Minerals

Although someone in the mining industry may use the term mineral to describe anything that is dug out of the Earth, and a dietitian might think of a mineral as something that supports bodily functions, to a geologist, minerals are the building blocks of rocks. Minerals can be composed of a single element or a group of elements, but they all possess five important characteristics:

  1. they form naturally,
  2. possess a definite crystalline structure,
  3. are a solid,
  4. have an easily identifiable chemical structure, and
  5. they are, with very few exceptions, inorganic (not produced by living organisms).
Both graphite (left) and diamond (right) are minerals composed of carbon. The minerals differ in the way their carbon atoms are arranged.

Mineral Properties

All minerals have a well-defined chemical composition. Some minerals are composed of a single element, such as gold, silver and copper, while others are composed of two or more elements, such a quartz. Minerals composed of multiple elements usually contain roughly the same ratio of elements. Quartz, for example, is composed of silicon (Si) and oxygen (O), and contains about twice as much oxygen as silicon, or a ratio of 2:1. Contaminants and substitutions can cause different samples of a single mineral to have slightly different chemical compositions, but the ratio of elements should remain approximately equal. 

Sodium chloride, also known as halite or table salt, crystallizes in cubic shapes and breaks at 90 degree angles.

To be considered a mineral, a material must form under natural conditions. Although a number of valuable materials have been produced in the laboratory that closely mimic those that occur naturally, they are not considered a mineral. Cubic zirconia, for example, are diamond-like materials created in a laboratory, and although they resemble diamonds chemically, they are not considered minerals (nor are they as valuable as real diamonds).

Minerals are solids with readily identifiable crystalline shapes. The atoms within minerals align to form organized and predictable patterns that are often identifiable to the naked eye as a distinct crystalline shapes. Sodium chloride (table salt) crystals, for example, form cubic shapes, giving salt its block-like appearance. Note that because minerals must be a solid, they can only exist within a given temperature range. Water, for example, is a mineral (ice) at temperatures below 0 degrees Celsius; above this temperature water becomes a liquid or a gas, and is therefore no longer a mineral.

Finally, the vast majority of minerals are not produced from living organisms and are therefore considered inorganic. Almost all minerals are formed from natural geologic processes. There are a few materials, however, that are recognized as minerals despite having formed within living organisms. Sugar, which is produced within sugarcane plants, is one such example.

How to Identify Minerals

Some minerals are harder than others, making it possible to distinguish some minerals based on their relative hardness. Hardness in this case refers to the ability of mineral to resist scratching. A simple way to test a mineral’s hardness is to try to scratch it with another mineral or material with a known hardness. A copper penny, for example, can scratch calcite but not fluorite. Apatite can be scratched with a piece of glass, but quartz cannot. Diamonds, the hardest of all naturally-occurring substances cannot be scratched by anything except other diamonds.

Yellow is an unreliable characteristic of minerals, as one mineral may come in several different colors. Pure quartz is clear, while this piece of impure quartz takes on a yellow hue.

There are thousands of different minerals on the Earth, some with very similar properties, often making it a challenge to distinguish one mineral from another. There are several physical and chemical characteristics a geologist might consider when trying to identify an unknown mineral. Here we consider a few of these characteristics, though you may notice that one obvious trait as been left out: color. Color may seem like an easy way to distinguish between minerals, but several minerals come in a variety of different colors, while other minerals may share the same color but have entirely different chemical compositions.

There are two hardness scales: a relative hardness scale known as the Mohs scale, and an absolute scale. The Mohs hardness scale uses 10 minerals arranged from 1 (least hard) to 10 (most hard). Because it is a relative scale, however, a mineral with a hardness of 2 is not necessarily twice as hard as a mineral with a hardness of 1; it simply means that 2 is harder than 1. The absolute hardness scale ranges from 0 to about 80. There is a large gap between diamond, with an absolute hardness of 80, and another very hard mineral, corundum (rubies and sapphires), which has an absolute hardness of about 20.

