Minerals amethyst

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 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. 

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... 

We then equilibrate the formation fluid, using data from Table 30.1. Since pH measurements from saline solutions are not reliable, we assume that pH in the reservoir is controlled by equilibrium with the most saturated carbonate mineral, which turns out to be witherite (BaCC U) or, for the Amethyst field, strontianite (SrC03). Using the Miller analysis, the procedure for completing the calculation is... 

Fig. 30.1. Volumes of minerals precipitated during a reaction model simulating the mixing at reservoir temperature of seawater into formation fluids from the Miller, Forties, and Amethyst oil fields in the North Sea. The reservoir temperatures and compositions of the formation fluids are given in Table 30.1. The initial extent of the system in each case is 1 kg of solvent water. Not shown for the Amethyst results are small volumes of strontianite, barite, and dolomite that form during mixing.

The Amethyst fluid is richer in strontium and calcium than the other fluids, but nearly depleted in barium. Celestite becomes saturated first, and more of this mineral forms from this fluid than from the Forties fluid. Anhydrite becomes saturated later in the mixing process and precipitates,... 

In the three simulations, the sulfate minerals form at mixing ratios related to their solubilities. Barite, the least soluble, forms early, when small amounts of seawater are added. The more soluble celestite forms only after the addition of somewhat larger quantities of seawater. Anhydrite, the most soluble of the minerals, forms from the Amethyst fluid at still higher ratios of seawater to formation fluid. 

Si02 is one of the most abundant compounds in nature. It forms a number of minerals and several varieties whose names are quite familiar agate, car-nelian, sard, amethyst, chalcedony, flint, and chert. All are composed of Si02 with only small or trace amounts of other elements or compounds included during crystallization. In many cases it is the additional components that impart the peculiar color, optical, or physical properties to these minerals. The names are familiar because since ancient times these minerals were used or commonly set in jewelry. All of the names are varieties of the mineral quartz, the stable form of SiOj, formed at ordinary temperatures and pressures. Si02 also forms several polymorphs, which are mentioned below. 

Quartz is found in several forms in all three major kinds of rocks—igneous, metamorphic, and sedimentary. It is one of the hardest minerals known. Geologist often divides quartz into two main groupings—course crystalline and cryptocrystalline quartz. Course crystalline quartz includes six-sided quartz crystals and massive granular clumps. Some colored varieties of coarse crystalline quartz crystals, amethyst and citrine, are cut into gem stones. 

Balitsky, V. S. Balitskaya, O. V. (1986) The amethyst-citrine dichromatism in quartz and its origin. Phys. Chem. Minerals, 13, 415—21. 

Cohen, A. J. Hassan, F. (1974) Ferrous and ferric ions in synthetic alpha-quartz and natural amethyst. Amer. Mineral., 59, 719-28... 

Lehmann, G. (1975) On the color centers of iron in amethyst and synthetic quartz a discussion. Amer. Mineral., 60, 335-7. 

One of the commonest forms of hole-excess center imparts color to the minerals smoky quartz and amethyst. These minerals are forms of silica, containing aluminium as an impurity. The AP+ substitutes for 81 +, and to preserve charge neutrality equal amounts of H+ are incorporated into the crystal. The smoky purple color arises in the electron deficient [A104] group. It is formed when an electron is liberated from a neutral [A104] group by ionising radiation is trapped on one of the H+ ions present. Other hole centers have been described in a variety of crystals. 

Amethyst Galleries. 1995. The mineral tyuyamunite. Amethyst Galleries, Inc. httD //mineral.galIeries.com/minerals/DhosDhat/tvuvamun/tvuvamun.htm. 

McLaren, A. C., Pitkethly, D. R. (1982). The twinning microstructure and growth of amethyst quartz. Phys. Chem. Minerals, 8, 128-35. 

Gemstones Beryllium is also associated with gemstones. A gemstone is a mineral that can be cut and polished for use in jewelry. Some typical gemstones are jade, sapphire, diamond, ruby, amethyst, emerald, spinel, moonstone, topaz, aquamarine, opal, and turquoise. Gemstones are often used as birthstones, which honor the month in which a person is born. (For instance, the birthstone for April is a diamond.)... 

Silicon is the second most abundant element in Earth s crust, exceeded only by oxygen. Many rocks and minerals contain silicon. Examples include sand, quartz, clays, flint, amethyst, opal, mica, feldspar, garnet, tourmaline, asbestos, talc, zircon, emerald, and aquamarine. Silicon never occurs as a free element. It is always combined with one or more other elements as a compound. 

About 400 varietal names that have been in use for centuries and are still current today (e. g. amethyst, ruby, sapphire, emerald) have been included in the text. Unfortunately it was felt necessary to retain about 300 names of poorly defined or inadequately characterized minerals it is hoped that my professional colleagues will assist in cleansing the classification by providing more complete data or by completely discrediting such minerals. 

The composition of pure quartz is close to 100% pure Si02 because the structure of the mineral is so compact and perfect that there is no room for silica replacement by any other element. Also, quartz is insoluble in all acids except HF, which further contributes to its purity. Quartz forms many micro- and cryptocrystalline varieties. Some of them are well-known as semiprecious stones (amethyst, citrine, agate, tiger-eye, etc.). 

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