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The Essence of Mineralogy

Chemical Elements

arrangement is only systematic on the scale of

a few tetrahedra. So a glass is ordered only on

a local scale. In order to be a mineral, a materi-

As stated above, the elements are the basic

building blocks for minerals, and for that matter,

al must be made up of an orderly arrangement

of atoms on scales of tens of units put together.

all materials. There are about 90 known natural-

ly occurring elements. The good news is that

Before we continue, there are two important

only eight of these—oxygen, silicon, aluminum,

points to be made about number 4 and 5 above.

iron, calcium, sodium, potassium, and magne-

In many ways these two descriptions of a miner-

sium—compose over 98% of the Earth’s crust by

al form the basis for understanding minerals.

weight (Figure 1.3). If you haven’t yet had a

The chemical formula portion of the mineral

chemistry course, or you’ve forgotten these,

description forms the field of crystal chemistry,

you’ll want to become familiar with the chemi-

while the atomic arrangement characteristic

cal symbols for these elements, which are shown

forms the field of crystallography. In fact, an ear-

in Table 1.1 and will be used throughout this

lier book on the subject of mineralogy was titled

book.

Because this group of eight elements forms the

Crystallography

and

Crystal

Chemistry:

Introduction (Bloss, 1994). The importance of the

majority of the crust, these elements, in turn,

concepts of crystallography and crystal chem-

make up the building blocks for the vast majority

istry and their interrelationships cannot be over

of rock-forming minerals. Let’s consider the

information in Table 1.1 and see what conclusions

stressed. We can view crystal chemistry as the

materials that nature has provided (i.e., the ele-

can be drawn from it.

ments) and crystallography as the way nature

Notice that each of the elements in the list has

has put these elements together to form minerals.

a number in the column to the right of it to indi-

Our goal will be to describe the basic rules that

cate the amount and type of charge on that ion.

are followed in selecting which elements to use

Atoms like oxygen, which have a negative

to build which minerals, and in turn how those

charge, are called anions (think of it as “a nega-

elements are arranged in these minerals. The cri-

tive ion”). These atoms have an overall negative

teria that nature has chosen to make these selec-

charge because they have more electrons (which

tions will be based upon certain physical charac-

are negatively charged) around the nucleus than

teristics (e.g., pressure and temperature) present

they have protons (with positive charges) in the

at the time of formation of the mineral. We can in

nucleus. On the other hand, some atoms have lost

turn determine the physical conditions at the

electrons relative to the normal stable element,

time when the mineral was formed, perhaps tens

and they are called cations. Such atoms have a

of millions of years ago. This is really the founda-

positive charge because the number of protons in

tion of the field of petrology, which is based on

the nucleus is greater than the number of elec-

the study of formation conditions of rocks using

trons around the nucleus (Figure 1.4).

characteristics we can learn from their minerals

and mineral assemblages.

Given these descriptions for defining a miner-

al, we can now proceed to thinking about what’s

really important to learn about minerals. In this

chapter, our goal is to present the concepts and

Oxygen 46.6%

vocabulary that we feel every geologist should

Silicon 27.7%

know about mineralogy. These terms are part of

the daily language of geology, and thus are wor-

thy of memorization. As noted in the Preface,

they form four groups that are the basic building

blocks for this textbook: chemical elements, crys-

Everything

else 1.5%

tal systems, optical systems, and the Big Ten

Magnesium

minerals. Each of these topics will be revisited

Aluminum 8.1%

2.1%

with its own chapter in the next section of this

Iron 5.0%

Potassium 2.6%

book, and with several chapters in the advanced

Calcium 3.6%

Sodium 2.8%

Figure 1.3. A pie chart showing the weight percents of the

latter portion of the text. Our goal here is to let

eight most abundant elements in the Earth’s crust. Silicon

you know, right at the beginning, what’s really

and oxygen are by far the most abundant elements in the

important about mineralogy and to get you used

Earth’s crust, so it is no surprise that combinations of Si

to some of its terminology. That’s why we have

and O are the fundamental building blocks of most com-

called this chapter “The Essence of Mineralogy.”

mon minerals.

Chemical Elements

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