Rock & Gem Kids: Dimorphs and Polymorphs


By Darryl Powell

The chemical formulas of cavansite and pentagonite are the same: Ca(VO)Si4O10 • 4H2O (hydrated calcium vanadium silicate). Why, then, do they have different names?

Two Form Crystallization


Mineralogists call cavansite and pentagonite “dimorphs”, which literally means “two forms”. Both cavansite and pentagonite crystallize in the orthorhombic crystal system. However, cavansite crystallizes in the orthorhombic dipyramidal class and pentagonite crystallizes in the orthorhombic pyramidal class. (Crystal systems are divided into classes. If you are really interested in crystallography, you can look up crystal classes on the internet or in a good mineralogy book.)

Cavansite and pentagonite crystals are very small and almost always grow in groups, often groups that look like small balls. Because their crystals are about the same size, and because they are both a bright, electric blue, it was assumed that all bright, electric-blue crystal groups were cavansite. A close-up examination of different “cavansite” specimens, however, revealed that there is actually a difference in the crystallography.

New Class for Pentagonite

In 1973, the name pentagonite was given to the orthorhombic pyramidal class in order to show that it is actually different from the very similar mineral cavansite. Calcite and aragonite are both CaCO3 (calcium carbonate). Calcite crystallizes in the trigonal system.
Aragonite crystallizes in the orthorhombic system. Calcite and aragonite are dimorphs.

Polymorphism refers to four or more mineral species that have the same chemical formula. Silicon dioxide (SiO2) is a great example. Quartz crystallizes in the trigonal system, tridymite crystallizes in the orthorhombic system, cristobalite crystallizes in the tetragonal system, coesite crystallizes in the monoclinic system, and stishovite crystallizes in the tetragonal system—and they all have the same chemical formula!


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