Silica History & General Information

For any mineral or group of minerals there are two general classes of facts to be ascertained: (1) the geometrical, physical, and chemical properties; (2) the mode of occurrence, association, and origin, or briefly what may be called the natural history of the mineral.
To the geologist, of course, the natural history of a mineral is its most attractive feature, but until the mineral is accurately defined and determined, the significance of its occurrence and origin may be lost sight of.
It is my purpose to discuss the various silica minerals. The silica minerals are with a single exception accurately defined; they have been produced in the laboratory; they constitute the most complete example of polymorphism known. They are, moreover, of considerable geological interest, and altogether we have a fairly good idea of the role that the silica minerals play in Nature.
Let us first consider briefly the silica minerals and their properties as a background for the discussion of their natural history.
The name silicon comes from the Latin word silicis which means flint.
Silicon is the second most common element in the Earth's crust, comprising 25.7% of the Earth’s crust by weight. It was discovered in 1824 by the Swedish chemist Jons Jakob Berzelius. It is shiny, dark gray with a tint of blue. Silicon, atomic number of 14, is a semi-metallic or metalloid, because it has several of the metallic characteristics. Silicon is never found in its natural state, but rather in combination with oxygen as a silicate ion (SiO4) in silica-rich rocks such as obsidian, granite, diorite, and sandstone. Feldspar and quartz are the most significant silicate minerals. Silicon alloys with a variety of metals, including iron, aluminum, copper, nickel, manganese and ferrochromium.
Silica is processed into two intermediate products- silicon and ferrosilicon. Silicon is known in the ferroalloy and chemical industries as “silicon metal.” The ultra pure form of silicon (>99.99% Si) is distinguished from silicon metal by the term “semiconductor-grade silicon.” The terms “silicon metal” and “silicon” are used interchangeably.
Silicon is used in ceramics and in making glass. Ferrosilicon is crushed into a variety of forms and sold as bulk metal. Depending on its intended use, it can be mixed with aluminum and calcium. It is a very heavy alloy. When it comes into contact with moist air or water, an explosive chemical reaction occurs in which hydrogen is released. Consequently there are very strict laws about the shipping of ferrosilicon it must be kept perfectly clean and dry.

<= A quartz crystal.
Silicon is considered a semiconductor. This means that it conducts electricity, but not as well as a metal such as copper or silver. This physical property makes silicon an important commodity in the computer manufacturing business.
Silica is in human connective tissues, bones, teeth, skin, eyes, glands and organs. It is a major constituent of collagen which helps keep our skin elastic, and it helps calcium in maintaining bone strength. Silica dust in mines has caused silicosis or a lung disease in miners. Wetting the area being mined and application of good ventilation has reduced the danger of lung disease. Some organisms like sponges and some plants use silicon to create structural support.

Chalcedony The microcrystalline form of silica known as chalcedony is very much like law quartz in its properties, but seems to be somewhat different from law quartz. It does not appear to grade into quartz, and may easily be distinguished from it in practically all cases.
Whether chalcedony is a distinct form of silica or simply a variety of quartz is uncertain.
A reasonable explanation is that chalcedony is an aggregate of silica fibers which are oriented in one direction only. In quartz the fibers are oriented in all directions. This assumes the thread structure advocated by Sosman.3

Opal Opal is a hydrogel mineral of colloidal origin. In some specimens we have evidence of its origin by desiccation of a gelatinous mass of silica. Opal is probably a solid solution of water in silica. When first formed it consists of two phases, silica and water, but in time the water gradually diffuses into the silica and a solid solution is formed. Opal is not, then strictly speaking, a colloid, but only colloidal in origin.
Opal is apparently one of the few strictly amorphous minerals, since it gives no X-ray diffraction pattern.

Lechatelierite The latest of the silica minerals to be described is lechateliérite, named by Lacroix in 1915 in honor of Henry Le Chatelier, the famous French chemist, whose work on silica and silicates is well known. Lechateliérite is naturally occurring silica glass. It has not been generally recognized as a distinct mineral.
In its properties lechateliérite is much like opal, but in origin it is absolutely different. As Miers has well said: "The essential characters of a mineral, moreover, are quite independent of its source or previous history . . . . ." This statement is important for crystalline minerals, but is probably not valid for amorphous substances such as opal and lechateliérite. At any rate, it is convenient to treat these two as distinct minerals. About a century ago Breithaupt distinguished two classes of amorphous substances, the hyaline or glasses and the porodine or those of colloidal origin. It is unfortunate that the term hyalite has been used as a variety of opal.
Lechateliérite is unique in that it is the only naturally occurring glass that is definite enough to be considered a mineral.
Silica glass, which is often incorrectly called "quartz glass", is now made on a commercial scale in a very pure transparent form. Silica glass possesses some unusual physical properties such as transparency to ultraviolet light and a remarkably small coefficient of thermal expansion. A rod of the glass 1 meter in length will expand only about 0.6 mm. for a rise of temperature of 1000°C.
The Silica Minerals In General
All the known forms of silica, and they are legion in number, are believed to be synonyms or varieties of the ones mentioned. Asmanite, found in meteorites, is a synonym of tridymite. Lussatite, considered by some to be distinct form, is probably a mixture of opal and fibrous chalcedony.
It seems probable that all the forms of silica capable of existence at atmospheric pressures are now known. But it is not safe to say that all the possible distinct forms of silica are known, for polymorphism is a general phenomenon of nature and there is no theoretical limit to the number of polymorphous modifications of any chemical substance.
The artificial production of the various forms of silica under specified conditions in the laboratory has been a great help in the proper understanding of their natural history.
Let us next consider the typical modes of occurrence of the various silica minerals with a view of determining, as far as possible, the conditions under which they were formed in nature.

Occurrence Of Opal Opal is a late secondary mineral found in seams and cavities of various rocks, especially of volcanic igneous rocks. It is found in sedimentary beds which are in part at least formed from diatoms, radiolaria, and sponge spicules.
It seems plausible to regard opal as a mineral which has formed rapidly. Our information concerning the formation of minerals of colloidal origin is rather meager.

Occurrence Of Low Quartz Low quartz is a typical hydrothermal mineral which is probably always formed below 575°C. Its temperature range is considerable. Its occurrence as secondary enlargements of sand grains in sandstones as well as numerous occurrences in sedimentary rocks proves that it may be formed at temperatures at little above the ordinary. Vein quartz usually has a prismatic habit.

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