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Rock that is heated to melting in meteor impacts and lava usually form crystals on cooling. However if cooling is rapid enough glass is formed.
While the word Glass is sometimes used to refer to any amorphous solid, the word is most specifically used to refer to a family of amorphous solids typically using tetravalent silica as the network-forming connection. Glass is a strong, but brittle, material.
Transparent forms of glass are the most commonly used, however, not all glass is transparent. Glasses may be colored by any number of different impurities. Iron ions are the most common form of impurity that produces dark glass; glass containing iron will be yellow, brown, or black depending on the iron content. (For example, obsidian, a naturally-formed volcanic glass, is a shiny opaque black; the color is from the iron content). Other transition metals can give glass bright colors.
Silica or silicate is the primary constituent of glass, which is abundant on the Moon. While a form of glass can be made from pure silica, the very high melting temperature of silica makes high-silica glass difficult to work with, and almost all commonly used glass contains other oxides. On Earth, sodium oxide is usually added to form low-cost soda-lime glass (commonly called window glass); boron oxides may then added to form the higher-cost borosilicate glass, which has a lower coefficient of thermal expansion and hence may be used in applications involving heating and cooling. Many other specialized glass formulations exist. The scarcity of water on the moon makes anhydrous glass practical. This type is considerably stronger than typical glass on Earth which is exposed to moisture.
Glass by itself is not commonly used as a structural material because it is brittle. One solution to this problem is to use glass fiber in a glass composite material. One form of glass fiber is called fiberglass. A composite material requires a matrix to hold the fibers together. In terrestrial glass-fiber applications, this matrix is usually an organic material, typically epoxy or polyester resin; the resulting structural material is sometimes called fiber-reinforced polymer (FRP) or glass-reinforced plastic (GRP). Metal-matrix composites and even glass-matrix composites, do exist, but are little used on Earth.
Glass fibers are also used to transmit light in the form of light-pipes and fiber-optics.
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