What is Calcite?
| Chemistry |
CaCO3, Calcium Carbonate |
| Class |
Carbonates |
| Group |
Calcite |
| 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
Specimens |
Calcite History & General Information
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 lime stones 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 Elmwoo`d,
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 lime stones 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.
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.
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.Calcite Physical Properties
| Hardness |
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 |
Approximately 2.7 (average) |
| Cleavage |
Perfect in three directions,
forming rhombohedrons. |
| Color |
Extremely variable but
generally white or colorless or
with light shades of yellow,
orange, blue, pink, red, brown,
green, black and gray.
Occasionally iridescent. |
| Fracture |
Uneven - Flat surfaces (not
cleavage) fractured in an uneven pattern |
| 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 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.
Best Field Indicators are
crystal habit, reaction to acid,
abundance, hardness, double
refraction and especially
cleavag |
| Luster |
Vitreous to resinous to dull
in massive forms. |
| Streak |
White |
| Transparency |
Crystals are transparent to
translucent |
| Crystal System |
Trigonal; bar 3 2/m |
| 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, phosphorescent,
thermo luminescence and
triboluminescent. |
Associated Minerals |
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. |
| Best Field Indicators |
crystal habit, reaction to
acid, abundance, hardness,
double refraction and especially
cleavage. |
Calcite Chemical Analyse
Physical & Chemical Analysis 56% CaO,
44% CO2
Specific Gravity: 2.7
Mn, Fe, and Mg may partially replace the
Ca
Industrial grade 99 % + pure snowy white
calcium carbonate limestone or marble.
Acid soluble. Provides fluids density
increase as well as secondary fluid loss
and lost circulation/seepage control.
Available in wide range of micron sizes.
Available in bulk form
| Purity |
99% Min |
| SiO2 |
0.6 %Max |
| Moisture |
1% Max |
| Mesh |
400, 600, 1500 |
| Packing |
25 kg. 4ply paper bags. |
Other specification according to API
or OCMA standards.
Calcite Properties
1. 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.
2. Fluorescence, phosphorescence:
Fluorescence, phosphorescence, thermo
luminescence 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.
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.
3. Acid Test: 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. |