jewelry. They are also very hard. Graphite is used as pencil lead and has a slippery feel.
Compare the diamond with the pencil lead inFigure3.5. Diamond and graphite are both
made of only carbon, but they are not the same mineral. The crystal structure of diamond
differs from the crystal structure of graphite. The carbon atoms in graphite bond to form
layers. The bonds between each layer are weak, so the sheets can slip past each other. The
carbon atoms in diamonds bond together in all three directions to form a strong network.
As a result, the properties of diamond differ from the properties of graphite.
Figure 3.5: Even though they are both made of carbon, diamonds and graphite have different
characteristics. ( 26 )
Groups of Minerals
Imagine you were in charge of organizing more than 100 minerals for an exhibit at a mu-
seum near you. You want the people who visit your exhibit to learn as much as possible
about the minerals they see. How would you group the minerals together in your exhibit?
Mineralogistsare scientists who study minerals. They use a system that divides minerals
into groups based on chemical composition and structure. Even though there are over 4,000
minerals, most minerals fit into one of eight mineral groups. Minerals with similar crystal
structures are grouped together.
Silicate Minerals
Silicate minerals make up over 90 percent of Earth’s crust. When you think of the Earth’s
crust, you may think of the people, animals or trees that live on the Earth’s surface. Yet
living organisms are made of organic matter and there is only a small amount of organic
matter in Earth’s crust. About 1,000 silicate minerals have been identified, making the
silicate minerals the largest mineral group.
Silicatesare minerals that contain silicon atoms bonded to oxygen atoms. The basic build-
ing block for all silicate minerals is called a tetrahedron, where one silicon atom is bonded
to 4 oxygen atoms (Figure3.6). Silicate minerals also often contain other elements, such as
calcium, iron, and magnesium.