Two keys, one for minerals and one for rocks, briefly present clues that
may aid the collector in identifying rocks and minerals found in
Illinois. In outline form, the keys are a guide to some of the easily
observable properties that various rocks and minerals display.
The rocks and minerals in the school set of “Typical Rocks and Minerals
of Illinois” are included, plus other relatively common ones you might
find in Illinois. Because of the great diversity of rocks and minerals
in this state, the keys are not conclusive. It is therefore suggested
you consult other more complete keys (such as that in Dana’s _Manual of
Mineralogy_) when identifying rocks and minerals that are either from
other states or are difficult to identify.
The minerals (p. 30-35) are arranged in two groups: 1) those with a
metallic luster, and 2) those with a nonmetallic luster. Each group is
arranged according to increasing hardness. Other characteristics such as
color, streak, cleavage, fracture, and composition are listed.
The rocks (p. 36-39) are arranged according to their reaction to dilute
hydrochloric acid applied to a scratched surface. (The acid reacts more
readily to powdered material produced by scratching the rock.) After the
reaction to acid has been determined, the texture and components of the
rock should be noted. Because rocks grade into one another, clear
distinctions are not always possible.