Dimorphism, Trimorphism, Solid Solutions, Gel or Colloidal Minerals

A number of cases are well known among minerals in which two or three different species have the same chemical composition but distinctly different physical properties. When one compound appears in two different forms, it is said to be dimorphous; when in three different forms, trimorphous. Carbon in the forms of graphite and diamond calcium carbonate as calcite and aragonite, iron sulphide as pyrite and marcasite, are familiar examples of dimorphism. The two minerals in each case differ from each other in such physical properties as crystallization, hardness, specific gravity, color reaction with acids, etc. Titanium oxide, TiO2, is trimorphous, since it occurs in the three distinct minerals, rutile, octahedrite and brookite. These instances furnish proof of the statement that the physical characters of a mineral depend not only upon its chemical composition but upon its crystalline structure as well.

Solid solution in mineral
In certain minerals small variations in composition occur that cannot be accounted for by the presence of impurities or by the isomorphous replacement of one element or radical by another similar element or radical. These extraordinary cases have been explained by the assumption that the mineral on crystallizing has formed a solid homogeneous solution with a small amount of foreign material. The relationship between a dissolved salt and its solvent. Examples of solid solution in minerals are furnished by nephilite, Na AlSiO, the analyses of which always show a varying excess of silica, and by pyrhotite, FeS, whose analyses show a small and varying excess of sulfur present.

Gel Mineral or Colloidal Mineral
There are a number of mineral substances whose analyses do not yield definitive chemical formula and further show no signs of a crystal structure. These are thought to be solid phases of colloidal material and have been called “gel-minerals.” Minerals may exist in a crystalloid phase with a definitive composition and molecular structure or, formed under different conditions, practically the same substance may occur as mineral gel. The mineral gels are formed under conditions of low pressure and temperature and are commonly substances of secondary origin formed during the process of weathering of the materials of the earth’s crust. They characteristically occur in mammillary, botryoidal, stalactitic and similar forms. The power of these minerals to adsorb other substances to a considerable extent accounts for their often wide variations in chemical composition. Limonite and opal are familiar examples of gel mineral.