New Mexico Mineral Symposium — Abstracts

Calcium, carbon, and oxygen are abundant elements in the earth’s crust that frequently combine to form calcium carbonate. Calcite is the stable form of CaCO₃ at the surface of the earth. Thus calcite is a widely occurring mineral that is common and abundant in all classes of rocks - igneous, metamorphic, and sedimentary. Sedimentary deposits of limestone and chalk are almost entirely composed of calcite; marble is metamorphosed calcite. Calcite occurs in silicate rocks, pegmatites, and hydrothermal veins; it precipitates in hot springs and caves and is even found in biologic settings. Rocks, such as granites, that are poor in calcium have little calcite.

Common — Because calcite occurs abundantly, many mineral collectors have access (either through the mineral marketplace or by collecting) to locations that produce large, beautiful crystals. Availability and abundance are often related to “low” price, a factor in assembling a specialized calcite collection. Thus, common is a key factor that explains collector interest in calcite.

Colorful — Calcite is colorless and transparent or white when pure; other colors are uncommon and highly desirable. When colored crystals are found, they are generally yellow to honey-colored. The cause for the yellow color is unknown but is thought to be associated with impurities of iron or perhaps to defect color centers.

Some of the most colorful calcite crystals result from the replacement of calcium by manganese, iron, zinc, cobalt, lead, strontium, magnesium, or barium. Although this substitution is typically on the order of only a few percent, it produces such notable calcite varieties as pink manganoan calcite, beige plumbian calcite, rose-red cobaltian calcite, and tan ferroan calcite. Colorful calcite is also produced by admixtures of other minerals.

Clarity — Clear and transparent crystals of calcite when included by such strongly colored minerals as boulangerite, celadonite, copper, hematite, and others can often produce striking specimens. The inclusions may occur either on an early calcite growth surface that has been overgrown by a later generation of clear calcite (a phantom), or it may be uniformly disseminated throughout the calcite crystal. Calcite may even incorporate up to 70% sand within its structure.

Whatever the cause for colorful calcite, all well-crystallized specimens are highly prized and generally priced accordingly. A calcite collection with a wide variety of colored calcites is a major step above a collection of “common” calcites.

Crystal structure — Calcite’s structure consists of alternating layers of calcium atoms and carbonate groups stacked along the c-axis. Calcium has octahedral coordination with six oxygens from six different CO₃ groups. The crystal structure and composition determine the properties and appearance that makes calcite so interesting to collectors.

Durability

If calcite has any negatives, they would be associated with durability. Calcite hardness (Mohs scale = 3) and cleavage (readily induced) require careful handling of crystals throughout the acquisition, curation, and display processes. In addition to the obvious need to avoid contact with harder objects, calcite should never be subjected to freezing conditions, as many crystals are included with small amounts of water. Internal fractures and external damage decrease the desirability of specimens.

Forms of Calcite

Calcite has been described as the mineral with the most forms; in fact more than 600 have been documented. A single crystal of calcite may be bounded by more than one form. It is not uncommon for three, four, or even five or more forms to be combined on a single crystal; thus, the combinations number into the thousands. Although this variety is greater than that of any other mineral, all of the forms of calcite fall into the following five groups. They are:

Pinacoid – An open form made up of two parallel faces that are each perpendicular to the c-axis.
Prism – An open form composed of six or twelve faces, all of which are parallel to the c-axis.
Rhombohedron – A closed form composed of six faces; three at the top of the crystal alternate with three at the bottom. The two sets of faces are offset by 60°.
Scalenohedron – A closed form with twelve faces grouped in symmetrical pairs, three pairs above and three below in alternating positions. In perfectly developed crystals, each face is a scalene triangle; the faces meet in a zigzag line around the crystal’s girdle.
Dipyramid – A closed form having twelve faces, six on the top of the crystal and six immediately below them on the bottom; each face is an isosceles triangle.
Habits of common calcite crystal aggregates can be described as fibrous, nodular, stalactitic, or botryoidal.

Twinning

When two crystals grow in a fixed relationship related to the structure of a mineral, they are described as twins. Typically, twins are uncommon and are highly prized by collectors. Calcite has four recognized twin laws, the most common of which occurs when the c-axes of the crystals lies in the same plane and is at 180° to one another. These are easily recognized by re-entrant notches along the contact at the basal plane.

Under the other three twin laws, the two crystals are inclined at an angle to one another, and frequently rapid growth occurs on faces where the two parts of the twin meet, making identification somewhat difficult. The second-most common twin law for calcite occurs when the c-axes of the twin are inclined at an angle of 127°29.5’ with respect to one another. Here the twin plane is parallel to a face of a shallow negative rhombohedron. These are often described as “butterfly” twins.

When the c-axes of the twin are inclined at an angle of 90°46’ with respect to one another, the twin plane is parallel to a face of the positive rhombohedron, which is defined by the cleavage of calcite. Occurrences of this calcite twin law are quite rare and are commonly referred to as “heart” and “axe-head” twins when rapid growth occurs.

Another rare twin law for calcite occurs when the twin plane is parallel to a face of a steep rhombohedron and the c-axes of the twin are inclined at an angle of 53°46’ with respect to one another and. These twins have been described as “fish-tail” and “bishop-hat” twins.
To put crystal structure into perspective, a calcite collection that contains specimens that are damage-free, has a wide representation of crystal forms, and includes the four twin laws is another major step above a collection of common calcites.

Collectible

Because calcite seemingly occurs everywhere, it has been possible to acquire and preserve excellent calcite crystals from mining ventures operated primarily for metals. As older mines became uneconomic and are closed, new mines in new places become the source for metals (and for calcite). Thus, availability of calcite cycles from boom-to-bust at every location.

Consequently, it is challenging to include older classics in a calcite collection without compromising one’s budget or quality standards. Specimens from the late 1800s and early 1900s mining operations in Germany, England, and Michigan are highly desirable and command high prices for even mediocre specimens.

In the late 1900s, mines in Mexico and Peru, at Tsumeb, and at many of the MVT deposits in the midwestern United States were producing seemingly inexhaustible quantities of minerals, including choice calcite. But where are these specimens now?

Today, mines in the former Soviet Union, India, China, and Africa are the source for much of the fine calcite now available in the marketplace. I suggest you not let this opportunity pass by. Of course, there will eventually be other new localities, but will they meet the standard of older and modern classic localities?

Summary

A well-documented calcite collection is, obviously, only as good as the specimens it contains. When careful attention is given to acquiring specimens without damage and those with a wide variety of crystal habits, including the four twin laws of calcite, the collection has a strong foundation. The addition of calcites colored either by substitution or from inclusions, and specimens from classic localities, both historical and modern, further enhance the collection. Ultimately, it is the collector’s taste in aesthetics that completes the collection to his/her satisfaction.