Mineral and Aquamarine

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  • Topic: Mineral, Gemstone, Beryl
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  • Published : October 12, 2008
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Aquamarine is an earth mineral in the beryl family. The word aquamarine is derived from latin and literally means water of the sea, or seawater. It was named by the Romans over 2000 years ago for its color which is varying shades of blue-green, like that of the ocean. Aquamarine is the birthstone of march and is said to be a good luck charm for scorpios.

Beryl Basics 1:

Aquamarine is a member of the beryl family. So I’ll start by giving you some basic information on beryls. A beryl is a class of earth mineral with the basic formula Be3Al2(Si6O18). Its chemical name is beryllium aluminum silicate. A few commonly known beryls are emeralds, heliodors and of course aquamarines.

Aquamarine Basics:
As I said before, an aquamarine crystal is a member of the beryl family so it has the same basic chemical formula as beryl but differs from other beryls in that it also contains iron as an impurity. Specifically, this iron is in a 2+ oxidation state. This Fe2+ cation causes aquamarine to have the blue-green appearance that it does. A pure beryl is clear and colorless so this proves that this impurity is the only factor determining the color. Heliodor, another beryl also contains Iron as the sole impurity but because the iron is in a different oxidation state, the 3+ oxidation state, it appears yellow. The unpaired d-electrons of the Fe2+ have an energy level that corresponds to this blue green color but those of Fe3+ have a different energy level that corresponds to the yellowish color of heliodor. This all relates back to the relationship between d-electron energy level and the absorbance wavelength that we learned about when studying coordination compounds such as those used as pigments. The multi-colored stone seen on the slide is an example of a beryl that is a mixture of both aquamarine and heliodor and contains a mixture of iron in its 2+ and 3+ oxidation states. In order to convert the stone to aquamarine, the gem can be heated. This causes the Fe3+ to convert to Fe2+ and the color of the stone will turn totally blue. Heat treatment is done to many gems to alter the color. Most aquamarines are treated in this way to bring them to the desired shade of blue.

Aquamarine Crystallography 1:
Aquamarine, is a silicate. The silicates are by far the largest group of minerals. Minerals composed of molecules containing varying amounts of silicon and oxygen and one or more metals are silicates. The construction component of all silicates is the tetrahedron, which is one oxygen atom equidimensionally placed around four silicon atoms. These tetrahedrons, when combined with other molecules or tetrahedrons form the silicate minerals.

The types of silicates are classified based on the interactive formation of the tetrahedrons. Each group of silicates has a set ratio of silicon and oxygen, although in some minerals the silicon atoms are replaced by other atoms.

Aquamarine Crystallography 2:
Aquamarine can be further classified as a cyclosilicate. A cyclosilicate is one of a group of silicate minerals that have their tetrahedrons linked into rings. Each silicon atom is bound by two oxygen atoms that are part of another tetrahedron. Each ring consists of three, four, or six linked tetrahedrons. In the case of aquamarine, its six linked tetrahedrons.

Aquamarine Crystallography 3:

Aquamarine has a hexagonal crystallography and has a strong tendency to be prismatic. This slide showcases an aquamarine’s crystal that exhibits both of these traits. Since the mineral is hexagonal, it has four axes, three of which are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners. In the image shown on the slide, the fourth axis is longer than the others but this need not be so, it could very well have been shorter.

Aquamarine Properties 1:
This slide outlines some of the basic...
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