Amethysts

Silicon makes up 25.7% of the earth s crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals. 

Quartz. When colorless, quart2 [14808-60-7] is also known as rock crystal. When irradiated, it becomes smoky from a color center associated with a ubiquitous Al impurity at about the 0.01% level. The name citrine [14832-92-9] is used when quart2 is colored by Fe, and irradiation of this can produce the purple-colored amethyst [14832-91-8] under certain circumstances (2). Although not signiftcandy lower priced than the natural materials, synthetic citrine and amethyst ate used in jewelry because of the abiUty to provide matched sets of stones from large, up to 7-kg, hydrothermaHy grown crystals. 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The name amethyrin refers to the fact that the dull-purple color of the protonated form of the macrocycle is that of amethyst stones, Formal oxidation or reduction products are the aromatic [22]hexaphyrin(l.0.0.1.0.0) or the [26]hexaphyrin(l.0.0,1.0.0), respectively. However, none of these products could be observed either as reaction products or as direct products of oxidation or reduction reactions. 

FIGURE 14.36 Impure forms of silica amethyst (left), in which the color is due to I e impurities agate (center) and onyx (right). 

The oxidation is slow in acidic solution but rapid in basic solution, where insoluble iron(III) hydroxide, Fe(OH)3, is precipitated. Although [Fe(H20)6]3+ ions are pale purple and Fe3 1 ions give amethyst its purple color, the colors of aqueous solutions of iron(III) salts are dominated by the conjugate base of [Fe(H20)g]3+, the yellow [Fe0H(H20)d2+ ion ... 

Quartz is the most abundant gangue mineral. It occurs commonly in Au-Ag and Pb-Zn deposits but is scarce in Cu deposits. Chalcedonic quartz coexisting with Au-Ag minerals occurs abundantly in Au-Ag deposits. Amethyst is generally rare and occurs as a late-stage mineral in Au-Ag and Pb-Zn deposits. 

Amethyst Vitreous Transparent or translucent Purple Banded or uneven... 

Tyrian purple was derived from the "purple snail," the common name for what, in reality, are several species of mollusks of the genus Murex. Each one of the mollusk species yielded a slightly different variety of purple. In Tyre, where the most prized purple dye was produced, Murex brandaris snails were those most abundant and generally used, while in Sidon, not far to the north of Tyre, an amethyst purple variety of the dye was obtained from... 

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of SK>2 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon J l... 

The Silicate Class. Amethyst Galleries, Inc., http //mineral.galleries.com/minerals/silicate/class.htm... 

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