A mineral is a naturally occurring, inorganic solid defined by a specific, relatively fixed chemical composition and an ordered internal atomic structure. This crystalline arrangement gives each mineral species a unique set of physical and chemical properties. Identifying a mineral specimen involves systematically testing these properties, moving from simple observation to more specific mechanical and chemical analyses.
Initial Visual Assessment
The first step in mineral identification involves observing the specimen without manipulation, focusing on properties like color, luster, and crystal habit. Color is the most immediately noticeable property, but it is often unreliable for identification because trace impurities can cause significant variation. For example, quartz can appear clear, purple (amethyst), or yellow (citrine) depending on the presence of minor elements or radiation exposure. Only a few minerals, such as the green copper carbonate malachite or the yellow native element sulfur, display a color that is consistently diagnostic.
Luster describes the quality and intensity of light reflected from a mineral’s surface. Luster is broadly categorized into metallic and non-metallic, which includes several subdivisions. Non-metallic lusters can be glassy or vitreous, like quartz, pearly, like some micas, or dull and earthy, which resembles dried dirt. Recognizing the type of luster can quickly narrow down the possibilities since it is a more consistent property than the mineral’s external color.
Crystal habit refers to the typical shape a mineral assumes when it is allowed to grow freely in an unrestricted space. Common habits include prismatic, cubic, or tabular forms. This external shape is a direct reflection of the mineral’s internal atomic structure. However, many specimens encountered are massive or granular, meaning the ideal crystal shape is not visible because growth was constrained by surrounding rock.
Mechanical Resistance Properties
Mineral identification progresses by testing how the specimen reacts to physical stress, revealing the strength of the atomic bonds. Hardness is defined as a mineral’s resistance to scratching and is measured using the relative Mohs Hardness Scale, which ranges from 1 (the softest) to 10 (the hardest). This scale is relative, meaning a mineral with a hardness of 7 will scratch all minerals with a lower number. The difference in absolute hardness between steps on the scale is not uniform, as the jump from level 9 (corundum) to 10 (diamond) is much greater than the difference between the lower numbers.
Field testing for hardness can be performed using common objects with known Mohs values to approximate the mineral’s rank. Common objects like a fingernail (2.5), a copper penny (3), or a steel nail (5.5-6.5) are used for comparison. To perform the scratch test, a point of the known object is pressed firmly and dragged across a clean, unmarked surface of the unknown mineral. If the test object leaves a permanent groove that cannot be wiped away, the mineral is softer than the test object.
Cleavage and fracture describe how a mineral breaks when subjected to stress, offering a powerful insight into its internal structure. Cleavage is the tendency of a mineral to break along smooth, flat planes of weakness within its crystal lattice, where atomic bonds are weakest. Cleavage is described by its quality and the number of directions, such as the single perfect cleavage of mica, which allows it to split into thin sheets. Fracture, conversely, is the irregular or random breakage of a mineral surface that occurs when the atomic bonds are equally strong in all directions.
Minerals like quartz, which lack planes of weakness, exhibit a characteristic conchoidal fracture, which resembles the curved, smooth surfaces of broken glass or a clam shell. Observing the number of parallel, flat surfaces on a broken specimen can distinguish cleavage from fracture, which produces jagged, splintery, or uneven surfaces. For instance, the two distinct cleavage planes in the feldspar group intersect at or near a 90-degree angle, a diagnostic feature that is separate from the mineral’s hardness.
Compositional and Density Indicators
The streak test provides one of the most reliable pieces of evidence for identification, revealing the color of a mineral when it is reduced to a fine powder. This test is performed by scraping the specimen across an unglazed porcelain plate, which has a hardness of about 7. Since the color of the powdered mineral is consistent, it is a more dependable property than the external color, which can be altered by weathering or surface impurities. For example, the mineral hematite can appear metallic silver, black, or reddish-brown in hand sample, but its streak is consistently a reddish-brown color, which is a definitive indicator.
Minerals that are harder than the porcelain plate will not leave a streak but will instead scratch the plate itself, indicating a Mohs hardness of 7 or greater. The streak test is particularly useful for metallic minerals, such as pyrite, which has a brassy yellow color but leaves a greenish-black streak, immediately distinguishing it from true gold, which leaves a yellow streak.
Specific gravity is a measurement that quantifies a mineral’s density by comparing its mass to the mass of an equal volume of water. While precise measurement requires specialized equipment, a simple “heft test” involves comparing the weight of the specimen to a similarly sized piece of a common mineral like quartz, which has an average specific gravity of 2.65. Minerals with heavy elements, such as galena, have high specific gravity values, sometimes exceeding 7.5, making them feel noticeably heavier than average minerals.
Final confirmatory tests involve examining special properties that are unique to a few mineral species. Magnetism is a distinctive property, with the iron oxide magnetite being the only common mineral that is strongly attracted to a simple handheld magnet. Another diagnostic test is the reaction to acid, where a drop of dilute hydrochloric acid causes carbonate minerals, such as calcite, to effervesce vigorously, releasing bubbles of carbon dioxide gas. Fluorescence, where a mineral glows under short-wave or long-wave ultraviolet light, is also characteristic of certain minerals, such as fluorite.