What is a mineral?

A mineral is a naturally occurring solid with a definite chemical composition and crystalline structure. Minerals provide the foundation of our everyday world. Not only do minerals make up the rocks we see around us in the West, they are also used in nearly every aspect of our lives. The minerals found in the rocks of the West are used in industry, construction, machinery, technology, food, makeup, jewelry, and even the paper on which these words are printed.

Minerals provide the building blocks for rocks. For example, granite, an igneous rock, is typically made up of crystals of the minerals feldspar, quartz, mica, and amphibole. Sandstone may be made of cemented grains of feldspar, quartz, and mica. The minerals and the bonds between the crystals define a rock’s color and resistance to weathering.

Metallic minerals are vital to the machinery and technology of modern civilization. However, many metallic minerals occur in the crust in amounts that can only be measured in parts per million (ppm) or parts per billion (ppb). A mineral is called an ore when one or more of its elements can be profitably removed, and it is almost always necessary to process ore minerals in order to isolate the useful element. For example, chalcopyrite (CuFeS₂), which contains copper, iron, and sulfur, is referred to as a copper ore when the copper can be profitably extracted from the iron and sulfur.

Non-metallic minerals do not have the flash of a metal, though they may have the brilliance of a diamond or the silky appearance of gypsum (CaSO₄•2H₂O). Generally much lighter in color than metallic minerals, non-metallic minerals can transmit light, at least along their edges or through small fragments.

Mineral Identification

Although defined by their chemical composition and crystal structure, minerals are identified based on their physical properties. A variety of properties must usually be employed in identifying a mineral, each eliminating possible alternatives.

Hardness is a very useful property for identification, as a given mineral can only exhibit a narrow range of hardnesses, and it is easily testable, which quickly and simply minimizes the number of possibilities.

Hardness is important because it helps us understand why some rocks are more or less resistant to weathering and erosion. Quartz, with a rating of 7 on the Mohs scale, is a relatively hard mineral, but calcite (CaCO₃), rating 3 on the Mohs scale, is significantly softer. Therefore, it should be no surprise that quartz sandstone is much more resistant to erosion and weathering than is limestone, which is primarily made of calcite. Quartz is a very common mineral in the Earth’s crust and is quite resistant due to its hardness and relative insolubility. Thus, quartz grains are the dominant mineral type in nearly all types of sand.

Color is helpful in identifying some minerals like sulfur, but it is uninformative or even misleading in others like garnet. Luster describes how light is reflected from a mineral’s surface and can range from adamantine, seen in diamonds, to dull or earthy (effectively no luster), like kaolinite. Crystal form, if visible, can also be diagnostic. For example, fluorite and calcite may appear superficially similar, but fluorite forms cubic crystals while calcite forms trigonal-rhombohedral crystals. Relatedly, crystals may have planes of weakness that cause them to break in characteristic ways, called cleavage. Or they may not, and instead display fracture when broken. Mica and graphite have very strong cleavage, allowing them to easily be broken into thin sheets, while quartz and glass (the latter not being a mineral) have no cleavage, instead displaying a distinctive curved fracture form known as conchoidal. The density of a mineral may also aid in identifying it (e.g., metals tend to be very dense). Finding the exact density is straightforward, but it does require measuring the volume of the sample. Placing an unknown mineral in water (or other liquid) to find its volume by displacement can be a risky undertaking since several minerals react violently with water, and many more break down with exposure. A mineral’s streak is obtained by dragging it across a porcelain plate, effectively powdering it. The color of the powder eliminates conflating variables of external weathering, crystal habit, impurities, etc. Some minerals are magnetic (affected by magnetic fields), while a few are natural magnets (capable of producing a magnetic field).

