Page 150 - Laboratory Manual in Physical Geology

silicon-oxygen tetrahedron: SiO 4 4–

2–
4+
Oxygen (O ) and silicon (Si ) atoms
naturally form covalent bonds (share O 2–
electrons). Each Si atom prefers to
bond with four relatively larger O 2– Si 4+
4–
atoms. This creates, SiO 4 , called a
silicon-oxygen tetrahedron.
O 2–
covalent O 2–
bonds
O 2–
The simplest ratio of atoms is two Silicon-oxygen tetrahedra
oxygens for each silicon: SiO 2 , polymerize—link together with one
called silicon dioxide or silica.
another to form long chains and clumps
that thicken magma and lava.
They can also bond with
metal atoms (like Al, Fe,
Mg, Ca, Na, K) in a
crystalline framework as
silicate minerals.

Quartz: SiO 2
Olivine: (Fe, Mg)SiO 4

FIGURE 5.1 Silica and silicate minerals. Silicon (Si) and oxygen (O) are, by far, the most abundant chemical elements in magma, lava,
and igneous rocks. They form silica (SiO ), which thickens magma and lava (makes it more viscous) and bonds togther, alone (as quartz), or with
2
metal atoms to make silicate minerals besides quartz in igneous rocks.

Mafic Minerals in igneous rocks are glossy black biotite , dark gray to
black amphibole , dark green to green-gray pyroxene , and
The name mafic refers to minerals with magnesium ( ma -)
olive-green olivine .
and iron (- fic ) in their chemical formulas, so they are also
called ferromagnesian minerals. They get their dark color
from the abundant proportion of iron and magnesium in Composition of Igneous Rocks
their chemical composition. The common mafic minerals
Composition of a rock refers to what it is made of.
Chemical compositon refers to the chemical elements that
make up the rock. This determines how the rock will react
ACTIVITY with materials of different compositon, such as whether
or not it will react with and decay (tarnish, dissolve,
5.3 Estimate Rock Composition chemically disintegrate) in air or water. It also determines
rock color. For example, ferromagnesian-rich rocks (iron-
THINK | What are igneous rocks composed of? and magnesium-rich rocks) generally have a dark color and
About It How is composition used to classify and ferromagnesian-poor rocks generally have a light color.
interpret igneous rocks? But the chemical elements in a rock are normally bonded
together in tangible materials like minerals that, in turn,

OBJECTIVE Determine the compositional group of make up most rocks. So the phsyical composition of rocks is
an igneous rock using methods of visual estimation a description of what visible materials they are made of, in
and point counting. whole or part. It is your job as a geologist, using your eyes
and simple tools (like a hand magnifying lens), to describe
PROCEDURES
and identify what physical materials igneous rocks are
1. Before you begin , read about Composition of made of.
Igneous Rocks and How to Assign Rock Samples
to Chemical Groups (p. 145). Also, this is what you Chemical Composition—Four Groups
will need :
Magmas, lavas, and igneous rocks are composed mostly
____ Activity 5.3 Worksheet (p. 145 ) and pencil
of the same eight elements that characterize the average
2. Then follow your instructor’s directions for composition of Earth’s crust. They are oxygen (O), silicon
completing the worksheet. (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium
(Ca), sodium (Na), and potassium (K).

Igneous Rocks and Processes ■ 131