Calcite

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Calcite mineral

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3.

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300°C, and vaterite is even less stable.

Calcite crystals are trigonal-rhombohedral, though actual rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.

Properties

It has a defining Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities.

The stone is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as dogtooth spar while the rhombohedral form is sometimes referred to as nailhead spar.

Single crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece to appear doubled. The birefringent effect was first described by the Danish scientist Rasmus Bartholin in 1669.

At a wavelength of 590 nm it has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively.[9] Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.

This mineral forms in a wide range of geological settings, including sedimentary deposits and hydrothermal veins. It is a key component of various rock types and has significant influence on the formation of caves and stalactites. As water seeps through layers of the Earth, dissolved calcium carbonate often re-precipitates to create magnificent crystalline structures underground.

Because of its softness and ease of cleavage, this material can be carved and shaped into ornamental objects. Historically, certain cultures valued the stone for decorative and practical purposes, using its translucent varieties for windows or even rudimentary lenses. In modern times, it continues to be admired for both its aesthetic appeal and practical applications.

Industrially, it is used in construction materials, as a filler in paints and plastics, and as a component in agricultural soil treatments. The mineral’s ability to neutralize acids makes it a valuable resource in various chemical processes. Collectors appreciate the vast array of crystal forms and colors, making it a popular specimen for private and museum displays.

From a symbolic perspective, some enthusiasts regard this crystal as a stone of clarity, suggesting that it helps organize thoughts and enhance concentration. Although these ideas stem from personal and cultural beliefs rather than scientific evidence, they contribute to the stone’s enduring appeal.

Calcite

FAQ

Where is this mineral typically found?

This mineral is abundant and found worldwide. Significant deposits occur in sedimentary rock layers, hydrothermal veins, and limestone formations.

How can I identify high-quality specimens?

High-quality pieces often display well-formed crystals, transparency, or vivid coloration. The absence of major cracks or inclusions can also indicate a desirable sample.

What is Iceland spar?

Iceland spar is a transparent variety prized for its optical clarity and strong birefringence. It has been historically used in scientific instruments and for special visual effects.

Is this stone suitable for everyday jewelry?

Because it has a Mohs hardness of 3, it can be prone to scratches and chips. It is often used in jewelry with protective settings or worn for occasional use rather than daily wear.

Can this mineral lose its color over time?

Some specimens may lose or alter color if they contain impurities that react to light, heat, or chemicals. Storing samples away from excessive light and heat can help preserve their appearance.

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