A mineral is a naturally occurring, inorganic solid defined by a specific, fixed chemical composition and a highly ordered, characteristic crystal structure. This precise internal atomic arrangement dictates all of the mineral’s external, observable traits. Geologists rely on these non-chemical physical properties to classify and identify the thousands of different mineral species found in the Earth’s crust. Analyzing these consistent, measurable characteristics provides the data necessary for accurate mineral identification.
Visual and Light Interaction Properties
Luster describes the quality and intensity of light reflected from the mineral’s surface, which is broadly categorized as either metallic or non-metallic. Minerals that look like polished metal, such as pyrite, have a metallic luster. Non-metallic types include vitreous (glassy), pearly, silky, resinous, or dull (earthy) appearances.
Color is often the first property noticed, but it can be highly unreliable for identification because minute chemical impurities can drastically change a mineral’s hue. Minerals whose color is consistent due to their fundamental composition, like the green of malachite, are said to be idiochromatic.
A far more reliable diagnostic tool is streak, which is the color of the mineral’s powder. The streak is obtained by rubbing the mineral across an unglazed porcelain plate, which pulverizes a small sample. While a mineral’s surface color may vary, its streak color is much more consistent because the powdered form eliminates the effects of crystal structure and surface condition. Many metallic minerals, like hematite, have a dark streak that differs significantly from their surface color, making this test useful for distinguishing between similar-looking specimens.
Structural and Mechanical Properties
A mineral’s internal atomic structure determines how it resists or yields to applied physical stress, defining its mechanical properties. Hardness is the measure of a mineral’s resistance to scratching or abrasion, which is directly related to the strength of the chemical bonds holding the atoms together. The Mohs Hardness Scale is a relative scale ranging from 1 (talc) to 10 (diamond). Testing involves scratching the mineral against objects of known hardness, such as a fingernail (about 2.5), a copper penny (about 3.5), or a glass plate (about 5.5).
Cleavage is the tendency of a mineral to break cleanly along flat, parallel surfaces, which correspond to planes of relative weakness within the crystal structure. This property is described by the number of distinct directions the mineral breaks and the quality of those surfaces (perfect, good, or poor). For instance, mica exhibits perfect cleavage in one direction, forming thin sheets, while minerals like halite break into cubes, showing three cleavage directions at 90-degree angles.
Fracture describes any breakage that occurs randomly or irregularly, rather than along predetermined planes of structural weakness. When a mineral lacks planes of weak bonding, it will fracture instead of cleave, producing an uneven or rough surface. One common type is conchoidal fracture, which creates smooth, curved surfaces resembling broken glass, often seen in quartz. Other fracture types include hackly (jagged, sharp edges) and splintery or fibrous.
Mass and Form Characteristics
The internal arrangement of atoms influences the macroscopic form a mineral takes when it grows and its mass relative to its volume. Crystal habit, or crystal form, refers to the typical shape a mineral develops when it is allowed to grow without physical restriction. This external shape is a direct reflection of the mineral’s orderly internal atomic structure. Common habits include prismatic (elongated columns), cubic, tabular (flat plates), or acicular (needle-like).
Specific gravity (SG) is a unitless ratio that compares a mineral’s mass to the mass of an equal volume of water. It is essentially a measure of the mineral’s relative density and is a non-destructive way to gauge its composition and atomic packing. Minerals with higher atomic weight elements packed closely together, like gold, will have a high specific gravity and feel noticeably heavier, or have more “heft,” than common silicate minerals.
Specialized Diagnostic Properties
A few minerals possess unique physical properties that, when present, serve as immediate and definitive identifiers. Magnetism is one such property, primarily found in iron-bearing minerals like magnetite, which is strongly attracted to a magnet. Some forms of magnetite, known as lodestone, are natural magnets and can attract small metallic objects.
Fluorescence and phosphorescence are optical properties where a mineral emits visible light when exposed to ultraviolet (UV) radiation. Fluorescent minerals, like fluorite or willemite, glow only while the UV light is on, while phosphorescent minerals continue to glow briefly after the UV source is removed. Other specialized properties include a distinctive taste, such as the salty flavor of halite, or a characteristic feel, like the soapy texture of talc. Certain carbonate minerals, such as calcite, exhibit double refraction, meaning they split a single ray of light into two, causing a double image when viewed through the crystal.