Rocks, being mixtures of minerals, are more complex than minerals and
are therefore classified in a more complicated way. The broadest
grouping of rocks is based on the origin of the rock rather than on the
minerals that compose it. In this scheme all rocks are divided into
three general groups, igneous, sedimentary, and metamorphic.
The igneous rocks are mentioned first because they are produced directly
from hot liquids that come from deep within the earth. These hot liquids
are essentially molten rock and are called magmas. When they cool, the
elements of the individual minerals come together and crystallize, as
water crystallizes into ice on a winter day.
Different minerals crystallize, or “freeze”, at different temperatures
so that if the magma cools slowly some individual grains have
opportunity to grow larger than others. If the magma cools quickly, as
does lava or basalt, the separate mineral grains will be small. Igneous
rocks are classified on the basis of the size and arrangement of the
individual crystals and the kinds of minerals present.
The glacial drift in Illinois contains many pieces of igneous and
metamorphic rocks, but most of the rocks native to Illinois are
sedimentary.
Some sedimentary rocks are made up of weathered fragments of other rocks
that have been moved by rivers, waves, winds, or glaciers. These
sediments have been deposited and later compacted or cemented by the
mineral matter carried in water moving through them. Such sedimentary
rocks are called clastic (meaning broken pieces) rocks, as opposed to
those formed by chemical precipitation from water and those that consist
of fossil remains.
Clastic sedimentary rocks are classified first on the basis of the size
of the grains of gravel, sand, silt, and clay of which they are
composed; on the type of deposition, such as glacial drift and
wind-blown silt or loess, that produced them; and then on the basis of
their mineral composition. Rocks such as gypsum and some of our
limestones were formed by chemical precipitation from sea water.
Metamorphic rock literally means rock that has changed form. The change
of form has been caused by heat and pressures that occur below the
surface of the earth or by heat from upward moving hot magmas or melted
rocks. Examples of metamorphic rocks are found in Illinois as boulders
and pebbles in the glacial drift.
Descriptions of the rocks and minerals represented by specimens in the
Geological Survey’s rock and mineral sets follow. The numbers preceding
the descriptions correspond to the numbers on the specimens in the sets.
Although the set includes the rocks and minerals that are most commonly
found in Illinois, many others can be collected. For that reason,
following the descriptions is a key for identifying other Illinois
minerals and rocks.
As a further aid to the beginner and the amateur, a list of equipment
useful in making a rock and mineral collection is given at the end of
the book.