diamond's tightly packed structure gives it a much higher density than graphite's layered structure.
- Eliminating Possibilities: By measuring a mineral's density, geologists can narrow down the list of potential minerals. For example, if a sample has a very high density, it's unlikely to be quartz (density ~2.65 g/cm³) but could be galena (density ~7.5 g/cm³).
Using Density in Practice
Geologists often use density in conjunction with other physical properties like hardness, cleavage, luster, and color for positive identification.
- Quantitative Measurement: The most precise method involves carefully measuring the mass of a mineral sample and its volume (often done using water displacement, known as the Archimedes method). The calculated density can then be compared to known values in mineral databases.
- Qualitative Estimation (Specific Gravity): A simpler method is estimating specific gravity, which is the ratio of the mineral's density to the density of water (which is approximately 1 g/cm³). Geologists can feel the relative "heaviness" of a mineral sample compared to a common mineral like quartz of similar size. Minerals with high specific gravity feel noticeably heavy.
Examples of Mineral Densities
| Mineral | Approximate Density (g/cm³) | Key Elements |
|---|---|---|
| Quartz | 2.65 | Silicon, Oxygen |
| Feldspar | 2.5 - 2.8 | Silicon, Oxygen, Aluminum, Potassium/Sodium/Calcium |
| Calcite | 2.71 | Calcium, Carbon, Oxygen |
| Pyrite | 5.02 | Iron, Sulfur |
| Galena | 7.4 - 7.6 | Lead, Sulfur |
| Gold | 19.3 | Gold |
Comparing the density of an unknown sample to established values is a fundamental step in mineral identification.
