Summary
Highlights
A mineral is defined as an inorganic, naturally occurring solid with a definite chemical composition and atomic structure. Key characteristics include being inorganic, natural, solid, having a specific chemical composition, and a specific atomic structure. Examples of common minerals are potassium feldspar, sulfur, muscovite mica, galena, calcite, olivine, and quartz.
Minerals are identified by various characteristics. Color can be useful but is often unreliable. Hardness is tested using the Mohs scale, with higher numbers indicating harder minerals. Luster describes how light reflects off a mineral's surface, typically metallic or non-metallic. Streak is the color of the mineral's powdered form. Cleavage refers to predictable breaks, while fracture indicates random breaks. Other characteristics include magnetism, taste, odor, fluorescence, and reaction to acid.
The New York State Earth Science Reference Tables provide a "Properties of Common Minerals" chart. This chart helps identify minerals by listing their luster, hardness, cleavage/fracture, color, streak, density, use, and chemical composition. Examples include Galena (metallic luster, soft, cleavage, gray-black streak, dense) and Olivine (hard, fracture, non-metallic, used in jewelry).
Minerals are the building blocks of rocks. Rocks are classified into three main types based on their formation: igneous (from magma or lava), sedimentary (from compacted sediments), and metamorphic (from intense heat and pressure). Igneous rocks form from the cooling and solidification of magma (underground) or lava (on the surface).
Extrusive or volcanic igneous rocks form when lava cools quickly on Earth's surface. Rapid cooling results in small or no visible mineral crystals, sometimes with a fine texture (crystals less than 1mm) like basalt or rhyolite. Non-crystalline or glassy rocks like obsidian form when cooling is extremely fast. Some extrusive rocks, like pumice, are vesicular, containing trapped air bubbles.
Intrusive or plutonic igneous rocks form when magma cools slowly beneath the Earth's surface. Slow cooling allows for larger crystal growth, typically between 1-10mm (like granite) or even over 10mm (like pegmatite). The Earth Science Reference Table for igneous rocks also categorizes them by color (lighter on the left, darker on the right), density (less dense on the left, more dense on the right), and composition (felsic, rich in silicon and aluminum, or mafic, rich in iron and magnesium). Mineral composition is also detailed, showing which minerals are commonly found in each rock type. The rock cycle illustrates that igneous rocks form from melting to magma and then solidification. Longer cooling times result in larger crystal sizes.
Sedimentary rocks form from the compaction and cementation of sediments (bits and pieces of other rocks). The New York State reference table for sedimentary rocks divides them into clastic, crystalline, and bioclastic types.
Clastic sedimentary rocks are made of compacted and cemented sediments, classified by sediment size. Examples include conglomerate (rounded, mixed-size sediments), breccia (angular, mixed-size sediments), sandstone (sand-sized), siltstone (silt-sized), and shale (clay-sized). Crystalline sedimentary rocks, like rock salt, form when minerals dissolved in water evaporate, leaving crystals behind.
Bioclastic sedimentary rocks are formed from the compaction and cementation of organic matter. Examples include limestone (from cemented shells, like coquina) and bituminous coal (from compacted plant remains). The rock cycle shows that existing rocks weather into sediments, which are then eroded, deposited, buried, compacted, and cemented to form sedimentary rocks.
Metamorphic rocks are formed from existing rocks exposed to intense heat and/or pressure. These conditions, often due to tectonic plate movement, cause rocks to change or morph. The New York State reference table for metamorphic rocks classifies them as foliated or non-foliated.
Foliated rocks primarily result from regional metamorphism, which is driven by pressure. An example sequence involves shale transforming into slate, then phylite, schist, and finally gneiss, with increasing heat and pressure. Foliation means minerals align due to pressure, and banding is a severe form of foliation seen in gneiss. This process can lead to melting if very deep, forming igneous rocks.
Non-foliated metamorphic rocks can be formed by pressure, like bituminous coal becoming anthracite coal, resulting in a denser rock. Others are formed by contact metamorphism, where intense heat from nearby magma or lava alters the rock, such as sandstone turning into quartzite, or limestone becoming marble. Metaconglomerate is another example. The key characteristic of metamorphic rocks is banding, which is unique to them.
The rock cycle demonstrates that any type of rock (igneous, sedimentary, metamorphic) can transform into any other type through various geological processes. For example, igneous rocks can weather into sediments (forming sedimentary rocks) or be exposed to heat and pressure (forming metamorphic rocks), or even melt and re-form as new igneous rocks. This continuous change has shaped Earth's history.