Calcite (sometimes called “tiff” locally in south-_western_ Missouri),
the essential mineral in limestone, can be recognized by several
definite characteristics:
1. It bubbles, “fizzes,” or _effervesces_ in dilute acid. See page 11.
2. It is easily scratched with a knife.
3. It breaks or cleaves into rhombohedral shapes, of which at least
one flat, glistening side is visible on every individual grain
in the broken surface of limestone.
4. It has a glassy luster on crystal and cleavage faces.
5. It crystallizes into six-sided crystal forms, which can be
differentiated from quartz (also six-sided) by tests (1) and
(2) above.
The one single test of calcite which is most diagnostic, and which
appeals to most persons, is number one above, effervescence of the solid
lump in dilute acid. The bubbles are filled by carbon dioxide gas which
comes from, and is freed from, the calcite by the reaction of it with
the acid. Calcite is calcium carbonate, CaCO₃.
[Illustration: A small calcite crystal from the Joplin region.]
Many Missourians have not realized that the ordinary, everyday limestone
(fine to coarse granular), which is so abundant here, is composed of a
mineral—calcite which makes up the grains. The strikingly beautiful
calcite crystals (displayed in museums) derived from the calcite crystal
caves found in some mines in the Joplin district are accepted without
question as _mineral_ specimens of calcite, but the idea that all of the
commonplace glistening grains in the local limestone are also mineral
grains is a new thought to most persons. A pure limestone is composed
entirely of calcite. Even impure limestones which contain subordinate
amounts of quartz sand, chert, clay, or iron oxide are in the main also
calcite. Dolomite and dolomitic limestones contain the mineral dolomite.
The mineral of ordinary marble is calcite; dolomite marble contains
dolomite. The cementing material in sandstone and a common accessory
mineral in shale are calcite. It is truly a wide-spread and abundant
mineral. Even the lime deposit in the bottom of the tea-kettle, the
water heater, boiler, or automobile cooling system is calcite, or
aragonite, a twin brother to calcite.
The use of calcite in the form of limestone is treated under limestone.
As for the use of large calcite crystals, they are sold as ornaments and
curiosities. Visitors to the Missouri State Fair may recall the exhibit
of a beautiful, reconstructed crystal cave which was lined with large
calcite crystals. Calcite crystals have been shipped in car-load lots to
beautify grottos, notably some in Iowa and Illinois, and are displayed
in almost all prominent museums.
Water-white (clear), optical-quality calcite crystals, which command a
high price, are relatively rare and have not been found in Missouri.
The optical property of calcite which accounts for its high value is its
ability to separate, or refract, every single ray of light passing
through it into two widely separated, easily distinguishable rays, hence
doubling their number. This is called double refraction, and is shown by
the double image of an object viewed through the calcite. Instruments
which polarize light may contain calcite crystals. The artificial
product, “Polaroid”, is used for a similar purpose.
[Illustration: Calcite cleavage rhomb, characteristic rhombohedral
shape. Note the double image due to high double refraction of
calcite.]