There's an old saying in mining, if it's not grown, it must come must come out of the Earth.
Each year the US produces 75 billion dollars worth of
mineral resources that include exotic metals such as gold
and silver, and more basic materials like sand and
gravel. As geologists we interpret minerals and rocks to
unravel the history of our planet and find and manage mineral resources to support economic
growth
We have to learning objectives for this lesson we are going to identify the surprisingly
short list earth's most popular elements and introduce you to our
friends the silicates, the most common mineral group in
the rocks below our feet.
We know that rocks are made minerals and minerals or composed of elements. Elements are the
primary ingredient so all earth materials. We consume elements in the food we eat, the
air we breathe and the water we drink. Elements can't be separated into simpler substances
by chemical means.
There are 92 naturally occurring element on Earth.
How many of these do you recognize?
Rocks and minerals are composed of combinations of these elements, but don't get too worried
about learning all of the names. Just 8 elements make up most
of the rocks in the Earth's crust and several these are also key ingredients in Earth's
core and mantle
Remember that earth is differentiated into three major compositional layers. Most of
these layers are made up of just four elements, iron, oxygen, silicon and magnesium in some
combination.
Dense elements such as iron and nickel sank to Earth's core during the early history of the
planet. The core and the mantle do contain silicon and oxygen, but these lighter elements
are much more abundant in the outer layers of Earth, and especially in the crust.
Scientists have discovered over four thousand different minerals and more are found each
year. However, only a few dozen minerals are relatively abundant in the Earth, that's a
lot easier to wrap your mind around.
We live on the planet surface, so we are most interested in the rocks below our feet that
we can use for resources and that break down to form our soils.
When we look more closely at a characteristic rock on Earth's crust we can see that it is
composed of different colored minerals. Analyzing the elements in these
minerals shows that oxygen and silicon make up 75 percent of the elements present. In
addition to oxygen and silicon there are only six other elements that
make up the majority of the Earth's crust. All six of these may be familiar
to you. We take in iron calcium, potassium and magnesium in our diet. Foods like spinach
are rich in all of these elements. We consume sodium as salt and we use aluminum in a variety
of forms every day
In combination with a handful of other elements, oxygen and silicon combine to form the most
common mineral group, the silicates. So, silicate minerals contain oxygen
and silicon, often with one or more of the other six elements we just mentioned. Let's
consider the rock granite, a common rock in Earth's crust. Granite contains several silicate
minerals. Some, like quartz, have a relatively simple composition and are composed exclusively
of oxygen and silicon. Granites also contain one or more varieties of the mineral feldspar
that will have additional elements such as potassium, sodium and aluminum. Most of the
minerals in granite are light-colored and may be white, pink, tan or clear. However,
some are darker. We see a difference in the elements that are present in these minerals.
They contain iron and magnesium and lower proportions of silicon and oxygen.
However, all of these minerals are silicates, as indicated by the presence of silicon and
oxygen in their chemical formulae.
The silicates are therefore composed of two nonmetallic elements. These non metallic elements
often give mineral group their names. These elements
may form bonds with metallic elements such as potassium and aluminum to create specific
varieties of minerals such as feldspar or muscovite mica.
Many silicate minerals begin life as elements in magma.
Silicates form as the magma cools and crystallizes to form different types of igneous rocks.
The original composition of the magma and the characteristics to the physical and chemical
environment will determine the type of igneous rocks that form.
Magmas with higher percentages of silicon and oxygen will erupt more violently than
those where these two elements make up a smaller proportion the total elements present.
Being able to identify the minerals present in igneous rocks allows geologists to unravel
the volcanic history of ancient mountain ranges, as well as evaluate the potential
hazards of modern active volcanoes.
These igneous rocks can be broken down into their constituent minerals by weathering processes
at Earth's surface. For example we can see large feldspar crystals
exposed here at the Black Canyon the Gunnison National Park in Colorado. These and other
silicate crystals weather out of the outcrop and are then washed
away into the river and carried downstream toward the
coast where they may become part of the beach. Eventually these mineral grains and fragments
may be cemented together to form a sedimentary rock such as sandstone.
Sand and gravel deposits are dominated by resistant silicate
minerals like quartz and feldspar. The US mines about seven billion dollars of sand
and gravel each year, most of which goes to make concrete or is used for road construction.
We to principal learning objectives for today. How confident are you that you complete both
of these tasks successfully?
Okay, that's it for us. Jen, how about we go and replenish our elements with some spinach?