CALCITE

Calcite is an extremely common carbonate, very important as a rock former, occurring in sediments and in igneous, metamorphic and sedimentary rocks. It is an important ore, with hundreds of uses in industry, such as in the manufacture of cement, as mineral filler in cosmetics, paints and rubbers, in steel, metallurgy, agriculture and many others.

Under the microscope, calcite in large, well-formed crystals (spatic) shows cleavage and twins; microcrystalline calcite (micrite) has no cleavage or twins. Macroscopically, it can be any color from black to white, passing through the entire rainbow; it can be zoned or banded. The crystals, even easily formed at room temperature, have more than 2400 combined crystal forms. The largest single crystals known were 7x7x2 and 6x6x3m and weighed 250 tons. Calcite has dozens of different habits. Very useful in its recognition is its strong effervescence with cold diluted (~10%) hydrochloric acid– just a drop is enough to obtain the effect. Calcite is trimorphic with aragonite and vaterite and forms a series with rhodochrosite. It can contain Fe, Mg, Mn, Zn and Co. Calcite has at least 20 varieties.

Calcite can fluoresce in red, blue, yellow, and other colors under both short-wave and long-wave UV light. It is phosphorescent, cathodoluminescent and thermoluminescent, but rarely triboluminescent.

1. Characteristics

Crystal system: Trigonal scalenohedral.

Color: All. From black to white, passing through the rainbow.

Habit: Dozens of different habits.

Cleavage: Rhombohedral, perfect {10-11}, with angles of 60 and 120 degrees to each other. 

Tenacity: Brittle

Twinning: Polysynthetic and pressure twins, very common. 

Fracture: Conchoidal.

Mohs Hardness: 3

Parting: No.

Streak: White.

Lustre: Vitreous, pearly on cleavage surfaces.

Diaphaneity: Transparent.

Density (g/cm³): 2.7

 

2. Geology and Deposits

Calcite is the most common carbonate, it can occur in practically all types of sediments and rocks, in the latter either as a primary or secondary mineral. In limestone, marble and carbonatite, it is the main rock-forming mineral.

It is very common as an accessory and can fill veins that cross rocks. It is a very common mineral associated to metallic ore minerals, in this case called “gangue”. The list of possibilities is very long; calcite can always be present because of its ease in dissolving and reprecipitating.

 

3. Mineral Associations

Calcite occurs in so many different paragenesis, as a primary or secondary mineral, that any list of associated minerals will always be incomplete. Calcite basically occurs in association with any other mineral.

 

4. Transmitted Light Microscopy

Refraction indices:  nε: 1.486  no: 1.658 

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, has no color or pleochroism.

Usually it is a bit darker, grayer, than colorless silicates like quartz and feldspars.

Relief: Relief varies from low to moderate to high every 90º when turning the stage in crystals with well defined cleavage. This phenomenon has been called “relief pleochroism” or “chagrin change” and is typical of rhomboedral carbonates (calcite, dolomite, aragonite, siderite, rhodochrosite and magnesite).

When microcrystalline, these carbonates do not show “relief pleochroism” or it is very difficult to perceive.

Cleavage: Rhombohedral {10-11} perfect in three directions. In the crystals there are two cleavages that intersect at angles of 60º and 120º.

When calcite is microcrystalline (“micrite”), no cleavage is observed.

Habits: Normally anhedral in thin section. When it crystallized in cavities, such as vesicles of volcanic rocks, it can be idiomorphic in crystals called “dog teeth” (= scalenohedral). Macroscopically it can show many hundreds of habits and forms.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Extreme birefringence, 0.172: very high colors, 4th order.

They are cream, brownish, with colored dots and bands.

Extinction: Symmetrical in relation to the two cleavages (the vertical string of the crosshair forms the bisector of the angle between the two cleavages in the extinction position). In the extinction position there is birefringent calcite dust generated by the polishing.

Elongation sign: Does not apply as calcite may be elongated in several directions.

Twins: On {01-12}. They are very common in large crystals and appear as straight, colored, parallel bands. These twins can often be seen even at PPL. The abundance of twins characterizes calcite; dolomite is practically untwinned.

Zoning: No.

CONVERGENT LIGHT

Character: U(-), but can be anomalously biaxial, mainly in metamorphic rocks.

2V angle: No. When anomalous from 0 – 15º.

Alterations: Calcite is very resistant, but it can be easily dissolved, allowing substitutions (pseudomorphoses!) for other minerals in spaces formerly occupied by calcite crystals.

May be confused with: other common carbonates such as dolomite, siderite, magnesite, rhodochrosite and aragonite. Among these, calcite is the carbonate with the most twins. But other analytical techniques are needed to confirm trigonal carbonates. Under a microscope it is only possible to identify these different carbonates with the use of a universal stage, a technique that requires a lot of experience.

