We collect minerals for many reasons, both scientifi c and aesthetic, whether they are crystallized or massive. We admire crystal groups, single crystals, and their beauty as creations of nature. By and large, we collect and display specimens dominated by groups of a particular mineral species, sometimes accompanied by other species. Such clusters of minerals are called an aggregate of crystals. Under the right circumstances and within those aggregate mineral groups are three general forms of crystals that exhibit twinning.
The fl uorites of this specimen are simple, but stunning purple cubes, but the sphalerite is a perfect Spinel Law twin.
THE ARKENSTONE GALLERY OF FINE MINERALS, IROCKS.COM
Crystals can twin in parallel or routine ways involving two crystals, or as in complex twinning, several crystals. To be considered as twins, the crystals must be in a fi xed identifi able relationship to each other. How this happens is most impressive. As crystals grow in solution, individual ions of a mineral gather to form a crystal. This is an orderly geometric process that produces a geometric pattern that results in an external crystal form that we recognize. Several factors infl uence how crystals grow, not the least of which is the direction of fl ow of the solution. This direction brings ions in contact with the growing crystal from a path that can infl uence growth. Such direction causes ions to accumulate fi rst on the side that receives the fl ow. This development can affect the orderly process of crystallization.
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