Calcite is truly a collector’s delight. There are many interesting forms as well as colourful and beautiful varieties found in nature. The following are the varieties based on crystal form, mode of formation and physical properties that are of significance for the museum collectors, tourists and industries.
(a) Travertine: It is a sedimentary rock. Travertine is a natural chemical precipitate of carbonate minerals (see also the chapter on limestone), typically aragonite, but often recrystallized to calcite, which is deposited from the water of mineral springs (especially hot springs) or streams saturated with calcium carbonate. When pure, travertine is white, but often is brown to yellow due to impurities. When carbon dioxide-rich water percolates through rocks in limestone areas, the water dissolves the limestone and becomes saturated with it. When the water resurfaces later, the
sudden drop in pressure and the change in temperature cause the water to release the carbon dioxide gas. The calcium carbonate then recrystallizes, often over minute underwater plants. The resulting rock is typically quite porous with numerous cavities.
(b) Calcareous tufa: When travertine is exceptionally porous, it is known as calcareous tufa (see also chapter on limestone).
(c) Iceland spar: It is the pure crystallized (rhombohedral) transparent variety of calcite. It is basically clear cleaved fragments of completely colourless (ice-like) calcite. It was originally discovered and named after Eskifjord, Iceland where it is found in basalt cavities. Most of today's Iceland spar comes from Mexico.
(d) Scalenohedron or dogtooth spar: 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, and hence the name. Beautiful clear colourless or amber-orange examples of this variety are considered classics and outstanding specimens come from Pugh Quarry, Ohio; Cornwall, England and Elmwood, Tennessee. However, it is found worldwide.
(e) Cave calcite: Calcite is the primary mineral component in attractive cave structures like stalactites and stalagmites, cave veils, cave pearls, soda straws and the many other different cave formations that millions of visitors to underground caverns enjoy. It is due to the ability of calcite to readily dissolve that these formations occur. Overlying limestone or marble is 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 in a variety of forms that are attractions for tourists.
(f) Mexican onyx: Mexican onyx is soft and is different from onyx which is a variety of quartz and is hard.
C
RITERIA OFU
SE1. Colour: Calcite is generally white or colourless but sometimes, due to inclusion of impurities, it shows variable colours like light yellow, orange, blue, pink, red, brown, green, black and grey. Occasionally, it is iridescent (i.e., varying in colour when seen in different lights or from different angles).
2. Lustre and transparency: Lustre is vitreous in crystals, and resinous to dull when in massive form. Crystals are transparent to translucent. Iceland spar is highly transparent (like ice).
3. Crystal system and cleavage: Calcite crystallizes in trigonal form, but its habits are highly 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. Twinning is often seen and results in crystals occurring in some unique shapes, making them attractive to people. Cleavage is perfect in three directions, forming rhombohedrons.
Calcite 43 4. Refractive index: Calcite shows strong birefringence with refractive index varying from 1.49 to 1.66 in normal light causing a significant double refraction effect. At a wavelength of about 590 nanometre, calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively (high degree of birefringence). Iceland spar is particularly remarkable for its 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 because the angle is affected by the speed of the beams. There is only one direction that the beams are both of the same speed and that is parallel to the C-axis or primary trigonal axis. By contrast, the direction perpendicular to the C-axis will have the widest difference.
5. Polarization: As white light waves propagate through space, the vibrations take place in all possible directions on all possible planes. But due to certain factors, these vibrations are modified so as to take place on a single plane only. This light vibrating on a single plane is called polarized light, and the phenomenon of such modification is called polarization. The polarizing factors are: (i) partial reflection, (ii) double refraction or birefringence, and (iii) absorption. When light is incident on an object, some of its rays are reflected back and some are refracted, and both the reflected and the refracted rays are polarized. When the object is doubly refractive, each of the two refracted rays is polarized. In case of absorption (as in case of a dark coloured object), light rays vibrating in all but one plane are absorbed within the object and the rays emerging out from it are thus polarized. Polarized light is used for seeing the colour of an object in thin section under microscope, otherwise in unpolarized light it will appear as almost colourless unless the object is very dark coloured. Calcite in general and Iceland spar in particular is a strong polarizer of light by double refraction. In fact, Iceland spar is used in demonstrating the polarization of light.
6. Luminescence: Many substances easily gain energy and emit light without being heated very much. They do this through a process called luminescence. Atoms of some luminescent materials emit light only during their exposure to exciting energy and they are called fluorescent. If the exciting energy is heat, then the phenomenon is called thermoluminescence. Atoms of some luminescent materials stay excited for some time before they de-excite and consequently, they glow in the dark long after they have received extra energy. They are called phosphorescent. Triboluminescence is a property that occurs in some objects when it is struck or put under pressure and in a dark room such objects glow. Fluorescence, thermoluminescence, phosphorescence and triboluminescence are shown by some specimens of calcite (e.g., specimens of massive calcite containing a small amount of manganese that is found in Franklin, New Jersey, USA fluoresce a bright red colour under ultraviolet light; some specimens of Mexican Iceland spar phosphoresce purple or blue colour after ultraviolet light source is removed).
