How to Recognize a Mineral
Minerals are found all over the world and often serve as a smaller component of rocks, though can also be found on their own.
Minerals are found all over the world and often serve as a smaller component of rocks, though can also be found on their own. Minerals are considered to be naturally occurring, inorganic solids, and nearly 5400 types of minerals are known today with dozens of new ones being discovered and described each year. Minerals are often defined by their chemical formula, but a variety of physical and chemical properties can be used to identify them.
Minerals are often referred to as crystals, even if they do not look like the typical crystal you might imagine. Oftentimes a mineral’s crystal shape is unique and is determined by the microscopic arrangement of its molecules. For example, quartz is a mineral composed of silica dioxide, and the arrangement of the silica and oxygen molecules causes quartz to exhibit a distinct six-sided crystal prism.
Minerals exhibit so many crystal shapes, ranging from slender and needle-like to cubic to rounded. However, some substances do not exhibit consistent crystal shapes and are said to be amorphous. This includes commonly known materials such as obsidian (volcanic glass), pearl, and opal. Because of this, these substances are said to be mineraloids instead of true minerals.
Some minerals are soft enough to scratch with your fingernails, like talc, while others are extremely hard. The hardness of a mineral is determined by the Mohs Hardness Scale, a scale that ranks from 1 to 10, with 10 describing the hardest minerals. Diamond is one of the few minerals that receives a 10 on the hardness scale, and it is so hard that is can be used to drill into hard metals and other minerals. Apatite, a mineral commonly used to manufacture fertilizer and also used as a common gemstone, lies right in the middle of this scale with a hardness of 5.
Sometimes minerals break cleanly along planes of weakness, and the ability for a mineral to do this is referred to as cleavage. Mica, a particularly flat mineral, cleaves into thin sheets, while halite (commonly known as table salt) cleaves into cubes. Not all minerals have the ability to cleave, leaving them to fracture randomly if broken, such as quartz.
Color can be an inconsistent indicator when trying to determine what mineral you are looking at. Color can be very distinctive for some minerals, like malachite which often exhibits a deep green color, or sulfur that is always a bright yellow. However, some minerals can vary greatly in their color, like calcite which can range from yellow to green to blue to white.
Minerals can take on various optical properties, including their ability to reflect light in a specific way. This property is called luster, and luster is often used to distinguish between metallic and nonmetallic minerals. However, other minerals have lesser-described lusters, like diamonds which are said to have a brilliant luster. Some minerals are said to be dull, meaning they exhibit very little luster at all, while others may be said to appear waxy, greasy, and even silky.
When you scrape a mineral across a hard surface, like concrete or porcelain, it may leave a colored streak mark. Sometimes a mineral exhibits a streak that is the same color as the mineral itself, while some streaks differ in color from the mineral they come from. Hematite is one such mineral that appears as a silver-gray color, but exhibits a rust-colored streak. Not all minerals leave a streak, however, including quartz.
Specific gravity is perhaps better described as a mineral’s density. A denser mineral is said to have a higher specific gravity than a less dense mineral, and as a result the mineral with the higher specific gravity may feel a bit heavier when held in your hand. Galena, a mineral that is partly composed of lead, possess a very high specific gravity.
Only some minerals are magnetic, and they all include iron in their chemical formula. Minerals that do exhibit magnetic properties may do so at different strengths. Some iron-bearing minerals are only weakly magnetic, including garnets from which rubies are derived. Other iron-bearing minerals are extremely magnetic, including magnetite and hematite. Typically, meteorites contain a lot of iron-bearing minerals and are extremely magnetic.
Also known as deformation, tenacity describes a mineral’s ability to bend under pressure. Some minerals bend easily, like many metals which can shaped easily, and some minerals break quickly when pressure is applied, making them very brittle. Some minerals even possess elastic properties, meaning they can be bent or stretched and then return to their original shape when the pressure is removed.
Many carbonate minerals will bubble when exposed to hydrochloric acid, whereas minerals with another chemical composition will have no reaction whatsoever. You can even tell the difference between calcite and dolomite depending on the speed of their reaction to hydrochloric acid.
Some minerals glow brilliantly when exposed to UV light, a property called fluorescence. As the name might imply, fluorite is known to fluoresce under UV light.
It is not common for minerals to be taken in by our other senses, but a few exhibit some unusual properties. Sulfur, a bright yellow mineral is often said to smell like rotten eggs, and halite is used as table salt, making it one of the few minerals with a specific taste. A few other minerals formed from evaporated water deposits, such a sylvite, are said to have a bitter taste.
A common debate occurs around water and ice. By definition, minerals are a naturally occurring inorganic solid, and liquid water breaks this rule. However, ice and snow are considered by many to be a mineral as long as it isn’t melted. The only exception to the rule is mercury, a metal that is naturally liquid at room temperature, but is still considered to be a mineral by many geologists.
