2 Minerals and Their Properties
Learn how minerals are defined and identified, how they form, and how their properties and combinations help explain the rocks they make up.
Minerals and Rocks: The Basics
A is a naturally occurring, inorganic solid with an orderly internal and a characteristic chemical composition. Its composition may be fixed, as in quartz, which is silicon dioxide , or may vary within limits, as in some groups. The atoms’ arrangement and the chemical bonds between them influence the ’s properties.
A is different: it is naturally formed material made of one or more minerals or other solid material. Minerals are made of elements. Quartz contains silicon and oxygen; halite contains sodium and chlorine, represented by ; and calcite contains calcium, carbon, and oxygen, represented by . Silicate minerals contain silicon and oxygen and are especially abundant in Earth’s crust. Feldspar and quartz are common examples.
Takeaway: Minerals are defined by their natural origin, inorganic nature, solid state, , and composition; rocks are materials made from minerals and sometimes other substances.
Identifying Minerals by Their Properties
No single visible feature is always enough to identify a . Color can be misleading because impurities may change it, and different minerals can look alike. Geologists therefore examine several properties together.
describes how the surface reflects light. Common descriptions include metallic, glassy, pearly, and dull.
is the color of the ’s powder, tested by rubbing it on an unglazed porcelain plate. It may differ from the ’s outward color.
measures resistance to scratching. The ranks minerals from talc at , the softest, to diamond at , the hardest. A can scratch materials softer than itself.
is splitting along smooth, repeated planes of weakness in a . describes breakage when does not occur, producing surfaces that may be irregular or curved.
Crystal form is the external shape a crystal develops when it has room to grow. Crowded crystals in many rocks may not show their full shapes.
Density, often compared using specific gravity, magnetism, and reaction to dilute acid can provide additional clues. Calcite commonly fizzes when exposed to dilute acid; such tests require appropriate supervision.
For example, a clear, glassy-looking sample is not necessarily quartz. Checking its , , and along with its appearance can help narrow down its identity.
Takeaway: Identify minerals by combining observations and tests rather than relying on color or appearance alone.
How Minerals Form
Minerals form when atoms or ions come together in an orderly . The minerals that grow depend on conditions such as temperature, pressure, available elements, and surrounding fluids.
Cooling magma or lava: As molten cools, minerals crystallize. Slow cooling underground often allows larger crystals to grow, while rapid cooling at the surface usually produces smaller crystals.
Precipitation from water: Dissolved substances can form crystals as water evaporates or as temperature or chemical conditions change. Halite can form when salty water evaporates; calcite can precipitate from water rich in dissolved calcium and carbonate.
Hot, -rich fluids: Heated water moving through cracks can deposit minerals as it cools or reacts with surrounding . These deposits can form veins.
Changes within existing rocks: During metamorphism, heat, pressure, or chemically active fluids can cause minerals to recrystallize or react to form new minerals. Weathering can also produce new minerals, such as clay minerals, from older ones.
Takeaway: formation is linked to the conditions and materials present, including melts, water, fluids, and changes within existing rocks.
Minerals as Building Blocks of Rocks
Most rocks are mixtures of grains. Their , the arrangement of their grains, and grain size help determine what kind of they are. Some rocks consist mostly of one : marble, for example, is mainly calcite. Other rocks also contain volcanic glass, fragments, or remains of organisms, so a is not always a collection of visible crystals.
Granite is an igneous commonly made of quartz, feldspar, and mica. Its minerals crystallized as magma cooled.
Basalt is an igneous commonly containing plagioclase feldspar and dark minerals such as pyroxene. Its makeup differs from granite’s because its source melt had a different composition and history.
Sandstone is a sedimentary made of cemented grains, often mainly quartz. Some sedimentary rocks instead form as minerals precipitate from water.
Marble is a metamorphic formed when limestone, which is rich in calcite, recrystallizes under heat and pressure.
Because minerals are building blocks of many rocks, identifying them can help geologists infer how a formed and what conditions it experienced.
Takeaway: A ’s minerals and grain arrangement connect its present-day features to the processes that formed it.