2.3: Minerals

Minerals Defined

A mineral is a naturally occurring, inorganic solid, with a definite chemical composition and a characteristic crystalline structure. Naturally occurring, means that anything man has created, like beautiful cubic zirconia created in labs used to resemble diamonds, does not count as a mineral. To be an inorganic solid, the mineral must not be composed of the complex carbon molecules that are characteristic of life and must be in the solid state (no vapor or liquid). This means that water in its liquid form is not a mineral, while water as a solid (ice), would be (if it is not man-made). A definite chemical composition refers to the chemical formula of a mineral. For most minerals, this does not vary (ex. halite is NaCl - sodium chloride, or table salt), though some minerals have a range of compositions, since one element can substitute for another of similar size and charge (ex. olivine has varying amounts of magnesium and iron). The atoms within minerals are lined up in an orderly fashion, like a stack of soup cans at a store, so the characteristic crystal structure reflects the internal arrangements of atoms. Using halite (NaCl) as an example again, the sodium and chlorine are in a predictable cubic grid, which is what we expect with a mineral. Opal is made of silicon, oxygen, and water, but the location of the atoms and molecules are not regular or predictable, so it is not a mineral. Since this is the only criteria of a mineral that opal does not meet, we call it a mineraloid.

Figure \(\PageIndex{1}\): The white crystal in the back is quartz. It is made of pure silicon and oxygen. The crystal in the front is also quartz but is purple due to the addition of extra elements. We call this amethyst, but really it is just a variety of quartz.

Minerals are not only important for their many uses, but also as the building blocks of rocks - igneous, sedimentary, and metamorphic. Just like words are made up of letters, rocks are made up of minerals. There are several thousand minerals, but we only focus on the most common ones. A few common minerals to mention are:

• quartz
• amphibole
• olivine
• feldspar
• pyroxene
• clay minerals (group of minerals that are very small)
• mica
• calcite
• halite

If it has been a while since you have taken a chemistry course here are a few reminders of important concepts you will need for this section of the text.

• An atom is the basic building block of matter.
• An element is a type of atom like hydrogen, oxygen, or gold. They are all generically atoms, but they are specific elements.
• Molecules are made of multiple elements.
• An ion is an atom or molecule having a positive or negative charge.
• A cation is a positively charged ion.
• An anion is a negatively charged ion.

Types of Minerals

Silicates

Most of the minerals that make up the rocks of the Earth’s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The building block of all these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. In silicate minerals these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks.

Figure \(\PageIndex{2}\): The Silica Tetrahedron – Building Block of All Silicate Minerals. Because the silicon has a charge of +4 and the four oxygens each have a charge of -2, the silica tetrahedron has a net charge of -4.

The simplest silicate structure, that of the mineral olivine, is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine the -4 charge of each silica tetrahedron is balanced by the addition of two iron or magnesium cations as they each have a +2 charge. Olivine can be either Mg₂SiO₄ or Fe₂SiO₄, or some combination of the two, which is written like this: (Mg,Fe)₂SiO₄, meaning that any proportions of Mg and Fe are possible.

Looking at the chart below, you can see other configurations are possible. For this course we are going to focus on sheet silicates and framework silicates.

Figure \(\PageIndex{3}\): Silicate Mineral Configurations. The triangles represent silica tetrahedra.

In sheet silicates, the silica tetrahedra are arranged in continuous sheets. There is even more sharing of oxygen between adjacent tetrahedra, and hence fewer positive ions are needed to balance the charge. Bonding between sheets is relatively weak, so these minerals tend to break into sheets. Clay minerals and micas (like biotite mica and muscovite mica pictured below) are very common sheet silicates. Clay minerals are interesting (and important to this class) because they do not like to allow water to pass through them and make infiltration difficult. At the same time when water is absorbed into clay minerals, it can hold a lot of water and associated ions.

Figure \(\PageIndex{4}\): Biotite Mica (left) and Muscovite Mica (right). Both are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite is dark like the other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene and amphibole), while muscovite is light colored. Each sample is about 3 cm across.

Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and in quartz. Feldspars include aluminum, potassium, sodium and calcium in various combinations. Quartz is made solely of silicon and oxygen where the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Each tetrahedron is bonded to four other tetrahedra (with oxygen shared at each corner of each tetrahedron), and as a result, the ratio of silicon to oxygen is 1:2 (quartz is SiO₂). Since the one silicon cation has a +4 charge and the two oxygen anions each have a -2 charge, the charge is balanced and there is no need for other ions to balance the charge.

Non-Silicate Minerals and Mineral Groups

While silicates dominate Earth's crust, there are other important minerals or mineral groups. Read through the table below. You do not need to memorize all of these groups and minerals, but you will want to use this table as a reference in the future.

