Limonite is iron oxide, with 3 molecules of water of crystallization (or constitution) associated with every 2 molecules of the oxide. If limonite is moderately heated the water is driven out and the resulting compound is hematite, the same oxide, but without the water. In this case and many other similar cases, as gypsum, opal, etc., we have two or more minerals resulting from the presence or absence of water in the mineral. The water molecules have a definite place in the arrangement of molecules which determines the structure of the mineral. Sometimes the water is driven out at a temperature around 212 F., in which case it is called, water of crystallization, but in other cases as gypsum, a considerably higher temperature is required to drive out the water, and then it is called, water of constitution. In all cases the removal of the water changes the arrangement of molecules and a new mineral results, with characteristics of its own. In this case limonite is only one of a series of minerals which have the Fe₂O₃ molecule as a basis, and that incorporate more or less water into their molecular construction as follows: Turgite 2Fe₂O₃·H₂O Goethite Fe₂O₃·H₂O Limonite 2Fe₂O₃·3H₂O Xanthosiderite Fe₂O₃·2H₂O Limonite Fe₂O₃·3H₂O Of these goethite is crystalline, the others non-crystalline. They may occur pure or in all sorts of mixtures, the mixtures usually being lumped under limonite. The limonite is far the commonest of the series, goethite is fairly common, but the others are rare as pure minerals. Limonite is found in all parts of all states and in every country. Though so common, it is by no means an important source of iron today, only about one percent of the iron mined in this country coming from this source, though in Germany, Sweden and Scotland it is relatively much more important. Goethite Fe₂O₃·H₂O Pl. 12 Occurs in lustrous brown to black orthorhombic prisms, usually terminated by low pyramids; hardness 5; specific gravity 4; color brown to black; streak brownish-yellow; luster imperfect adamantine; opaque. Goethite, named for the poet Goethe, who was interested in mineralogy, is much less abundant than limonite or hematite, but occurs with them, when they are in veins. Its usual form is an orthorhombic prism with the edges beveled, and a low pyramid on either end. The crystals usually grow in clusters, making a fibrous mass, often radiated, in which case it is known as “needle iron stone”; or the prisms may be so short as to be almost scales; when, because of the yellowish-red color, it is called “ruby mica”. It is found in many states, including Connecticut, Michigan, Colorado, etc. Hematite Fe₂O₃ Pl. 13 & 14 _specular iron_