The Mineral CALCITE
Chemistry: CaCO3, Calcium Carbonate
Uses: In cements and mortars, production of lime, limestone is used
in the steel industry; glass industry, ornamental stone, chemical and optical
uses and as mineral specimens.
Calcite's Physical Properties
Calcite , which gets its name from " chalix " the Greek word for lime,
is a most amazing and yet, most common mineral. It is one of the most common
minerals on the face of the Earth, comprising about 4% by weight of the
Earth's crust and is formed in many different geological environments.
Calcite can form rocks of considerable mass and constitutes a significant
part of all three major rock classification types. It forms oolitic, fossiliferous
and massive limestones in
sedimentary environments and even serves as the
cements for many sandstones and
shales . Limestone becomes
marble from the
heat and pressure of metamorphic events. Calcite is even a major component
in the igneous rock called
carbonatite and forms the major portion of many
hydrothermal veins. Some of these rock types are composed of better than
99% calcite. Why would a collector be interested in such a common mineral?
Because of its extraordinary diversity and beauty!
With calcite so abundant and so widely distributed it is no wonder that
it can be so varied. The crystals of calcite can form literally a thousand
different shapes by combining the basic forms of the positive rhombohedron,
negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons,
various scalahedrons, prism and pinacoid to name a few of the more common
forms. There are more than 300 crystal forms identified in calcite and
these forms can combine to produce the thousand different crystal variations.
Calcite also produces many twin varieties that are favorites among twin
collectors. There are also phantoms, included crystals, color varieties,
pseudomorphs and unique associations. There simply is no end to the varieties
There are several varieties of calcite and it would be impossible to
describe them all. However there are a few standouts. Possibly the most
well known of calcite's varieties is its most common form, the classic
or "Dogtooth Spar" as it is sometimes called. This variety
appears as a double pyramid or
but is actually a distinctly different form. The point of the scalenohedron
is sharp and resembles the canine tooth of a dog, hence the name. Beautiful
clear colorless or amber-orange examples of this variety are considered
classics and outstanding examples come from Pugh Quarry, Ohio; Cornwall,
England and Elmwood, Tennessee but the variety is found worldwide.
Not necessarily a variety of calcite, cave formations are certainly
a unique aspect of calcite's story. Calcite is the primary mineral component
in cave formations. Stalactites and stalagmites, cave veils, cave pearls,
"soda straws" and the many other different cave formations that millions
of visitors to underground caverns enjoy are made of calcite. It is the
fact that calcite is readily dissolved that these formations occur. Overlying
limestones or marbles are dissolved away by years and years of slightly
acidic ground water to percolate into the caverns below. In fact the caverns
themselves may have been the result of water dissolving away the calcite
rich rock. As the calcite enriched water enters a relatively dry cavern,
the water starts to evaporate and thus precipitate the calcite. The resulting
accumulations of calcite are generally extremely pure and are colored if
at all, by very small amounts of iron or other impurities.
Mexican onyx is a variety of calcite that is used extensively
for ornamental purposes. It is carved into figurines and is so popular
that almost every child in the USA owns a small onyx animal or two. Carvings
such as vases, bookends, plates, eggs, obilisks, pyramids and statues are
all popular. It is not the same onyx as the
variety of onyx which is a little more precious (it is used in jewelry)
and is banded white and black. To avoid confusion it is best to refer to
it as Mexican Onyx. Mexican onyx is banded with multiple orange,
yellow, red, tan, brown and white colors that have marble-like texture.
The carvings are quite attractive and affordable; a rare combination!
Another variety is the so called "Iceland Spar", which is basically
clear cleaved fragments of completely colorless
(ice-like) calcite. Originally discovered and named after Eskifjord, Iceland where
the calcite is found in basalt cavities. In rock shops around the world,
iceland spar is available in large quantities and at affordable prices
and are popular among children. Most of today's iceland spar comes from
The iceland spar displays the classic cleavage form of calcite,
Iceland spar was and is used for optical equipment and during World War
II it was a strategic mineral as it was used for the sighting equipment
of bombardiers and gunners. It is iceland spar that best demonstrates the
unique property of calcite called double refraction.
refraction occurs when a ray of light enters the crystal and due
to calcite's unique optical properties, the ray is split into fast and
slow beams. As these two beams exit the crystal they are bent into two
different angles (known as angles of refraction) because the angle
is affected by the speed of the beams. A person viewing into the crystal
will see two images ... of everything. The best way to view the double
refraction is by placing the crystal on a straight line or printed word
(the result will be two lines or two words). There is only one direction
that the beams are both the same speed and that is parallel to the C-axis
or primary trigonal axis. Rotation of the crystal will reveal the direction
in the crystal that is parallel to the C-axis when the line or word becomes
whole again. By contrast, the direction perpendicular to the C-axis will
have the greatest separation. The extremely high index of refraction of
calcite that causes the easily seen double refraction is also responsible
for the interference colors (pastel rainbow colors) that are seen in calcites
that have small fractures.
are other important properties of calcite. Although not all specimens demonstrate
these properties, some do quite well and this is diagnostic in some cases.