Table \(\PageIndex{1}\): The Main Mineral Groups and Some Examples of Minerals in Each Group

Group	Definition	Examples
Oxides	minerals with oxygen as their anion, but they exclude those with oxygen complexes such as carbonate (CO3), sulphate (SO4), or silicate (SiO2)	hematite (iron-oxide – Fe2O3), corundum (aluminum-oxide Al2O3), water-ice (H2O)
Sulphides	minerals with the S^-2 anion and are the most important ores of lead, zinc, copper and molybdenum	galena (lead-sulphide – PbS), pyrite (iron-sulphide or fool's gold – FeS2), chalcopyrite (copper-iron-sulphide – CuFeS2)
Carbonates	minerals in which the anion is the CO3^-2 complex	calcite (calcium-carbonate – CaCO3), dolomite (calcium-magnesium-carbonate – (Ca,Mg)CO3
Halides	minerals with anions that are the halogen elements (the minerals in the second last column on the right side of the Periodic Table) chlorine, fluorine, bromine etc.	fluorite (calcium-flouride – CaF2), halite (sodium-chloride – NaCl)
Sulphates	minerals with the SO4^-2 anion	gypsum (calcium-sulphate – CaSO4·H2O), barite (barium-sulphate – BaSO4) (Note that sulphates are different from sulphides. Sulphates have the SO4^-2 ion while sulphides have the S^-2 ion)
Native elements	minerals include only one element (bonded to itself), such as gold, copper, sulphur or carbon (which could be graphite or diamond)	gold (Au), diamond (C), graphite (C), sulphur (S) , copper (Cu)

The common non-silicate minerals are:

• hematite
• pyrite
• calcite
• dolomite
• halite
• gypsum

Figure \(\PageIndex{5}\): These are samples of the mineral pyrite, also known as "fool's gold" as it looks like gold. Pyrite has a much lower density than gold (it is lighter) and is harder than gold (pure gold is very soft which is why it is mixed with other metals when used in jewelry).

Mineral Properties

When you identify a mineral, you use clues gathered from your observations of its physical properties. The properties are things like color, density, and how hard the mineral is. In a general geology course, you would explore about 10 properties of minerals. For our purposes, we will focus on just a few. Physical properties can vary within the same minerals, so you must be careful.

Color, the color of the mineral in its regular crystal form, is a property that often varies in each mineral. Quartz is a mineral that comes in many colors as shown in Figure \(\PageIndex{1}\). Purple-colored quartz is called amethyst, the white quartz is called milky quartz, and the clear quartz is called quartz crystal.

Specific gravity references a mineral's density and is the ratio of a mineral’s weight to the weight of an equal volume of water. A mineral with a specific gravity of 3 would weigh three times as much as water. Most minerals have a specific gravity around 2.7. Some minerals like pyrite (fool’s gold) shown in Figure \(\PageIndex{5}\) are noticeably dense with a specific gravity of 4.9-5.2. Gold's specific gravity is much higher at 19.3. Minerals with a high specific gravity will feel heavy. Those with a low specific gravity will feel light.

Hardness describes the resistance of a mineral to being scratched by another material and is controlled by the strength of the bonds between atoms. That other material could be a piece of metal or another mineral. If mineral A scratches mineral B, A is harder. If it does not, B is harder. In geology, hardness is based off a scale of 1 to 10 created by a mineralogist named Friedrich Mohs. Mohs’ scale lists ten minerals from softest to hardest as follows.

• 1 - Talc
• 2 - Gypsum
• 3 - Calcite
• 4 - Flourite
• 5 - Apatite
• 6 - Feldspar
• 7 - Quartz
• 8 - Topaz
• 9 - Corundum
• 10 - Diamond

Figure \(\PageIndex{6}\): These are two different micas, biotite (dark) and muscovite (light), which are common rock-forming minerals. They show one plane of cleavage. The silica tetrahedra are bonded strongly along the flat sheets but weakly between sheets such that it tends to break along those flat planes. They show one plane of cleavage as they have one orientation of natural breakage.

Each mineral on the scale can scratch any mineral having a lower number. So, quartz can scratch minerals 1-6 but cannot scratch 8-10. The hardness of quartz is related to the fact that all the bonds between the silica tetrahedron that make it up are equally strong.

Figure \(\PageIndex{7}\): These are two different micas, biotite (dark) and muscovite (light), which are common rock-forming minerals. They show one plane of cleavage. The silica tetrahedra are bonded strongly along the flat sheets but weakly between sheets such that it tends to break along those flat planes. They show one plane of cleavage as they have one orientation of natural breakage.

One last property of minerals is cleavage. As minerals are broken (such as when they are hit with a hammer, for example), some may break along smooth flat planes known as cleavage. This happens because bonds within a mineral may not be of the same strength, so when a mineral breaks it does so along these weaker areas. This results in flat cleavage planes. Micas, as pictured in Figure \(\PageIndex{7}\), break into these flat sheets for this very reason. The strong bonds between silica tetrahedra occur along the sheets. The weaker bonds are what hold the sheets together. Some minerals do not contain zones of weakness because either all bonds are the same strength, or the weaker bonds are not aligned within a plane. In this case it will not have cleavage, but rather breaks in a random fashion. Quartz is a great example of this as all of the bonds between the silica tetrahedron that make it up are equally strong.

Media Attributions

• Figure \(\PageIndex{1}\): Luz Mendoza, Unsplash
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Left: Steven Earle, CC BY 4.0; Right: Cubed Shaped Crystal by Hans-Joachim Engelhardt, CC-BY-SA-4.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F..._KUBUS_50P.jpg
• Figure \(\PageIndex{5}\): Renee Kiffin, Unsplash
• Figure \(\PageIndex{6}\): USGS, Public Domain
• Figure \(\PageIndex{7}\): Harmon D. Maher Jr., Usage Allowed for non-profit educational purposes