A mineral is a naturally occurring, inorganic solid with a defined chemical composition and a characteristic ordered atomic structure. This specific internal arrangement and fixed chemistry give rise to the unique set of physical properties used for identification. There are thousands of known mineral species, and while many look similar, each possesses a distinct fingerprint of properties determined by its internal structure. Mineral identification relies on systematically testing and observing these stable physical characteristics to differentiate one species from another. The methods range from simple visual inspection to more involved mechanical and chemical tests.
Initial Visual Characteristics
The process of mineral identification begins with a simple, non-destructive observation of the sample’s surface properties. Color is the first characteristic noted, though it is often considered the least reliable property for definitive identification because trace impurities can drastically alter a mineral’s appearance. For example, pure quartz is colorless, but minute amounts of iron can produce the purple hue of amethyst or the yellow of citrine.
Luster describes how light is reflected from the mineral’s surface and is a more consistent visual property than color. Luster is generally divided into two main categories: metallic, which resembles a polished metal surface, and non-metallic, which includes several sub-types. Common non-metallic lusters include vitreous (glassy), pearly, silky, and earthy (dull and non-reflective).
The external shape of a well-formed mineral, known as its crystal habit, also provides an immediate indication of its internal atomic structure. If a mineral is given ample space to grow, it will develop characteristic geometric forms such as cubic, prismatic, or tabular. However, many minerals grow in confined spaces, resulting in an irregular or massive shape that obscures the crystal habit.
Testing Mechanical Resistance
Geologists proceed to test the mineral’s resistance to mechanical stress, which provides reliable data. Hardness, defined as a mineral’s resistance to scratching or abrasion, is quantified using the relative Mohs Scale of Hardness, which ranks ten reference minerals from 1 (talc) to 10 (diamond).
In the field, this scratch test is performed using common objects of known hardness to narrow the possibilities. A human fingernail has a hardness of about 2.5, a copper penny is around 3.5, and a steel knife blade is approximately 5.5. If an unknown mineral can be scratched by a steel nail but not by a copper penny, its hardness is determined to be between 3.5 and 5.5.
The way a mineral breaks under stress further reveals its internal structure. Cleavage is the tendency of a mineral to break along smooth, flat planes of structural weakness due to weaker atomic bonds. Cleavage is described by the number of directions and the quality of the break, such as perfect, good, or poor, and the angles between the planes.
If a mineral has equally strong bonds in all directions, it will instead show fracture, which is an irregular or uneven breakage pattern. A distinctive form of fracture is conchoidal fracture, which produces smooth, curved surfaces resembling broken glass, often seen in quartz or obsidian.
Density and Chemical Reactions
Further testing involves properties related to the mineral’s chemical composition and density. Streak is the color of the mineral in powdered form, determined by rubbing the sample across an unglazed porcelain plate. This property is reliable because, unlike the external color, the streak color is unaffected by impurities or surface weathering. For instance, hematite can appear silver, red, or black in hand sample, but it consistently produces a reddish-brown streak.
Specific gravity is a measure of the mineral’s density compared to the density of water, indicating how heavy a mineral is for its size. Most silicate minerals have a specific gravity between 2.5 and 3.0. Minerals containing heavy metals, such as galena, have a much higher specific gravity, which is often noticeable simply by lifting the sample. Though precise measurement requires laboratory equipment, an estimate of specific gravity is a useful field observation.
Some minerals possess unique chemical properties that allow for quick identification. The classic example is the effervescence test, which involves placing a drop of dilute hydrochloric acid on the sample. Carbonate minerals, such as calcite, contain the carbonate ion (\(\text{CO}_3\)) which reacts with the acid to release carbon dioxide gas, producing a visible fizzing or bubbling. A vigorous reaction with cold, dilute acid is a specific chemical confirmation for the mineral.