Quartz amethyst

Silicon dioxide. Because silicon dioxide is so readily available in nature, there is only infrequent need to produce this compound in either the laboratory or the plant. Many familiar materials consist of silicon dioxide (or silica) in varying degrees of purity. Thus, flint, agate, amethyst, quartz, onyx, opal, granite, petrified wood, sand, and sandstone are all materials that consist entirely or largely of silica. You may recognize that all these materials have a variety of common uses and should recall, from preceding discussions, cases in which silicon dioxide is used in certain commercial chemical processes. 

Figure 8.2. a-fringe patterns due to Brazil twin boundaries in amethyst quartz. BF image g = 10Tl. Note the reversal of contrast at successive boundaries. (From McLaren and Phakey 1966.)... 

Figure 8.2 shows an array of Brazil twin boundaries parallel to one of the major rhombohedral planes, say (TiOl), in amethyst quartz, imaged... 

Dauphine twin boundaries are sometimes parallel to low-index planes, but very often there appears to be no crystallographic control. This was certainly the case for the Dauphine twin boundaries observed between the Brazil twin boundaries in amethyst quartz by McLaren and Riakey (1969). 

In addition to the black spot contrast, most crystals of neutron-irradiated amethyst quartz showed black lines, which were subsequently identified as the tracks of particles produced by the fission of uranium impurity atoms (see Section 8.13.3),... 

The rate at which electron radiation damage occurred in neutron-irradiated specimens also varied from one variety of quartz to another. Whereas synthetic quartz (dose Dq) became amorphous after a few seconds of exposure to a focused electron beam, amethyst quartz (dose < Do) was more stable in the electron beam than the sample before neutron irradiation. 

Hgure 8.59. Hssion-fragment tracks in neutron-irradiated amethyst quartz. Note the double image of the track Afi and the long-range strain effects in the matrix at C. (From Phakey 1967.)... 

Electron microscope study of Brazil twin boundaries in amethyst quartz. phys. stat. sol., 13, 413-22. 

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

Beithelot M (1907) Synthesis of amethyst quartz Researches on the color, natural or artificial, of some precious stones under radioactive influences. Compt rend 143 477-488... 

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. 

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. 

In addition to silica (silicon dioxide SiO ), the crystal form of silicon is found in several semiprecious gemstones, including amethyst, opal, agate, and jasper, as well as quartz of varying colors. A characteristic of quartz is its piezoelectric effect. This effect occurs when the quartz crystal is compressed, producing a weak electrical charge. Just the opposite occurs when electric vibrations are fed to the crystal. These vibrations are then duphcated in the crystal. Quartz crystals are excellent timekeeping devices because of this particular characteristic. 

Silicon dioxide (SiO ) is the most abundant compound in the Earths crust. Known as common sand, it also exists in the forms of quartz, rock crystal, amethyst, agate, flint, jasper, and opal. It has many industrial uses. 

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. 

Sihca, the other most important class of sihcon compounds, exists as sand, quartz, flint, amethyst, agate, opal, jasper, and rock crystal. It is discussed separately under Sihcon Dioxide. Silicates and silica have many applications... 

Quartz occurs in granite, sand, crystals, and sandstone. Quartz also has several crystalline varieties such as purple amethyst, colorless rock crystal, and yellow citrine. Flint, agate, and chert, etc. are other forms of quartz. Quartz is an excellent insulator. It does not break under temperature changes because of its low coefficient of expansion. Fused quartz transmits ultraviolet light. 

Another interesting natural example of colour centres lies in the colour of smoky quartz and amethyst. These semi-precious stones are basically crystals of silica, Si02, with some impurities present. In the case of smoky quartz, the silica contains a little aluminium impurity. The substitutes for the Si " in the lattice, and the electrical... 

Various names, such as amethyst, citrine, and smolqr or black quartz, have been used for colored rock-crystal, whereas for the cryptocrystalline aggregate of quartz, names such as chalcedony and jasper are used. Agate and cornelian, for example, are types of chalcedony that have specific textures or colors. In this chapter, we analyze how a variety of morphologies of high-temperature and low-temperature quartz appear, and how textures of polycrystalline aggregate seen in agate and other crystals are formed. 

Large short-prismatic quartz crystals develop toward the center of a void. In many cases, these crystals are amethyst, in which Brazil twins and Brewster fringes are universally observed. 

The destruction of color centers (1,3) by heating can result in bleaching or fading. Examples are brown or blue topaz, red tourmaline, smoky quartz, and some yellow sapphire. In other instances there may be a color change as when amethyst turns into yellow citrine, or when the heating of a brown topaz reveals the presence of a previously hidden Cr-derived color in a pinked topaz. These changes can usually be reversed by an irradiation treatment. 

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