Most minerals can be identified by process of elimination after examining a few Review of these properties and consulting a mineral identification guide. Mineral testing kits often include several common objects used to test hardness: a porcelain streak plate, a magnet, and a magnifying glass. Some minerals have rare properties, which may be more difficult to test. For example, there are minerals that exhibit luminescence of all types, giving off light due to a particular stimulus. Some minerals are radioactive, usually due to the inclusion of significant amounts of uranium, thorium, or potassium in their structure. Carbonate minerals will effervesce when exposed to hydrochloric acid. Double refraction describes the result of light passing through a material that splits it into two polarized sets of rays, doubling images viewed through that material. For example, a single line on a sheet of paper will appear as two parallel lines when viewed through a clear calcite crystal.

Mineral Formation

Mineral deposits in the West range in age from over one billion years old to just a few thousands or tens of thousands of years, and their occurrence is related to different processes that operate in different geologic environments. Economically recoverable mineral deposits are formed by geologic processes that can selectively concentrate desirable elements in a relatively small area. These processes may be physical or chemical, and they fall into four categories:

Magmatic processes separate minor elements of magma from the major elements and concentrate them in a small volume of rock. This may involve the early crystallization of ore minerals from the magma while most other components remain molten, or late crystallization after most other components have crystallized. Magmatic processes responsible for the formation of mineral deposits in the West are usually associated with igneous intrusions (formed during mountain building events, rifting, and volcanic activity), which can range in composition from granite (felsic) to gabbro (mafic). Metamorphism may also cause recrystallization of minerals and concentration of rare elements. Under conditions of extreme high-temperature metamorphism, minerals with the lowest melting temperatures in the crust may melt to form small quantities of pegmatite magmas.

Hydrothermal processes involve hydrothermal solutions that dissolve minor elements dispersed through large volumes of rock, transport them to a new location, and precipitate them in a small area at a much higher concentration. Hydrothermal solutions are commonly salty, acidic, and range in temperature from over 600°C (~1100°F) to less than 60°C (140°F). Some of these fluids may travel very long distances through permeable sedimentary rock. Eventually, the hydrothermal fluids precipitate their highly dissolved load of elements, creating concentrated deposits.

Sedimentary processes gather elements dispersed through large volumes of water and precipitate them in a sedimentary environment, such as sedimentary layers on the ocean floor or layers of sediment on lakebeds. Sedimentary mineral deposits form by direct precipitation from the water.

Weathering and erosion break down large volumes of rock by physical and chemical means and gather previously dispersed elements or minerals into highly concentrated deposits. Residual weathering deposits are mineral deposits formed through the concentration of a weathering-resistant mineral, while other minerals around it are eroded and dissolved. In contrast, mineral deposits formed by the concentration of minerals in moving waters are called placer deposits. In the Western US, placer deposits occur in rivers and streams that carry lighter sediment downstream but leave behind heavy minerals such as gold. Placer deposits can also occur on coastal beaches. The erosion of areas of small, low-grade gold veins and the subsequent concentration of the gold as stream sediment while the other minerals were carried downstream produced hundreds of placer deposits that were mined throughout the Western US during the 19th century. Mining of these deposits continues today.

Minerals in the West

The history of the Western States is driven in large part by the promise of mineral wealth. The rush for gold began in the 1840s, when much of the territory that would become California was ceded to the United States by Mexico. Gold discoveries in California resulted in Americans from the east and immigrants from around the world flocking to the area. The idea of acquiring wealth by finding gold became a backdrop for new communities and towns. As an area became either overcrowded or mined out, these gold seekers moved to new areas, eventually finding silver and gold in Nevada, and then gold and other minerals in Washington and Oregon. And finally, in the late 19th century, they headed to the Klondike and Alaska in search of new strikes. The minerals that early prospectors sought in rivers and streams are still being mined in many regions of the West.

Many Western mineral deposits are the products of several different concentration processes, sometimes operating tens or hundreds of millions of years apart. There are a variety of geologic environments in which these mineral-concentrating processes have operated over the past billion years to produce the abundance and diversity of mineral deposits found in the West today (Figure 5.2), including passive continental margins (Arctic Coast of Alaska), volcanic island arcs (Aleutian Islands), mountain-building events (Sierra Nevada, Cascades, Brooks Range), and basins formed by rifting events (Basin and Range of Nevada).