Physical Properties of Minerals

Every physical property a mineral displays - its weight, its hardness, the way it breaks, the colour it shows under different lights - follows directly from its crystal structure and chemical composition. This is not a coincidence. Structure and composition constrain properties in the same way that the rules of architecture constrain what a building can look like: within the limits of allowed structural and chemical variation, different samples of the same mineral will display similar properties. The practical consequence is powerful: physical properties can be used to identify minerals, because two samples with the same structure and composition will behave the same way when handled, scratched, broken, or illuminated. ^[1]

Some physical properties have direct economic consequences. Diamond and graphite are both polymorphs of carbon - identical in composition - yet their physical properties could hardly be more different. Diamond’s extraordinary hardness makes it indispensable as an abrasive and cutting material in industry, and its optical clarity and brilliance make it a prized gemstone. Graphite’s extreme softness makes it useful as a lubricant and as the marking material in pencils. The contrast between two minerals sharing the same chemistry makes clear that structure, not composition alone, governs properties. ^[1]

Four Property Groups

Physical properties are conveniently organised into four groups based on what aspect of the mineral they reflect. ^[1]

Mass-dependent properties - density and specific gravity - reflect how much material is packed into a given volume. They are controlled by the atomic weights of the constituent elements and the tightness of the packing arrangement in the crystal structure. ^[1]

Mechanical cohesion properties - hardness, tenacity, cleavage, fracture, and parting - reflect how the crystal structure resists mechanical disruption. The strength and geometry of chemical bonds in the structure control all of these. ^[1]

Light-interaction properties - luster, colour, streak, and luminescence - reflect how the mineral interacts with electromagnetic radiation. They depend on the electronic structure of the constituent atoms and ions and on how light is absorbed, reflected, transmitted, or re-emitted. ^[1]

Other properties - magnetism, electrical behaviour, taste, odour, and reaction with acid - reflect specific aspects of the mineral’s chemistry or electronic structure that do not fit neatly into the first three groups.

Density

Density (ρ) is formally defined as mass (m) distributed per unit volume (v):

Density is expressed in grams per cubic centimetre (g/cm³). It is a fundamental property because it ties the chemical nature of the mineral - which elements it contains - to the geometric nature of the crystal structure - how tightly those elements are packed.

Specific Gravity

Specific gravity (G) is the unitless ratio obtained by dividing a material’s density by the density of water at 4°C: ^[1]

Specific gravity is dimensionless - it is a ratio of two quantities with the same units, so the units cancel. Because the density of water at 4°C is 0.999973 g/cm³, which is essentially 1 g/cm³, the numerical value of specific gravity is virtually identical to the numerical value of density expressed in g/cm³. The practical advantage of specific gravity over density is that it can be measured without knowing the absolute mass or volume of the sample - only the relative buoyancy in water is needed. ^[1]

Controls on Density and Specific Gravity

Two independent factors control how dense a mineral is: the atomic weights of the elements present, and how tightly those elements are packed together in the crystal structure. ^[1]

The Packing Index

The packing index (PI) measures the fraction of a unit cell’s volume that is actually occupied by atoms or ions. It is defined as: ^[1]

where V_C is the total unit cell volume and V_I is the summed volume of all the ions in that unit cell calculated from their ionic radii. ^[1] This treats atoms and ions as spheres with defined radii - a useful approximation even though real electron distributions are not perfectly spherical.

For most minerals the packing index lies between 35 and 74. This means that 35-74% of the crystal structure consists of atoms or ions, and the remainder is equivalent to empty pore space. The theoretical maximum for identical spheres packed as efficiently as possible is 74 - this is the packing density of cubic closest packing and hexagonal closest packing. That most ionic minerals fall below 74 reflects the fact that their anion frameworks are relatively open, with cations sitting in the interstitial spaces between larger anions rather than every site being equivalently filled. ^[1]

Pressure, Packing, and Specific Gravity

As a general rule, minerals that form at high pressure have higher packing indices than minerals of the same composition that form at lower pressure. Pressure compresses the structure, forcing atoms closer together and eliminating interstitial space. The pair of Al₂SiO₅ polymorphs provides a clear illustration: kyanite is the high-pressure polymorph with a packing index of 60.1 and a specific gravity of 3.6, while andalusite, which forms at lower pressure, has a packing index of only 52.3 and a specific gravity of about 3.1. Same composition, different packing, measurably different density - the packing index difference of about 8 units translates directly into a specific gravity difference of about 0.5. ^[1]

