|CaCO3, Calcium Carbonate
|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.
|extremely variable but generally white or colorless
or with light shades of yellow, orange, blue, pink,
red, brown, green, black and gray. Occasionally iridescent.
|vitreous to resinous to dull in massive forms
|Crystals are transparent to translucent
|trigonal; bar 3 2/m
|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 or
pseudo-hexagonal. Twinning 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.
|perfect in three directions, forming rhombohedrons.
|3 (only on the basal pinacoidal faces, calcite has
a hardness of less than 2.5 and can be scratched by
|approximately 2.7 (average)
|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 , phosphorescent,
thermoluminescence and triboluminescent.
|numerous but include these classic associations: Fluorite
, quartz , barite , sphalerite , galena , celestite
, sulfur, gold, copper, emerald, apatite, biotite, zeolites,
several metal sulfides, other carbonates and borates
and many other minerals.
|Baptiste Lake North Occurrences, Bear Lake Occurence,
Bessemer Mine, Bower's Point Roadcut, Davis Hill, Davis
Quarry, Dwyer Mine, Egan Chute, Faraday Hill Roadcut,
Goulding-Keene Quarry, Grace Lake Roadcut, Morrison
Quarrv. Musclow Occurrence. Saranac Mine York River
Best Field Indicators:
|crystal habit, reaction to acid, abundance, hardness,
double refraction and especially cleavage.
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? 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 of calcite.
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 scalenohedron
or "Dogtooth Spar" as it is sometimes called.
This variety appears as a double pyramid or dipyramid
, 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 quartz
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 Mexico . The
iceland spar displays the classic cleavage form of calcite,
the rhombohedron . 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. Double 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. Fluorescence , phosphorescence
, thermoluminescence and triboluminescence 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 Franklin, New Jersey where the
massive calcite is enriched in a small amount of manganese
and fluoresces a bright red under UV light. Some Mexican
iceland 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. Why?
Because 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 or siderite
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.
Aragonite and vaterite 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.