A mineral is a naturally occurring solid with a specific chemical composition and a highly ordered atomic arrangement. Identifying a mineral relies on its innate physical properties, which reflect this internal crystalline structure. These characteristics provide a reliable way to distinguish minerals, even without sophisticated laboratory equipment. The process involves systematically observing and testing the sample’s intrinsic traits to narrow down its identity.
Initial Assessment: Color, Luster, and Habit
Initial identification involves visually examining the mineral’s surface color. Color can be misleading because trace impurities often cause wide variations within the same mineral type; for example, pure quartz can be colored purple or yellow by impurities. However, some minerals, known as idiochromatic minerals, possess a color that is consistently diagnostic, such as the deep yellow of sulfur or the green of malachite.
Luster describes the quality and intensity of light reflected from the mineral’s surface. Lusters are broadly divided into metallic, which appears like polished metal (e.g., pyrite), and non-metallic. Non-metallic lusters include vitreous (glass-like), pearly (iridescent sheen), or earthy (dull and non-reflective). Observing the mineral’s luster is generally a more reliable diagnostic tool than its surface color.
Crystal habit refers to the typical shape in which a mineral grows. While perfect crystals are uncommon, many minerals display characteristic growth forms that aid in classification. These forms can be described as prismatic (long columns), cubic, or massive, where the mineral lacks any distinctive external crystal shape. A specimen’s overall shape helps categorize it, even if the individual crystal faces are not fully developed.
Determining Resistance to Scratching
The Mohs Hardness Scale measures a mineral’s resistance to scratching or abrasion, reflecting its internal bonding strength. This ordinal scale ranks ten common minerals from 1 (talc, the softest) to 10 (diamond, the hardest). The fundamental principle is that any material on the scale can scratch all materials with a lower ranking.
This scale can be approximated using common objects with known hardness values. A human fingernail has a hardness of about 2.5, while a copper penny is 3.5. Window glass or a steel knife blade falls around 5.5 to 6.5. If the mineral can scratch glass, its hardness is greater than 5.5.
The technique requires pressing the known hardness tool firmly against an inconspicuous surface of the mineral. The goal is to determine if a permanent groove is left behind, which indicates that the tool is harder than the mineral being tested. By systematically testing the sample against materials with increasing hardness, one can accurately place the unknown mineral within a narrow range on the Mohs scale.
Using Breakage and Powder Color for Clues
The way a mineral breaks reveals the internal arrangement and relative strength of its atomic bonds. Cleavage describes the tendency of a mineral to break smoothly along flat, defined planes of weakness within its structure. For example, the mineral mica exhibits perfect cleavage, separating into thin, flexible sheets. Cleavage is defined by the number of distinct planes and the angles at which they intersect.
In contrast, fracture occurs when a mineral breaks irregularly, without any flat, smooth surfaces. Minerals with equally strong bonds in all directions, like quartz, typically exhibit a distinctive conchoidal fracture, which creates curved, shell-like surfaces. The presence of smooth, planar surfaces indicates cleavage, even if the rest of the break is jagged.
The mineral’s streak, which is the color of its powder, is a highly reliable property. This test is performed by scraping a corner of the mineral across an unglazed porcelain streak plate. Unlike the variable surface color, the streak color is consistent because it represents the mineral’s true color in its finely powdered form. For instance, hematite may appear black and metallic, but its streak will always be a characteristic reddish-brown.
Specialized Tests for Confirmation
When basic visual and hardness tests are insufficient, specialized properties can provide definitive confirmation. Specific gravity is a measure of the mineral’s density compared to the density of water. A mineral with a high specific gravity feels noticeably heavier than a piece of similarly sized quartz or calcite. This property helps differentiate minerals that look alike but have vastly different compositions.
Another diagnostic test involves applying a drop of dilute hydrochloric acid to the mineral surface. Carbonate minerals, such as calcite, react immediately with the acid, producing visible effervescence or fizzing. This reaction is caused by the release of carbon dioxide gas and serves as a definitive test for the presence of the carbonate chemical group.
A few minerals exhibit the unique property of magnetism, which is a definitive identifier for those specific types. Only a small number of minerals, most notably magnetite, are strongly attracted to a common magnet. Testing for magnetism is a quick and straightforward way to confirm the identity of these iron-bearing minerals.