Dolomite is more often idiomorphic than calcite.

Titanite shows higher relief, yellowish to brown colors, faint pleochroism and is biaxial.

 

Calcite 01: In PPL, large calcite crystals exhibiting the rhombohedral cleavage typical of carbonates. This type of calcite is called “spatic” (from the old German word “spat” = mineral that has cleavage). Crystals with such development are typical of coarse-grained marbles, carbonatites and hydrothermal veins, but can occur in virtually any geological environment.
Calcite 01: now in XPL, the same section of the previous image. The high-order interference color of calcite and other carbonates is difficult to classify; a creamy, yellowish or brownish color.
In the center of the image, microcristalline calcite filling the porosity of a sandstone. This kind of calcite is calles "micrite", never shows relief pleochroism nor cleavage, but the interference colors match the color of well-developed crystals. PPL, quartz grains in gray.
In XPL, euhedral calcites (scalenohedrons) together with clay minerals (green) in a vesicle (gas bubble) of a volcanic rock. The volcanic rock shows plagioclase crystals (white/gray lamellae)
Calcite in XPL, in the extinction position, showing twins, which are these colored, parallel straight bands. There can be two sets of twins in a crystal.
Calcite in a sedimentary rock (limestone) in XPL, both as a constituent of oolites (spherical/ellipsoidal shapes) and in the matrix too.
In XPL, calcite in a marble showing an unusual elongated grain texture tending to form parallel aggregates.
Extremely fine-grained limestone in XPL (in PPL the image is almost the same). Calcite (could be dolomite!) show very small equigranular crystals. The opaques are made of idiomorphic magnetite. Calcite replaces a material with low birefringence (clay?), with remnants in the form of oval particles, somewhat concentric, which could be chamosite derived from bertierine.
In XPL, marble with large crystals without preferred orientation and without twins, suggesting it can be dolomite. Other analytical methods are required to identify the minerals here.
Another marble with large grains without orientation, similar to the one in the previous image. But here are many twins, suggesting that it is calcite.
In XPL, in a volcanic rock, gas bubbles (vesicles) entirely filled with microcristalline calcite. There are no idiomorphic calcite crystals in the vesicles.
Calcite 02: in PPL, “dog tooth” calcite scalendohedrons grown on the wall of a vesicle in a volcanic rock. The calcites show good cleavage. Chalcedony dyed green by clay minerals was deposited on them and the rest of the cavity (vesicle) was filled with microcrystalline silica also dyed green.
Calcite 02: the same section as in the previous figure, now in XPL. As the chalcedony spherules has a radial structure and straight extinction, a "cross" can be seen in each one of the spherules.

5. Reflected Light Microscopy

Carbonates, including calcite, are easy to recognize under Reflected Light. The situation is the same as for Transmitted Light: the mineral is identified as a rhombohedral carbonate, but it is not known which one it is, as in addition to calcite it could be dolomite, siderite, magnesite, rhodochrosite and others. Calcite usually is characterized by a large number of twins.

Sample preparation: the polishing of calcite is very simple, it is easy and quick to obtain. It is of excellent quality.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.

Pleochroism: Very strong in shades of gray, easy to observe.

Reflectivity: Very low (~4%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Very strong anisotropy, very evident, in gray. It’s masked by the internal reflections, but it’s usually easy to see.

Internal reflections: Generalized, clear, white to milky.

May be confused with: other carbonates, which have practically the same characteristics under the Reflected Light microscope.

General Characteristics: 

Rhombohedral cleavage is easily visible when the grains are large.

Polishing pits are present when crystals are large.

Polysynthetic twins are very conspicuous, easy to see, both in PPL and in XPL. If the carbonate has many twins, it is probably calcite.

Calcite 01: Calcite in carbonatite in PPL. On the left of the image, an apatite grain. In PPL, the very strong pleochroism of calcite produces two very distinct shades of gray.
Calcite 01: the same section as in the previous figure, now in XPL. Calcite exhibit clear, white and milky internal reflections, but usually anisotropy is visible. Apatite with yellow internal reflections.
Calcite with many twins in XPL. In many cases calcites have twins in large quantities, mainly in tectonized ores. These twins can also be seen in PPL and are diagnostic for calcite, as dolomite has twins much less frequently.
In XPL+2°, safflorite (anisotropy in orange and blue), nickeline (anisotropy in sky blue and orange brown) and twinned calcite (twins in this case look like the polysynthetic twins of plagioclases).
Calcite 02: in XPL, hydrothermal vein with large calcite crystals. Calcite has the typical twins and characteristic anisotropy.
Calcite 02: from the same vein of the previous image, now in PPL, detail of the calcite crystals with twins. The strong pleochroism is diagnostic for carbonates, not only for calcite!
<