Rhodochrosite (#13)

Cluster of lustrous gemmy scalenohedrons.
Africa N'Chwaning II mine, near Kuruman, Cape Province, South Africa 1 ¼" x ¾" x ¾"

 

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The name rhodochrosite comes from two Greek words meaning "rose colored." When the color of a mineral is so distinctive that virtually all specimens have a similar color, the mineral is called idiochromatic, meaning "self-colored." In other words, the color is caused by some element that is essential to the composition of the mineral, rather to an impurity. Most such elements belong to the transition metals: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, although Sc and Zn do not cause color. If the color of a mineral is caused by a minor impurity, it is called allochromalic. Rhodochrosite is idiochromatic - pale pink to deep rose red like this specimen, all caused by manganese, Mn.

The chemical formula for rhodochrosite is MnCO3, so we would classify the mineral as a carbonate. The most common of the carbonate minerals is calcite, CaC03. Rhodochrosite and calcite are isostructural - that is, they have the same crystal structure. Other minerals of this same family are smithsonite (ZnCO3), siderite (FeC03), and magnesite (MgCO3), and there are a few much less common natural members of the group. If you were to see a drawing of the crystal structure of any one, it would be indistinguishable from that of the others. Because the metal atoms (Ca, Zn, Fe, etc.) have slightly different sizes, you would see differences in the distances between atoms, but these would be too small to distinguish on a drawing.

So, how big are atoms? We express their sizes in A (Angstroms) or nm (nanometers). One Angstrom is 1 o.a centimeters, or 0.00000001 cm. A nanometer is ten times that - a 1 preceded by 6 zeros. Either way, it is incredibly small. We measure the distances between atoms in several ways, but the most common is by X-ray diffraction - a technique commonly used in many geology, chemistry, and physics departments in academia and industry.