Minerals agate

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. 

Put a small amount of your solid into a tiny agate mortar and add a few drops of mineral oil. 

Occurrence. Silicon is the second most abundant element in the earth s crust, after oxygen (about 28% by weight). It occurs mainly in oxides (quartz, agate, opal, etc.) and a great variety of silicate minerals (feldspar, clay, mica, olivine, etc.). 

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. 

A variety of mineral-type materials are inorganic polymers [Ray, 1978]. Silica [(SiCLL] is found in nature in various crystalline forms, including sand, quartz, and agate. The various crystalline forms of silica consist of three-dimensional, highly crosslinked polymer chains composed of Si04 tetrahedra where each oxygen atom is bonded to two silicon atoms and each silicon atoms is bonded to four oxygen atoms. Silicates, found in most clay, rocks, and... 

Section 1.5.5). Quartz is one of the commonest minerals in the earth, occurring as sand on the seashore, as a constituent in granite and flint and, in less pure form, as agate and opal. The silicon atom in all these structures is tetrahedrally coordinated. 

Ooeumnae.—In the pure state in many minerals, as quarts, agate, < tc. 

Numerous processes have been proposed for extracting potash from felspar, leucite, alunite, and other minerals rich in this substance, but the cost is so great that very few proposals yet made ofier promise of successful competition with the Stassfurt deposits. This is even the case with alunite, where mere calcination to 1000° drives off water and sulphuric acid, leaving water-soluble potassium sulphate, and alumina. Humphry Davy in his paper On Some Chemical Agencies of Electricity (1807), indicated in Cap. Ill, found that when water was electrolyzed in cavities contained in celestine, fluorspar, zeolite, lepidolite, basalt, vitreous lava, agate, or glass, the bases separated from the acid and accumulated about the cathode. It is therefore probable that if water with finely divided potash minerals in suspension were electrolyzed, the alkali would be separated in a convenient simple way. 

Next to materials of the glass-ceramics type, many varieties of chalcedony, such as agate, carneol, onyx, sardonyx, heliotrope and jasper, exhibit similar changes in hardness resulting from different consolidation of the cryptocrystalline structure of silica among mineral individuals. 

As agate is an impure variety of quartz it has the same physical properties as llial mineral It is named from the river Achates in Sicily where it has been known from the time of Theophrastus. 

Onyx is a striped variety of the common silicate mineral called agate, with alternating black and white bands. It comes mainly from India and South America. Sardonyx is a variety of onyx with brown and white bands. 

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.). 

Another classical sampling technique for solids is the mineral oil mull preparation (18). In this technique, a small amount of sample ( 1 mg) is placed into an agate mortar. To this, a small amount of mineral oil is added and the sample and oil mixed to an even consistency. The mixture is then placed onto an IR optical window and sampled by the IR spectrophotometer. One advantage to this tech-... 

Occurrence.—In tho pure atato in many minerals, os quarts, agate, c. 

Although methods are available for synthesizing silicon dioxide, there is no practical reason for doing so. The abundant quantities of silicon dioxide found in the earth s crust are sufficient to satisfy all industrial needs. Among the minerals and earths that contain silicon dioxide in an uncombined form are quartz, flint, diatomite, stishovite, agate, amethyst, chalcedony, cristobalite, and tridymite. 

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