PHYSICAL PROPERTIES:
Chemistry: CaCO3
Composition: Calcium Carbonate
Class: Carbonates
Group: Calcite
Crystal system: trigonal
Fracture: conchoidal
Hardness: 3 on clevage face
Specific gravity: 2.7
Refractive Index: 1.49 and 1.66
Luster: Glassy to dull
Streak: white
Cleavage: perfect in three directions
Color: Colorless, white, pale tints
Transparency: Crystals are transparent to translucent
Associated Minerals: Some of the
more common minerals associated with calcite are. Apatite, barite, biotite,
celestite, copper, fluorite, galena, gold, quartz, sphalerite, sulfur,
zeolites, several metal sulfides, other carbonates and borates and many
other minerals.
COMPOSITION:
Calcium Carbonate ( 56% CaO, 44% CO2; Mn, Fe, and Mg may partially replace
the Ca).
DISTINGUISHING CHARACTERISTICS:
Fluorescence, phosphorescence, thermoluminescence
and triboluminescence are other important properties of calcite.
Calcite is frequently fluorescent, a small amount of manganese is
enough to make it glow red under some wave lengths of ultraviolet light.
Such as those from Franklin, New Jersey. 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 should occur in most specimens, but is not easily
demonstrated. In a dark room strike the specimen and it should glow.
Calcite is not the only calcium carbonate mineral. There are no less
than three minerals or phases of CaCO3. Aragonite
and vaterite are polymorphs 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, 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 much softer
on the base than on its cleavage face. It is number 3 of the Mohs scale.
But it can be scratched with the fingernail on the basal plane (about
2.5).
ENVIRONMENT:
Calcite forms in all types of occurrences, with all classes of
rock. Calcite is the primary mineral
component of limestone and its metamorphic form is marble. It forms
oolitic, fossiliferous and massive limestones in sedimentary environments
and even serves as the cements for many sandstone's and shale's. Most
calcite is white, though various impurities may tint it almost any color,
even black. Calcite is commonly a late
vein mineral and may need to be removed with a very dilute hydrochloric
acid solution to reveal well-formed crystals of other minerals.
CRYSTAL DESCRIPTION:
Calcite is the stable form of the widely distributed mineral calcium
carbonate (CaCO3). It is noted for its beautiful
varieties and its perfect crystals. Crystal Habits are extremely variable
with almost any trigonal or pseudo-hexagonal form possible. One of it's
most well known crystal forms is the scalenohedron or "Dogtooth"
Other common crystal forms are rhombohedron, hexagonal prism and pinacoid.
There are more than 300 crystal forms identified in calcite and these
forms can combine to produce literally a 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. Calcite is polymorphous,
(having the same chemical formula but different crystal structure) with
the minerals aragonite and vaterite as well as with several other forms
that apparently exist only under extreme experimental conditions. Calcite
comes in many other forms too. Such as massive, fibrous, concretionary,
stalactitic, nodular, oolitic, stellate, dendritic, granular and so
on.
TESTS:
Easily scratched, dissolves in cold dilute hydrochloric acid
or vinegar with effervescence. 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. Aragonite dissolves as easily, but has a different
crystal form and no cleavage. When heated, aragonite crumbles to powder
and loses its fluorescence. Even when not previously fluorescent, calcite
usually becomes so after heating
LOCALITIES:
The list of Localities we have seen from our dealers,
books and web sites seems to be endless. Some of the older more famous
localities are Palm Wash, California; Missouri- Kansas-Oklahoma ( tri
state lead district); Iceland (the original Iceland spar). Oolitic calcite
sand forms on the shores of Great Salt Lake. Also the Pugh Quarry, Ohio;
Rosiclare, Illinois; Franklin, New Jersey; Elmwood, Tennessee; Germany;
Brazil; Guanajuato, Mexico; Cornwall, Durham and Lancashire, England;
Bombay area of India and many Brazilian and African localities. As well
as others around the world with their own unique varieties
USES:
In cements, mortars, and the production of lime. Limestone
is used in the steel industry; glass industry and as the ornamental
stone marble. Calcite is also used in chemical and optical uses and
as mineral specimens. Flawless transparent calcite is used in optical
instruments especially in geological (polarizing) microscopes.
FACTS & HISTORY:
Calcite gets its name from "chalix" the Greek word for lime.
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 is the primary mineral component in cave formations. Stalactites
and stalagmites, cave veils, cave pearls, "soda straws" and the many
other different cave formations are made of calcite.
Mexican onyx is a variety of calcite that is used extensively for
ornamental purposes. It is not the same onyx as the quartz variety
of onyx
Another well known variety is "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.
Iceland spar is used for optical equipment and during World War II it
was a strategic mineral. Iceland spar also best demonstrates the unique
property of calcite, called double refraction. Double refraction occurs
when light enters the crystal and calcite's unique optical properties,
split the light into fast and slow beams. As these two beams exit the
crystal they are bent into two different angles. This is the term known
as angle of refraction. Because the angle is affected by the speed of
the beams. A person looking into the crystal sees two images
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