Calcite is a common rock but it forms a fantastic variety of
aesthetic crystals.
Limes tone, composed mainly of calcite, is a common sedimentary rock
underlying large areas on every continent.
It is also the most soluble of the
sedimentary rocks.
Rain water with carbon dioxide in it will dissolve limestone
at the earth's surface at the rate of about one foot in two
hundred years
On a geologic time scale, this is extremely rapid. Large amounts
of calcium carbonate are thus constantly carried to the sea to be
deposited once more as limestone and once again to begin the cycle.
In limestone regions it is not only the surface of the rock that is
dissolved. As the rain water works its way downward, it enlarges the
cracks in the rocks through which it moves, forming large channelways
for underground drainage.
The extensive systems of caves and caverns found
in many parts of the world have been formed in this way.
If water with its dissolved calcium carbonate emerges as a spring
from a hillside, some of the calcium carbonate forms a porous deposit
called travertine or calcareous tufa, made up mostly of calcite.
The calcareous material en-crusting the objects over which the water
flows is frequently the petrifying material of fossil plant and
animal remains.
Most travertine is a gray friable or crumbly mass possibly
colored by small amounts of impurities. The town of Yellow Springs,
Ohio, gets its name from a travertine deposit colored a yellowish red
by a small quantity of iron.
Calcium carbonate separated from the warm water emerging from the
ground at Mammoth Hot Springs in Yellowstone National Park has formed
the famous van-colored terraces there.
At Carlsbad (Karlovy Vary), the celebrated spa in Bohemia,
Czechoslovakia, similar deposits of Sprudeistein have formed from the
numerous hot springs.
Although most travertine is soft and crumbling, some is cohesive
and hard enough to be cut and polished for use in floors and
wainscoting in public buildings.
Such decorative material comes mostly from Tivoli near Rome, Italy.
It is the principal building stone of Rome and is used for decorative
purposes throughout the world.
After channels arid cavities have appeared in a limestone rock, a
decrease in rainfall may cause precipitation, that is, the formation
of deposits rather than further solution.
Only small amounts of water may enter the cavern
and drip slowly from the rooftop.
Evaporation will then cause the calcium carbonate in the water to
build stalactites downward from the roof and stalagmites upward
from the floor.
If they meet, a column is formed.
Cave deposits may have the appearance of hanging draperies, festoons,
or other fanciful shapes.
Water seeping on the floor of a cavern may build up a layered
translucent deposit resembling a frozen cascading waterfall.
Although much less common than calcite, the mineral aragonite,
another form of calcium carbonate, may be precipitated as cave deposits.
In some Mexican caverns alternating layers of calcite and aragonite
form an attractive decorative material. Fashioned into small objects
it is sold under the name of Mexican onyx.
Similar material used by the early Egyptians was
called "Oriental alabaster."
Long after the formation of a cave, solutions rising from depths may
encrust the floor, roof and walls with well-formed crystals
containing elements completely foreign to the surrounding limestone.
The best exhibit specimens of many minerals have been removed from
such crystal-coated caverns.
An ancient series of adjoining caverns exists in the region
surrounding the meeting point of Missouri, Kansas and Oklahoma. This
area, known as the Tri-State district, has long been a major producer
of lead and zinc.
The ore minerals galena and sphalerite were deposited in rock
cavities only a few hundred feet beneath the surface.
The district has not only furnished superb specimens of these
minerals but equally fine specimens of many others. The mineral
marcasite, which is iron sulfide (FeS2), is found in exceptionally
fine crystal groups in this district.
It is found in stalactites covered with crystal faces;
or it may occur in aggregates called "cockscomb marcasite."
In the same association, pinkish crystal crusts of dolomite with a
pearl-like luster called "pearl spar" coat crystals of
minerals formed at an earlier time.
Sprinkled at random on all the crusts are tiny, well-formed,
brass-yellow crystals of chalcopyrite.
Similar deposits in limestone of such crystallized minerals are known
in many parts of the world, most strikingly at Bou Beker, Morocco on
the Algerian border, where mining of lead-zinc ore has revealed the
same associations.
Calcite
Calcite, a common associate of ore minerals, is found
in many mining districts.
Some of the most famous localities for beautiful crystals are:
Cumberland, Durham, Lancashire and Cornwall in England; the Harz
Mountains and the silver mines of Freiberg and Schneeberg in Germany;
and Pribram in Czechoslovakia.
But in no place is calcite found more abundantly
in well formed crystals than in the American Tri-State district.
The region is especially noted for clear to amber-colored calcite
crystals of many shapes and sizes; the long dimension of some
crystals is as much as six feet.
Since the mineral is of little interest to miners,
many caves lined with calcite have remained intact.
No experience can be more thrilling to a mineralogist than to enter
such a cavern and see the light of his torch reflected from a myriad
of crystal faces.
Calcite is unique in the diversity
of its crystal forms.
When chemically pure, it is clear and colorless and called Iceland spar
from its occurrence in cavities in lava flows near Helgustadir on
Eskefiord, Iceland.
It is more commonly milky white but may be tinted red, green, blue or yellow.
Interesting crystals of calcite enclosing grains of sand are found in
the Bad Lands of South Dakota and at Fontainebleau in France. These
"sandstone crystals" are formed by calcite crystallizing in
sand. A single crystal continuity may extend for several inches,
incorporating as much as sixty per cent quartz sand.
Clear calcite shows better than any
other common mineral the property of double refraction,
that is, the breaking of light into two rays, each of which produces
a separate image.
In 1828 William Nicol, taking advantage of this double refraction,
constructed a prism from calcite that eliminated one of the rays,
permitting the other to emerge as plane polarized light.
Ordinary light vibrates in all directions at right angles to its
path; plane polarized light also vibrates at right angles to the
path, but in a single direction.
As an integral part of the polarizing microscope, this "nicol
prism" gave a great impetus to the study of minerals and an
understanding of the optical properties of crystals.
One of the most noticeable features of calcite is an
excellent rhombohedral cleavage that permits it to break into
rhomboidal fragments.
The first speculation regarding the internal structure of crystals
was brought about by a study of this cleavage. In 1782, Rene' Just
Hatiy, a professor at the University of Paris, was horrified when a
friend accidently shattered a prized calcite crystal by dropping it
on the floor.
However, the accident had its rewarding side, for Hatiy noted that
both large and small fragments had the same shape, and suggested that
the cleavage rhombohedron is the primitive form out of which all
calcite crystals are built. By analogy he suggested that other
crystals are also built of primitive forms of varying shapes, a
premise that with modifications is held today.
Calcite is a much prized mineral to
collect. It comes in over 26 different crystal habits and almost as
many colors. Some collectors have a separate cabinet where they only
collect calcite.