Minerals and Rocks

Minerals and rocks are the stuff that the Earth is made of, and understanding what they are made of and how they are classified is essential to understanding geology.  This short supplement summarizes the major properties of rocks and minerals that you will need to know for Hazards of the Earth's Surface.

You've already learned in Supplement 1 that the crust and mantle of the Earth are made up of about 8 major elements or 7 major oxides:

Elements O
Si Mg Al Fe Ca Na K
Oxides
SiO2
MgO Al2O3 FeO CaO Na2O K2O

In addition, the atmosphere  of the Earth contains abundant H (hydrogen) and C (carbon), which can be incorporated into rocks and minerals at or near the surface of the Earth.  Finally, there are other "trace" elements that occur in smaller concentrations.  Man of these are economically valuable, and knowing what minerals they are housed in and where they occur is very important to society: e.g., Au (gold), Cr (chromium), Li (lithium), Ni (nickel).

The organization of matter in the Earth can be summarized in a nutshell:

    Elements are organized into minerals, and
    Minerals are organized into rocks

This organization is demonstrated in this cartoon:
 

 

The only exceptions to these statements are natural glass and organic products, which also are constructed of elements and can be incorporated into rocks.

 

Minerals

A mineral is a naturally formed compound with a chemical formula and a structure.  It must have both to be a mineral.

This mineral has structure.

This non-mineral doesn't have structure, even though it is composed of the same elements in the same proportions, and therefore has the same formula.

It is the geometric arrangement of elements in a mineral that gives rise to the geometric arrangement of crystal faces that can be observed in all minerals given a large enough specimen.  Two notable naturally formed substances that do not have structure are glass and organic material.

A mineral is specified by a formula and an arrangement of atoms.  For example, we could say, "MgSiO3 consisting of Si atoms surrounded by 4 oxygen atoms at the apices of a tetrahedron, with the tetrahedron linked to form long single chains and the Mg atoms linking the chains together".  This is cumbersome; it's much easier to simply say: enstatite!  Saying a mineral is enstattie is equivalent to stating its formula and structure.  This is why geologists have invented so many mineral names.

Remember a mineral needs both a formula and a structure.  Therefore, it is inappropriate to refer to CaCO3 as a mineral, or C as a mineral, or SiO2 as a mineral.  In fact, each of these formulas can occur in minerals with more than one structure, and are examples of polymorphism.  Polymorphs are minerals with the same formula but different structures, such as diamond and graphite, which share the formula C.  The carbon atoms in graphite are arranged in layers with strong bonds within the layers but weak bonds between them, allowing graphite to be very easily broken through the weak inter-layer bonds.  In diamond, all of the carbon-carbon bonds are very strong.  This results in two minerals that share the same formula but have very different properties: diamond is the hardest mineral known, and graphite is among the softest.  CaCO3 occurs as two very common minerals, calcite and aragonite, while the formula SiO2 can be found in no fewer than 5 minerals, one of which is very common on the surface of the Earth (quartz), and some of which are only found under the high pressures present in the mantle (stishovite, coesite).

 

Practice Quiz 4.1.  Which of the following is a mineral?

 

Practice Quiz 4.2.  The minerals kyanite and andalusite are polymorphs.  Kyanite has triclinic symmetry, which is a way of describing how its atoms are arranged, and the formula Al2SiO5.  What can you say about andalusite? 

 

Silicate Minerals

If you look closely at the cartoon diagram above, you will notice that silicon (Si) and oxygen (O) have been grouped together in a special way, such that there are always 4 oxygen atoms around each silicon atom. In fact, this relationship is true everywhere in the crust and uppermost mantle, and makes thinking about silicate minerals--the great majority of minerals--much easier.  Instead of trying to describe how the oxygen and silicon atoms are arranged together in a mineral structure, geologists need only describe how the SiO4 groups are linked together.  In addition, the relative sizes of the atoms in the diagram above are approximately to scale, from which you can see that by volume minerals are mostly oxygen, with other atoms crammed into the crevices and nooks between the oxygen atoms.

