A most versatile mineral

kw: mineralogy, minerals, crystals, morphology

In the local rockhound club there are two pairs of collectors who specialize in a single mineral. One pair of women collects only gypsum in all its varieties, and a pair of men collects only calcite. Calcite, in particular, is known to have more than 300 crystal habits.

These six thumbnail images illustrate just 2% of the crystal habits of calcite. In reading order: a bipyramid, stacks of scalene plates, pyramidal iceland spar, nailhead spar, two scalenohedral crystals, and a hexagonal column with flat terminations.

What is a crystal habit? Depending on the symmetry of the chemical structure and the underlying details of how molecular units fit together, a mineral can have various preferred faces that make up its crystal form.

For example, table salt usually crystallizes as little cubes. If you dissolve some and let the solution evaporate slowly, you'll get some milky cubes big enough to see. However, if you add a little swimming pool acid (hydrochloric acid) to the solution, and let it evaporate, the cubes' corners will be replaced by small triangular faces, and with strong enough acidity, the triangles will dominate, making double-pyramid-shaped crystals will tiny squares at the corners, or perhaps no square faces at all. The predominance of squares or of triangles express different crystal habits of table salt.

Calcite has a more complex symmetrical system than table salt does. It is a trigonal mineral, meaning that its basic crystal form is either a 3-sided prism or the bipyramid seen at top left.
It has a strong tendency for the opposite trio of faces to form, so pseudo-hexagonal shapes such as the column at lower right are also common. The trigonal form is most easily seen in the plates at upper right. Each side of a plate is composed of three faces that come together at a low angle; the faces of the other side are rotated 60°. These plates are actually terminations of the nailhead spar sort, without any intervening material.

The columnar prism faces are parallel to the rotational axis of the crystals, the c axis. The flat plate that terminates the prism at lower right is perpendicular to the c axis. In between, there are three common angles at which triangular faces can lie, the steep angle seen at upper left, the very low "nailhead" angle seen at upper right, and one in between. Depending on the acidity or alkalinity (the pH), and the concentration of various other soluble minerals, these many kinds of crystal faces can develop in all sorts of combinations, which leads to the many crystal habits of calcite. So far as I know, calcite has the greatest number of crystal habits of any mineral.