VITRAIN (from the Latin word "vitrum", which means "glass" * EN: vitrain; DE: Vitrit, Vitrain; FR: vitrain; ES: vitrain; RU: витрен) is one of the main constituent parts of the fossil coals, which is having the strong lustre, the conchoidal and semi-conchoidal fracture, or the smoothed fracture. Vitrain is formed during the changing of the lignin-cellulose tissues of the plants, as the result of the decomposition under the conditions of the water-flooded peat bogs, in case of the insufficient access of oxygen. Vitrain exists within the coals in the form of the lenses or strips of the different thicknesses. Vitrain is the least "ash content" constituent part of the coal. Vitrain of the coals, which differ according to the degree of the metamorphism, has the different chemical composition. With the transition from the brown coals, to the hard coals, and further to the anthracites, there is increased within the coals the content of carbon, there are decreased the contents of hydrogen and oxygen, and there is decreased the yield of the volatile substances. They distinguish the structureless vitrain, namely, the homogeneous gelified fragment with the sharp contours, without the indications of the cellular structure of the vegetable tissues, and the structural vitrain, namely, the gelified fragment, which has has preserved the outlines and traces of the cellular structure of the vegetable tissues. According to the system of the Geological Institute of the Academy of Sciences of the USSR, both varieties of vitrain belong to the microcomponents of the group of telinite. There has introduced the term "vitrain" during the 1919 the English scientist Marie Stopes.