Now that we know some things about identifying minerals based on their physical characteristics,
we should learn how to group similar minerals together based on their chemistry. This is
important because one of the major differences between the different types of mineral-forming
geological environments is their chemistry. For example, mafic rocks, which are depleted
in silica and rich in calcium, magnesium, and iron, tend to be composed of different
classes of minerals than felsic rocks, which are rich in silica, aluminum, and sodium,
and depleted in magnesium and iron. In addition, a mineral’s chemistry
directly affects its lattice structure due to differences in atomic radii and bond lengths.
This means that from a clear macroscopic property, like a mineral’s cleavage, we can potentially know
something about its structure, which we can relate to its chemistry, and which we can use to know
something about the geologic environment in which it formed. Of course, this does not always work,
however it is a simple logical framework that geologists use to determine a rock’s history.
Minerals are divided into eight classes based primarily on the dominant anion
in their chemical formula. The eight classes are as follows: native elements, oxides,
halides, sulfides, carbonates, sulfates, phosphates, and silicates. Moreover,
an additional class, mineraloids, is reserved for geologic materials that lack a crystal structure,
and therefore are not minerals. Many of these terms should already be familiar from our study
of chemistry, but either way, let’s make sure we contextualize them in our study of geology.
Let’s begin with the simplest class of minerals: native elements. As the name suggests,
they are naturally occurring elements that have a crystalline structure. They are divided
into metals, semimetals, and nonmetals. Native metals will be of the greatest interest to us,
as they form most of the native elements. Remember that what makes a metal a metal is the propensity
to have delocalized valence electrons that are shared among all its atoms. Most native elements
are relatively unreactive, which is evidenced by their existence in such oxidizing environments
as Earth’s surface, where oxygen and water are readily available to form oxides and hydroxides.
Some examples of native metals are copper, gold, silver, and platinum. The native semimetals
are bismuth, antimony, arsenic, selenium, and tellurium. And the only two native nonmetals
are sulfur and carbon. There is an entire field of science called metallurgy that is completely
based on the study of metals. One can imagine the importance of metallurgy in the modern
age when considering all of the different properties of metals that are required for
specific applications, such as electrical conductivity, superconductivity, rigidity,
flexibility, magnetism, and high melting point. For example, electric heating elements require
an alloy that is ductile with a high electrical resistance and melting point, whereas solder,
which is used to connect wires to circuit boards, must have a low melting point and low resistivity.
Furthermore, you wouldn’t even be able to watch this video without the single-crystal silicon in
your computer’s central processing unit. Thus, the incredible importance of understanding minerals.
Since most minerals contain oxygen, such as with silicates and carbonates,
the oxide class is restricted to minerals where one or more type of cation is ionically bonded
only to oxygen. Oxide minerals can be broken down into two groups: simple oxides, which only contain
cations of identical charge, and the spinels, which contain cations of dissimilar charge.
The simple oxides can be further grouped based on the charge of their cation, which is usually
between one and four. Some examples of simple oxide minerals are cuprite, a monovalent copper
oxide, periclase, a divalent magnesium oxide, hematite, a trivalent iron oxide, and rutile,
a tetravalent titanium oxide. Another instantly recognizable monovalent oxide mineral is hydrogen
oxide, which is more commonly known as ice. The spinels are a bit more complicated, having both
a divalent and trivalent cation in their chemical formula. Magnetite is an iron oxide like hematite,
but it contains both ferric and ferrous iron, having a chemical formula of (Fe3+)2(Fe2+)(O2-)4.
The difference between magnetite and hematite is clearly highlighted by their magnetic properties,
as magnetite is strongly attracted to a magnetic field, whereas hematite is not.
An important subgroup of the oxides is the hydroxides,
which are composed of cations that are bonded to a hydroxide ion.
They are commonly produced by chemical weathering, or alteration of other minerals as they react with
precipitation and groundwater near the surface. Bauxite, which is the dominant ore of aluminum,
is composed of several aluminum oxy-hydroxide minerals that form in tropical environments where
abundant precipitation and high temperatures intensely leach everything out of the rocks
except for aluminum, which is one of the most immobile and insoluble geologic materials.
Another simple class of minerals with ionic bonding is the halides,
which are composed of a cation bonded to a halogen, typically fluorine or chlorine. The
halides are the most ionic of the minerals and are usually very soluble in water. Halite, or sodium
chloride, is a common halide mineral and it often precipitates during the evaporation of seawater.
Our final class of minerals that contain a single anion are the sulfides. Unlike the
oxides and halides, the bonding in sulfides is very complex and can range from ionic-covalent to
metallic-covalent depending on what it is bonding with. Most sulfide minerals are composed of sulfur
bonded to a transition metal, and often display many of the properties of metals. Incidentally,
both galena, lead sulfide, and pyrite, iron disulfide, are semiconductors with a metallic
luster. Many sulfide minerals are both associated with and hosts of numerous metallic ore deposits.
For example, the Bushveld Intrusion, which is a large intrusive igneous formation in South Africa,
possesses a layer of sulfide minerals that contain 65 percent of the world’s reserve
of platinum group elements. So that covers four out of
the eight classes of minerals. Let’s move forward and check out the other half now.