Minerals are natural solids found throughout Earth. Understanding their distinct characteristics helps us comprehend Earth’s geological processes and appreciate the materials that compose its crust. These fundamental building blocks of rocks have a wide range of uses, from construction materials to components in electronic devices.
What Makes a Mineral?
Minerals are defined by specific criteria. To be classified as a mineral, a substance must be naturally occurring, forming through geological processes without human intervention. It must also be inorganic, distinguishing it from materials derived directly from living organisms. Furthermore, a mineral must be a solid under typical Earth surface conditions.
Minerals have a definite chemical composition, meaning specific elements are in fixed proportions. They also possess an ordered atomic arrangement, or crystalline structure, which leads to predictable external shapes. Substances lacking this internal order, such as obsidian, are termed mineraloids rather than true minerals.
Fundamental Physical Properties
Minerals exhibit various physical properties that allow for their identification and differentiation.
Hardness
Hardness measures a mineral’s resistance to scratching. The Mohs Hardness Scale, ranging from 1 (softest, like talc) to 10 (hardest, like diamond), is commonly used. A mineral can scratch any mineral with a lower Mohs hardness number.
Luster
Luster describes how light reflects off a mineral’s surface. This can range from metallic, appearing like polished metal, to various non-metallic lusters such as vitreous (glassy), dull or earthy, silky, or greasy. For instance, quartz typically displays a vitreous luster, while galena exhibits a metallic sheen.
Color
Color is often the first property noticed, but it can be misleading for identification because many minerals can exhibit a range of colors due to impurities. For example, quartz can be clear, white, pink, or purple. Some minerals, however, consistently show a characteristic color, like the deep blue of azurite.
Streak
Streak refers to the color of a mineral’s powder, obtained by rubbing the mineral across an unglazed porcelain plate. This property is more reliable than overall color because the streak color is consistent even if the mineral’s hand specimen color varies. Hematite, for example, can be black or reddish-brown, but its streak is always reddish-brown.
Cleavage and Fracture
Cleavage and fracture describe how a mineral breaks. Cleavage is the tendency of a mineral to break along flat, parallel surfaces, reflecting planes of weakness in its atomic structure. Minerals can have one, two, three, or more directions of cleavage, creating distinct geometric shapes. Fracture, in contrast, occurs when a mineral breaks irregularly, without smooth, planar surfaces. Different types of fracture include conchoidal (shell-like curves), splintery, or uneven.
Density
Density, or more precisely specific gravity, indicates how heavy a mineral feels for its size. Specific gravity is the ratio of a mineral’s density to the density of water, making it a unitless value. Minerals with higher specific gravity, like galena, feel noticeably heavier than minerals of similar size but lower specific gravity, such as quartz.
Crystal Habit or Form
Crystal habit or form describes the characteristic shape a mineral’s crystals tend to grow into, given ideal conditions. This external shape reflects the internal arrangement of atoms. Common habits include prismatic (elongated), bladed, tabular, cubic, or acicular (needle-like).
Unique Characteristics
Beyond the fundamental properties, some minerals display unique characteristics that aid in their identification.
Magnetism
Magnetism is a property where certain minerals are attracted to a magnet. Magnetite, a common iron oxide mineral, is a notable example, exhibiting strong magnetism. Other minerals, like pyrrhotite, may show weaker magnetic responses.
Fluorescence and Phosphorescence
Fluorescence occurs when a mineral emits visible light after being exposed to ultraviolet (UV) light, and the glow stops when the UV light source is removed. Phosphorescence is similar, but the mineral continues to glow for a period even after the UV light is turned off. Fluorite and calcite are minerals that can sometimes exhibit these luminous effects.
Effervescence
Effervescence is the reaction of a mineral with acid, typically dilute hydrochloric acid, producing bubbles of carbon dioxide gas. This reaction is characteristic of carbonate minerals like calcite, which fizzes vigorously when a drop of acid is applied. Dolomite, another carbonate, will only fizz if powdered first.
Other Unique Properties
Some minerals can be identified by taste, though this method is not recommended due to potential toxicity. Halite, or rock salt, is a well-known example with a salty taste. Striations are fine parallel lines or grooves on the crystal faces of certain minerals, often visible on pyrite or plagioclase feldspar. Double refraction is an optical property where light passing through a mineral is split into two rays, causing a double image when looking through it. Calcite is a prime example of a mineral that exhibits strong double refraction.
Identifying Minerals Through Their Properties
Geologists and mineral enthusiasts use a combination of physical properties to identify unknown minerals. No single property is usually sufficient for conclusive identification, as many minerals share characteristics. Instead, a systematic approach involves testing several properties to narrow down possibilities. For example, observing a mineral’s color, then determining its streak, and subsequently testing its hardness provides a clearer picture. Checking for cleavage or fracture patterns further refines the identification process. Simple tools, such as a streak plate, a magnifying hand lens, a steel nail or glass plate for hardness tests, and a small bottle of dilute acid, are commonly employed in the field. By carefully noting multiple attributes and comparing them to known mineral characteristics, one can reliably identify a mineral specimen.