Composition and Specific Gravity

The atomic weights of the elements present also control density independently of packing. Minerals built from heavy elements - those with large atomic numbers and masses - are denser than minerals of similar structure built from lighter elements. When a mineral displays extensive ionic substitution, specific gravity typically varies systematically with composition: as heavier ions replace lighter ones, density increases; as lighter ions replace heavier ones, density decreases. This relationship is used in reverse - measuring specific gravity can constrain the composition of a solid solution series mineral in the field. ^[1]

Measuring Specific Gravity

The Jolly Balance

The most common method uses a Jolly balance, a device suited to samples weighing a few grams. The sample is weighed first in air (m_a) and then again while suspended in water (m_w). The difference (m_a - m_w) equals the weight of water displaced, which by Archimedes’ principle equals the weight of a water volume identical to the sample’s volume. Specific gravity follows directly: ^[1]

The Pycnometer

For grains too small for a Jolly balance, a pycnometer is used - a small flask with a ground-glass stopper containing a fine hole. The sample is weighed (m_a), the pycnometer filled with water is weighed (m_p), and the pycnometer containing both water and sample is weighed (m_p+s). The water displaced by the sample has a weight of m_a + m_p - m_p+s, and specific gravity is calculated from that displacement. ^[1]

The displacement formula is: ^[1]

Heavy Liquids

A third approach uses heavy liquids - dense fluids of known specific gravity. A mineral grain placed in a heavy liquid will sink if it is denser than the liquid, float if it is lighter, and remain suspended if it matches the liquid’s specific gravity exactly. ^[1]

Common heavy liquids and their approximate specific gravities at ~25°C are: ^[1]

Bromoform, tetrabromoethane, and diiodomethane have low viscosity and are easy to handle, but all are toxic and must be used in a fume hood. The tungstate liquids are substantially less toxic but have higher viscosities that make them harder to work with. The density of any heavy liquid can be reduced by adding a solvent - acetone for the halogenated liquids, water for the tungstates - to target a specific gravity that matches the mineral of interest. ^[1]

Hefting

In the field, a rough but practical estimate of specific gravity is obtained by hefting - holding a sample in the hand and comparing its perceived weight to its visible size. Quartz (G = 2.65) serves as the mental benchmark for an average specific gravity. Samples that feel noticeably heavy for their size have a higher gravity; those that feel light have a lower one. With practice and comparison to reference samples, reasonable estimates are achievable. ^[1]

Miscellaneous Properties

Beyond the main property groups, several other characteristics are useful in mineral identification. ^[1]

Taste

Minerals that dissolve readily in water may have a perceptible taste. Dissolved ions stimulate taste receptors on the tongue. Halite (NaCl) has a characteristically salty taste, while sylvite (KCl) is also salty but with a distinctly more bitter quality. Samples handled repeatedly in a student laboratory may pick up a salty taste from perspiration, so it is best to test a fresh surface. ^[1]

Odour

Even at room temperature, thermal vibration can dislodge surface molecules from weakly bonded minerals and carry them to odour receptors. Minerals with ionic, covalent, or metallic bonding are generally too strongly held together to produce a detectable odour at room temperature. Minerals with van der Waals and hydrogen bonding may have a characteristic smell - clay minerals are widely perceived to have an earthy or argillaceous odour for precisely this reason. ^[1]

Feel

Feel encompasses surface texture and grain-size perceptions. One feel property that is directly structural is the greasy feel produced by van der Waals bonding. Van der Waals bonds are extremely weak - a finger pressed on graphite’s surface breaks them and allows the mineral to smear, which registers as a greasy or slippery sensation. ^[1]

Reaction with Acid

Many carbonate minerals react visibly with dilute HCl. The standard test solution is one part concentrated HCl to nine parts water. The reaction with calcite is: ^[1]

Calcite reacts vigorously; dolomite [CaMg(CO₃)₂] reacts only when powdered to increase surface area. Some sulfides, such as sphalerite, release H₂S when attacked by the acid - an unmistakable rotten-egg odour. Field geologists routinely carry dilute HCl to distinguish calcite-bearing rocks from those without. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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