As mentioned earlier, all minerals have a particular crystal shape, also called a habit. While a number of different minerals may form a similar crystalline shape, many are sufficiently unique to aid in identification. Some minerals exhibit block-like shapes such as salt, while others form bladed, banded, prismatic, or botryoidal crystals. Some minerals actually have more than one habit. Pyrite (“fools gold”), for example, may exhibit either four- or five-sided polygons.

Minerals also exhibit particular optical properties, including luster, transparency, streak, and color (not reliable). Luster refers to the ability of a surface to reflect light. Surfaces with higher luster reflect more light and appear “shiny,” while surfaces with low luster reflect less light and appear “dull.” Minerals that appear metallic are said to have a metallic luster, while those with a tarnished metal look have a submetallic luster.

Pyrite, also known as “fools gold” has a metallic luster.

Minerals that do not appear metallic have a non-metallic luster. Transparency refers to the ability of light to pass through a material. Minerals that are transparent are “see-through;” that is, they allow light to pass through them well enough such that images are visible. Clear diamond and quartz are examples of transparent minerals.

Some minerals are translucent, which means that light can pass through, but you cannot see images through them. Minerals that light cannot pass through are opaque. Streak refers to the color of the powdery residue that is left behind when you rub a mineral sample on an unglazed porcelain plate known as a streak plate. The color of the streak may not match the color of the mineral, and while the color of the mineral may vary from sample to sample, the color of the streak usually remains the same.

Hematite (ore of iron) scratched against a streak plate

When you hit or smash a mineral, it will often break is a predictable manner. This tendency to break in particular way is called cleavage. Cleavage occurs because some bonds between atoms within a mineral’s crystalline structure are weaker than others. When sufficient stress is applied to a mineral, it will break apart or fracture where these bonds are the weakest.

While some minerals exhibit perfect cleavage along one or more planes, others exhibit no cleavage at all (break in unpredictable ways), and still others exhibit imperfect cleavage — they are less predictable, sometimes cleaving in one way, and sometimes in another. Mica, for example, exhibits a very simple form of perfect cleavage, breaking in very thing, parallel sheets. Halite (salt) has three planes of cleavage and typically breaks into smaller polygons with 90 degree angles.

Finally, minerals have widely varying densities, which is the amount of mass per unit volume. Some minerals, such as gold, are very dense, while others like talc are much less so. Density of a mineral is often stated as its specific gravity, which is a ratio of a minerals weight to that of water. Many of the common rock-forming minerals have a specific gravity of between 2 and 3. Quartz, for example, has a specific gravity of 2.65. Galena, the source material for lead, is much denser with a specific gravity of 7.5. Gold is exceptionally dense with a specific gravity of 20.

Types of Minerals

All minerals fall within two broad camps: the silicates and the non-silicates. Why only two groups?

Silicate minerals are composed of silicon-oxygen tetrahedrons.

As it turns out, the vast majority of the Earth is composed of silicate minerals, and most rocks are composed of silicates. Silicate minerals are those that contain silicate and oxygen atoms, such as quartz (in fact, pure quartz is composed only of silicon and oxygen). The building block of silicate minerals is the silicon-oxygen tetrahedron, which is composed of four oxygen atoms bonded to a single silicon atom at the center.

Common groups of silicate minerals include the feldspars, which comprise about 50 percent of the Earth’s crust, quartz, micas, olivine, pyroxene, and amphibole (hornblende). Granite, a common type of igneous rock (forms from cooling magma or lava) is composed primarily of silicate minerals including feldspar and quartz.

The non-silicates are a very diverse group of minerals that make up only about 8 percent of Earth’s crust. The major groups of non-silicate minerals include the carbonates, halides, oxides, sulfides, sulfates and single element minerals like gold, sulfur and graphite. The carbonate minerals all contain the carbonate ion (CO3). The most common carbonate is calcite (CaCO3), or calcium carbonate, which makes up a large portion of the common rocks limestone and marble. The halides include halite, which is table salt, as well as fluorite and sylvite. The oxides include the two major ores of iron, hermatite and magnetite, corundum (rubies, sapphires), and water ice (yes, h20!). Common sulfides include galena (an ore of lead), pyrite, and chalcopyrite (ore of copper). Sulfates are often used in the construction industry; both gypsum and anhydrite are used in plaster.

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