7. Water solubility: Calcite is practically insoluble in water having solubility of only 0.0013 gm/100 gm of water.
8. Hardness: Hardness is variable on different faces from less than 2.5 to 3 on Mohs scale.
9. Specific gravity: Approximately 2.7.
10. Mechanical properties: Calcite grains possesses low modulus, high tensile strength (ability to elongate without breaking) and good slump-resistance (resistance to heavy fall while at the same time transferring pressure).
U
SESThe important uses of calcite are: 1. Optical instruments
2. Glass
3. Ceramic glaze
4. Decoration and ornamentation 5. Synthetic marble
6. Water treatment 7. Adhesive
These uses are elaborated as follows.
1. Optical instruments: Iceland spar is used in the optical instruments for its transparency, strong birefringence with consequent ability to polarize light and to split images.
(a) Microscope: The transparent crystals of Iceland spar free from flaw is greatly valued in the optical industry for the manufacture of nicol prism in polarizing microscopes. Iceland spar for this purpose should have a high degree of purity (totally free from both cloudy inclusions and cavities of foreign substances), perfect crystalline structure and transparency, and the mineral pieces must be of at least 2.54 cm. long by 1.27 cm. thick (2 inch cube is preferred) and colourless. It should be free from internal iridescence due to incipient cracks along cleavage planes and from twinning other than parallel to the base.
(b) Distance-measuring instrument: The double image of a distant object viewed through a crystal of Iceland spar produces parallax effect, i.e., the apparent displacement of an object as seen from two different points that are not on a line with the object. By rotating the crystal, this parallax can be brought to zero and at that position of the crystal the two images coincide with the actual object. By calibrating the distances with the angle of rotation of the crystal, the distance of an object can be measured. Instruments based on this principle were used by bomber pilots and gunners during World War II (1940-1945) for estimating distances of targets. During that time, Iceland spar used to be classified as a strategic mineral.
(c) Quarter-wave plate for optical instruments: A wave plate or retarder is an optical device that alters the polarization state of a light wave passing through it by shifting the phase of light wave between two perpendicular polarization components. A typical wave plate is simply a birefringent crystal with a carefully chosen thickness. A quarter wave plate can change polarized light to circular light and vice versa. When plane polarized light passes through this plate, it is split into two components with relative shift of one quarter wave length. This shift causes light wave to propagate in helical fashion. Thus circular polarized light is produced. Birefringence of calcite makes it suitable for this application in polarizing microscope, in photo- elastic stress/strain benches for modelling stress distribution in bridges and buildings, in helium-neon laser systems, as beam-splitters etc. For this purpose, plates 10-25
Calcite 45 mm diameter and thickness around 0.025 mm are used. The exact thickness, however depends on the wave length of light selected. Blue light requires thinner plate than red light.
2. Glass: The composition of typical common colourless soda-lime glass is sodium calcium silicate [18Na2O.2MgO.8CaO.72SiO2]. Essentially, the process of manufacture of
glass consists in melting a mixture of 47% silica sand, 14% soda (Na2CO3), 12% dolomite
(CaCO3.MgCO3), 3.5% CaCO3, 2.5% carbon (coke) and the balance broken waste glass
(called cullet) at 1400-15000C. The CaCO3 is generally in the form of limestone, but often, it
is partly replaced by calcite to adjust the purity. The lime introduced into the charge in the form of calcite, limestone and dolomite acts as a flux (along with soda and broken glass) for lowering the melting point of silica from 15800C to 14000C. The low-melting soda and broken glass become liquid first and silica (quartz) being soluble in boiling soda, dissolves in this liquid to form low-melting sodium silicate which then reacts with the CaO and MgO to form glass. The calcium silicate component imparts great chemical stability to the glass and also reduces the latter’s thermal expansion (for more details. See the chapter on silicon).
Fe2O3 and TiO2 form low-melting iron-titanate glass causing blisters in the products and
consequent increase in porosity. Besides, Fe2O3 makes the product coloured (even in coloured
glass in which iron oxide is added later, its presence in the initial charge hampers control of the colour). TiO2 also has a high melting point and it will unnecessarily increase the firing
temperature. Both these constituents are therefore objectionable.
A little MgO may not objectionable, because it also forms a component of the charge. Lime has a tendency to form crystals and addition of magnesia helps prevention of this devitrification. The CO2 of the carbonates of calcium and magnesium are expelled reducing
them to CaO and MgO. But calcite is primarily added to introduce a high proportion of lime for balancing its ratio with MgO and hence it should contain as high CaO as possible. So MgO-content in the calcite should be minimal.