The SiO4 group that forms the basis of all silicate minerals is not shaped like a square in the cartoon, but rather is shaped like a tetrahedron with Si in the center and O at each of  the four apexes, and is called the silicon (or SiO4) tetrahedron.  A tetrahedron is a 4-pointed solid with 4 identical isosceles triangular faces:
 

This is what a tetrahedron looks like.

This is what the silica tetrahedron looks like.  There is an oxygen atom centered at each apex, and a silicon atom buried in the middle.  It is the building block of silicate minerals.

In silicate minerals the SiO4 tetrahedrons are either isolated from one another, or linked with other SiO4 tetrahedrons in various ways.  The patterns of linkage provide the basis for the classification scheme for silicate minerals, and very often mirrors the large-scale properties of the mineral.  For example, in the mica minerals the SiO4 tetrahedrons are linked together into sheets, one SiO4 tetrahedrons-thick, that extend infinitely in all directions, or until the end of the crystal.  The sheets themselves are held together by weaker forces acting through the other atoms in the mineral, Al, Fe, Mg, and K, that are positioned between the sheets.  This arrangement of the SiO4 tetrahedrons causes mica minerals their distinctive property of breaking easily into thin sheets. 

In this class we will not worry about learning the different ways in which SiO4 tetrahedrons can be linked together, but you should be aware that the SiO4 tetrahedron is the basic building block of all silicate minerals.
 

Practice Quiz 4.3.  Which of the following statements is not true about the silicon tetrahedron?

 

Rocks

Rocks are assemblages of one or more minerals, and may also include glass and organic material.  Most rocks contain between 3 and 6 minerals, in various abundances, though some may have as few as one and others as many as 15 or so.  Rock is different from sediment, which consists of loose, separate fragments of minerals and rocks that isn't bound together.

Geologists recognize 3 types of rocks: igneous, sedimentary, and metamorphic.

Most rocks near the surface of the Earth are igneous and sedimentary.  In addition, most of the surface of the Earth isn't covered by rock at all--it's covered by sediment. 
 

Practice Quiz 4.4.  What is a rock?

 

Practice Quiz 4.5.  Under what conditions might an igneous rock contain glass?

 

Practice Quiz 4.6.  Which of the following is not a sedimentary rock?

 

Practice Quiz 4.7.  Where do metamorphic rocks form?

 

 

A Language Analogy of Elements, Minerals, and Rocks

For fun, one can look to language for an analogy for the combination of elements into minerals and minerals into rocks.  Let elements be described as letters, minerals as words, and rocks as sentences.  All sentences must start with a capital letter and end with a period, explanation point, or question mark.  For example, this could be the analogy of a rock with 3 minerals:

    Go eat snails.

Just as there are words with only one  letter (a), there are minerals with just one element (diamond, graphite).  And just like there are sentences with just one word (Never!) there are rocks with just one mineral (limestone, consisting of the mineral calcite). 

Non-minerals like magmas and aqueous fluids have elements but not minerals, thus consist of a string of upper-case letters without punctuation:  LAGSSIAEOTN

Igneous rocks form when a magma crystallizes to form minerals:  LAGSSIAEOTN  -   Go eat snails.

Melting is the opposite of crystallization:   Go eat snails.  -   LAGSSIAEOTN

In partial melting, only a portion of the minerals in the rock are transformed into magma:  Go eat snails.  -   Go snails.  +  AET

Sedimentary rocks form either when minerals crystallize out of water; i.e.,  ICUQK -  Quick!

or when pre-existing rocks weather to produce new minerals: Go eat snails.  -  (not) + ISGLAAES
[
this is the weathering part, where the parentheses ( ) means that the mineral occurs as a sedimentary particle, not bound in a rock]

. . . followed by the recombination of these minerals into a sedimentary rock:   (not)  -  Not! 

Metamorphic rocks form from pre-existing igneous or sedimentary rocks when the elements are rearranged to form new minerals:  Go eat snails.  ->  Tail no gases.

 

Practice Quiz 4.8.  Using the language analogy for minerals and rocks, what does the following pair of words represent: spin, pins

 

Practice Quiz 4.9.  What process does this analogy represent: Time to sleep.  -  To sleep.  +  EIMT

 

Practice Quiz 4.10.  Which of the following analogies could represents the process of weathering of a sedimentary rock consisting of just one mineral?