One notable case of fluorescence occurs at
New Jersey where the massive calcite is enriched in a small amount
of manganese and fluoresces a bright red under UV light. Some
spar can fluoresce a nice purple or blue color and unique specimens will
even phosphoresce (continue to glow) after the UV source has been removed.
Triboluminescence is supposedly a property that should occur in most specimens,
but is not easily demonstrated. It occurs when the specimen is struck or
put under pressure; in a dark room the specimen should glow when this happens.
The best property of calcite is the acid test.
calcite always will effervesce (bubble) when even cold weak acids are placed
on specimens. Even the cement in sandstones will effervesce assuring the
geologist of identification of the cementing mineral. The reason for the
bubbling is in the formula below:
CaCO3 + 2H(+1) -------> Ca(+2)
+ H2O + CO2 (a gas)
The carbon dioxide gas (CO2)
is given off as bubbles and the calcium dissolves in the residual water.
Any acid, just about, can produce these results, but dilute hydrochloric
acid or vinegar are the two recommended acids for this test. Other carbonates
such as dolomite
do not react as easily with these acids as does calcite and this leads
to differentiating these somewhat similar minerals more readily.
Calcite is intricately tied to carbon dioxide in another way. Since
many sea organisms such as corals, algae and diatoms make their shells
out of calcite, they pull carbon dioxide from the sea water to accomplish
this in a near reverse of the reaction above. This is fortuitous for us,
as carbon dioxide has been found to be a green house gas and contributes
to the so called "green house gas effect". Environmentally then, calcite
is very important and may have been quite important to the successful development
of our planet in the past. By pulling carbon dioxide out of the sea water,
this biological activity allows more of the carbon dioxide in the air to
dissolve in the sea water and thus acts as a carbon dioxide filter for
he planet. Environmentalists are now actively engaged in determining if
this activity can be increase by human intervention to the point of warding
off the "green house gas effect". A significant amount of calcite precipitation
in sea water is undoubtedly inorganic, but the exact amount that this contributes
is not well known. Calcite and other carbonate minerals are very important
minerals in the ocean ecosystems of the world.
Calcite is not the only calcium carbonate mineral. There are no less
than three minerals or phases of CaCO3.
are polymorphs (latin for "many shapes") with calcite, meaning they
all have the same chemistry, but different crystal structures and symmetries.
Aragonite is orthorhombic,
vaterite is hexagonal
and calcite is trigonal.
Aragonite is a common mineral, but is vastly out distanced by calcite which
is the more stable mineral at most temperatures and pressures and in most
environments. Vaterite on the other hand is extremely scarce and rarely
seen. Aragonite will over time convert to calcite and calcite pseudomorphs
after aragonite are not uncommon.
Calcite is truly one of the best collection type minerals. There are
lots of interesting forms and varieties as well as colorful and beautiful
specimens to collect. It is generally easy to identify using its rhomohedral
cleavage, reaction to acid and double refraction and makes for a great
classroom example of these properties. If it is not the significant mineral
on a specimen, it might be an accessory to other wonderful minerals and
only enhancing their attractiveness. With its many different forms, environments,
associations and colors, a collector could never have all possible
combinations of calcite covered.
Color is extremely variable but generally white or colorless or
with light shades of yellow, orange, blue, pink, red, brown, green, black
and gray. Occasionally iridescent.
Luster is vitreous to resinous to dull in massive forms.
Transparency: Crystals are transparent to translucent.
Crystal System is trigonal; bar 3 2/m
Crystal Habits are extremely variable with almost any trigonal form
possible. Common among calcite crystals are the scalenohedron, rhombohedron,
hexagonal prism, and pinacoid. Combinations of these and over three hundred
other forms can make a multitude of crystal shapes, but always trigonal
is often seen and results in crystals with blocky chevrons, right angled
prisms, heart shapes or dipyramidal shapes. A notch in the middle of a
doubly terminated scalenohedron is a sure sign of a twinned crystal. lamellar
twinning also seen resulting in striated cleavage surfaces. Pseudomorphs
after many minerals are known, but easily identified as calcite. Also massive,
fibrous, concretionary, stalactitic, nodular, oolitic, stellate, dendritic,
granular, layered, etc. etc.
Cleavage is perfect in three directions, forming rhombohedrons.
Fracture is conchoidal.
Hardness is 3 (only on the basal pinacoidal faces, calcite has a
hardness of less than 2.5 and can be scratched by a fingernail).
Specific Gravity is approximately 2.7 (average)
Streak is white.
Other Characteristics: refractive indices of 1.49 and 1.66 causing
a significant double refraction effect (when a clear crystal is placed
on a single line, two lines can then be observed), effervesces easily with
dilute acids and may be fluorescent,
Associated Minerals are numerous but include these classic associations:
several metal sulfides,
and many other minerals.
Notable Occurrences include Pugh Quarry, Ohio; Rosiclare, Illinois;
New Jersey; Elmwood, Tennessee; Brush Creek and other Missouri,
Wisconsin, Kansas and Oklahoma localities, USA; Andreasburg, Harz
and Saxony, Germany; Brazil; Guanajuato, Mexico; Cornwall,
Durham and Lancashire, England; Bombay area of India; Eskifjord, Iceland;
many African localities as well as others around the world with their own
Best Field Indicators are crystal habit, reaction to acid, abundance,
hardness, double refraction and especially cleavage.