The industries specify calcite containing 95% (min) CaCO3, 0.15% (max) Fe2O3 and
TiO2 each and 2% (max) MgCO3 with powder size ranging from 180-850 micron.
3. Ceramic glaze: The purpose of glazing is to provide a uniform firmly adhering coating on the surface of the ceramic body concealing defects such as pinholes, bubbles etc. Glaze may be raw glaze or fritted glaze. Raw glaze consists of insoluble material applied as such (soluble components crystallize in the mixture and cause blemishes on the treated surface), while fritted glaze is heated beforehand to cause chemical change in the components. The glaze is made predominantly of quartz and feldspar. The ingredients are mixed, finely ground, and mixed with water, and this mixture is the glaze. The fine grinding ensures that the materials do not settle quickly and remain in suspension in the water long enough for them to deposit as a uniform coating on the body. The moulded raw body of the product is dipped into the glaze and then fired to a temperature of about 14000 C. After firing, it becomes lime and combines with the silica of quartz and the alumina of feldspar to form calcium alumino-silicate which is a hard glassy substance (cf. blast furnace slag) making the glazed surface hard and impervious.
For lowering the fusion temperature of the mix limestone is often added, and to improve the lime content of the mix, calcite is invariably added.
As regards specifications, since calcite is added to improve the CaO input in the mix, a high content of CaCO3 is desirable and impurities should be minimal. Industries usually
specify 97% (min.) CaCO3 content in calcite. The super white calcite of 500 micron or 30
mesh is generally used.
4. Decoration and ornamentation: Although beautiful crystals with attractive colours often showing iridescence are available, calcite is not regarded as a gemstone because of its softness (Mohs hardness 2.5-3). However, it is sometimes used in jewelery where it is not subject to rough handling, and great care has to be taken while cutting and polishing it. But for the same reason of softness, some of the varieties of calcite are used for sculpting decorative artefacts. Mexican onyx is one such variety preferred by the sculptors. It is used extensively for ornamental purposes. It is carved into inexpensive decorative objects like figurines, vases, bookends, plates, statuettes etc. Mexican onyx is banded with multiple orange, yellow, red, tan, brown and white colours that have marble-like texture.
5. Synthetic marble: Synthetic marble is a strong, homogeneous, dense, translucent product which can be cast into different shapes and sizes. It finds use in bathrooms and as structural components. It is made by casting and heat-curing (in presence of a catalyst) a finely ground mixture of alumina and resin matrix with some filler material. The filler constitutes 50-85% of the weight of the mixture and includes calcite, silica, oxides of antimony and titanium and talc. The refractive index of the mixture vis-à-vis the resin matrix is so manipulated as to give an illusion of depth. Calcite used is in the form of powder of size less than 250 microns (60 mesh).
6. Water treatment: In the conventional process, calcite is added to water to form Ca(OH)2 which increases the pH to around 6-7, and the conversion is very slow. Hence, this
process can be suitable for treating small volumes of still water. However, for treating large volumes of flowing water, now-a-days, caustic magnesia is preferred (see also chapter on magnesite).
7. Adhesive: Adhesive is an organic or inorganic substance capable of bonding together other substances by surface attachment. Generally, high-purity white dolomite is added to certain adhesives with a view to: (i) adjusting the colour, (ii) reducing shrinkage due to internal stress by virtue of its moderate hardness, and (iii) maintaining flexibility by virtue of its porosity. But, for high-performance adhesive where even a minute shrinkage associated with cracks is not permissible, dolomite is mixed with pure and white powdered calcite because of the latter’s low modulus, ability to elongate without breaking and resistance to heavy fall while at the same time transferring pressure. Thus made adhesives are used for grouting ceramic tiles. Due to low water-solubility of calcite, such adhesives can be used in wet conditions.
8. Other uses:
(a) Rubber: It is used in powdered form as a filler as substitute of precipitated calcium carbonate or PCC to increase whiteness and hardness (see chapter on limestone). (b) Textiles: In pulverized form it is used as a filler mainly on account of white colour of
Calcite 47 (c) Paint: In the same way as above, it is used as an extender in white paint.
(d) Chemicals: For making lime-containing chemicals like bleaching powder etc., calcite is sometimes used as a substitute of limestone-based lime. Calcite for this purpose, should contain at least 99% CaCO3 and no more than 0.5% of Fe2O3, 2 ppm of As
and 10 ppm of Pb.
(e) Cosmetics: It is used as filler in ultra-fine form and the calcite should have 97% CaCO3, 200 ppm Fe, 10 ppm (max.) Cu, 100 ppm (max.) Mn and 0.2% (max)
moisture.
(f) Electrode: Sometimes, calcite is added as a flux and the industries specify 95% (min) CaCO3, 0.01% (max) P, 0.035% (max) S and 2% (max) SiO2.