A mineral is a naturally occurring, inorganic solid defined by a specific, fixed chemical composition and a highly ordered internal atomic arrangement. Unlike rocks, which are aggregates of one or more minerals, each mineral species possesses a unique set of measurable physical properties. These properties are direct reflections of the mineral’s internal structure and chemical makeup, providing the necessary tools to accurately determine its identity.
The Role of Color and Luster
The most immediate properties observed are color and luster. Color, however, can be an unreliable diagnostic tool because trace chemical impurities can drastically alter a mineral’s hue. For instance, quartz, which is chemically pure silicon dioxide, can appear violet, pink, or smoky gray due to minor substitutions or radiation damage.
Minerals whose color is caused by a main element are idiochromatic, such as the consistent green of copper-containing malachite. Conversely, allochromatic minerals like quartz derive their color from trace elements not essential to their structure. Luster describes the quality of light reflected from a mineral’s surface and is a more consistent property.
Luster is broadly categorized as metallic, giving a reflective sheen like polished metal, or non-metallic. Non-metallic lusters include vitreous (glassy, like quartz), pearly (iridescent, like mica), silky (fibrous), or dull and earthy (non-reflective). This visual property helps narrow down identification and provides a quick assessment of the mineral’s composition and internal bonding.
Defining Structural Appearance: Crystal Habit and Form
A mineral’s appearance when it grows unobstructed is described by its crystal form and habit. The highly organized, repeating pattern of atoms within the solid dictates the external geometric shape of the crystal. When a mineral is allowed to grow freely, it develops smooth, flat faces that meet at specific, characteristic angles.
The term crystal habit describes the common shape a mineral tends to adopt. Common habits include prismatic (elongated like a pencil, seen in tourmaline and quartz) or equant (blocky, seen in pyrite). Other descriptive habits are acicular (needle-like) or botryoidal (resembling a bunch of grapes). Observing the external form is a powerful identification method because the habit is a direct, reliable expression of the mineral’s ordered internal structure.
How Minerals Break: Cleavage and Fracture
The way a mineral breaks under stress reveals the relative strength of its internal atomic bonds. Cleavage is the tendency of a mineral to break along specific, smooth, flat planes of atomic weakness. The quality of cleavage can be described as perfect, good, or poor, defined by the number of directions and the angles at which these planes intersect.
Mica, for example, exhibits one direction of perfect cleavage, allowing it to be peeled into thin sheets. Halite (rock salt) breaks into perfect cubes because it has three cleavage directions that meet at 90-degree angles. If the atomic bonds are equally strong in all directions, the mineral displays fracture, which is an irregular break instead of cleavage.
Fracture can be described by several distinct appearances that are not along a plane of weakness. Conchoidal fracture is a characteristic breakage that produces smooth, shell-like curved surfaces, commonly observed in quartz and glass. Other fracture types include hackly (jagged), uneven (rough and irregular), or splintery.
Properties Revealed by Testing: Hardness and Streak
Hardness and streak are two properties requiring physical interaction and are essential for definitive mineral identification. Hardness is defined as a mineral’s resistance to being scratched, quantified using the Mohs Scale of Hardness. This is an ordinal scale ranging from 1 (talc) to 10 (diamond).
Field identification often uses common objects as reference points on the Mohs Scale. A fingernail has a hardness of about 2.5, a copper penny is approximately 3.5, and glass ranges from 5.5 to 6. The streak test determines the color of a mineral’s powder, revealed by rubbing the specimen across an unglazed porcelain plate.
The streak is often a more reliable indicator than the mineral’s surface color, as it is unaffected by surface weathering or trace impurities. For instance, hematite can appear metallic silver, black, or red, but its powder is consistently reddish-brown. If a mineral is harder than the porcelain plate (about 6.5 on the Mohs Scale), it will scratch the plate instead of leaving a colored powder.