OpalOpal 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.

High quartz High quartz is the original mineral of persilicic igneous rocks present because there is an excess of silica left over after the affinities of silica for the various oxides are satisfied. At atmospheric pressure under laboratory conditions it is formed above 575°C. But in nature it has probably been formed at lower temperatures because of the influence of mineralizers. 575°C.As a point on the geological thermometer cannot be accepted too literally.
High quartz usually shows a bipyramidal habit due to the minor development of the prism faces.

Transformations of the Silica Minerals When the silica minerals are studied in the laboratory, there are found to be two types of inversion:
1) Inversions of the law and high forms of quartz, tridymite, and cristobalite respectively, which take place rapidly.
2) Inversions of quartz, tridymite, and cristobalite from one into another. - These inversions are sluggish, and so are of geological importance.
The inversion temperatures of the various polymorphous forms of silica have been determined under laboratory conditions by the Geophysical Laboratory.
These laboratory experiments are of course extremely important, but the mineralogist and petrographers are primarily interested in the natural occurrences and transformations.
I next wish to present the results of my study of some of the natural transformations of the silica minerals.

Average Composition of the Crust

Eight elements combine to make most minerals and rocks found in the Earth's crust. The elements are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The letters in the parentheses are abbreviations. You may already be somewhat familiar with some forms of these elements. Silicon combines with two atoms of oxygen (in the nomenclature of chemist, SiO₂, or silica) to make the mineral quartz, which is common on many mainland beaches. Silica is also used for window glass. Aluminum is used for soda cans. Iron is used for the frame of automobiles and reacts with oxygen in the air to produce rust. Magnesium is used in flares. Calcium is a major component in our teeth and bones. Sodium is in table salt. Potassium is used in fertilizer. Titanium is combined with other metals to produce alloys, which are stronger metals.

This definition excludes the thousands of compounds invented by humans in laboratories because these compounds are not naturally occurring. Compounds that are found in only plants or animals are also excluded. Liquids are excluded because they are not crystalline; their atoms are free to move. Minerals can be a single element, like diamond, which is made of carbon of compounds of two or more elements, like quartz, which contains one silicon and two oxygen atoms. Definite composition indicates that a chemical analysis of a given mineral will always produce the same ratio of elements. For example, quartz will always have one silicon for every two oxygen atoms. Therefore, minerals can be expressed by chemical formulas, such as SiO₂ for quartz.

Crystalline Silica

Industrial sand, also called silica sand, contains a very high percentage of silica, in the form of quartz, and is distinguished by its high degree of purity. Industrial sand deposits are thus less common than deposits mined for construction sand and gravel. Industrial sand is derived from hard rock quartzite formations and in unconsolidated beds of silica sand.
Industrial sand must meet stringent quality requirements since it is used as the principal ingredient in the manufacture of glass, and in foundry cores and molds for metal castings. Industrial sand also is an ingredient in paints, refractory products and specialty fillers. It is used in water filtration, abrasives, for enhancing production of oil and gas, and in specialty construction applications.
Industrial sand also satisfies recreational needs, such as golf courses, tennis courts and ball fields. It is used in municipal water filtration, in residential pool filters and sand boxes. Because of the ubiquitous occurrence and many commercial uses of silica, potential exposure to silica dust is widespread.

Occupational Exposure Occupational exposure to crystalline silica occurs by breathing silica-containing dusts present in many industries, such as: mining and quarrying; steel, iron, and other metal foundries; abrasive blasting; construction; glass and ceramics; paint and pigments; and, quarrying and crushing stone.