MINERALOGY. - The science of minerals. Its principal task was originally to describe and identify them; today it tends above all toward a better understanding of their chemical-physical structure, their properties, and their origin.
I. MINERALS
From the Celtic meine, crude metal. A designation that indicates the natural unitary constituent parts of the lithosphere. Simple bodies, solid (crystallized) or liquid (mercury) inorganic compounds, isomorphic mixtures of two or more elements or compounds, certain solid organic substances (amber, succinite, fichtelite, melite, etc.) and certain amorphous substances (opal p.p., collophane) are considered minerals.
Fossil coals and petroleum as such are mixtures and therefore are to be counted among rocks. Volcanic glasses, since at equilibrium reached (devitrification, crystallization) they normally give rise to aggregates of different minerals, are habitually considered rocks. Meteorites can be regarded as aggregates of minerals from other celestial bodies.
It is worth observing that the chemical composition of some minerals is not always defined and constant. This is due to the phenomenon of isomorphism (v. CRISTALLI), whereby the atoms or ions of certain elements can in the crystal lattice be replaced, to a variable extent, sometimes at will, by atoms or ions having a close ionic radius and equal polarizing power, or else normally vacant spaces in the crystal lattice can be occupied by interstitial addition. Thus, for example, by the term olivine is meant not a mineral of defined composition but a series of rhombic isomorphic mixtures whose composition can vary between the two end members: forsterite Mg₂ SiO₄ and fayalite Fe₂ SiO₄, through replacement in the crystal lattice of Fe⁺⁺⁺ ions with Mg⁺⁺⁺ ions.
Crystallized minerals can present themselves in the form of single individuals: more often the individuals are found associated in groupings or concretions. The latter are due to successive deposition, sometimes rhythmic, from solutions. Groupings and concretions can present a characteristic form: thus one speaks of groupings in rosettes of iron (hematite), discoidal, spheroidal: of concretions of mammillary form and of fibrous-radiate and concentrically zoned structure (alabasters, agate, onyx), of stalactitic, coralloid form, of pisoliths, of ooliths.
In many cases the crystals, depending on the speed of formation and growth of the crystal nuclei, cannot develop completely in polyhedral form and mutually limit one another. Thus one has the crystalline aggregates of individuals of the same species or of a different species, which can take the name of granular, saccharoidal, bacillary, fibrous (often with radiate structure), lamellar, scaly, squamous, sheaf-like aggregates, and so on.
Aggregates of minerals very often assume the extent of a geological unit and then take the name of rocks; in this sense a rock represents the normal mode of occurrence of minerals.
Most of these, in fact, owe their origin to the processes of rock formation (v.). Thus one distinguishes minerals of magmatic origin in the broad sense (that is, also comprising pneumatolytic and hydrothermal minerals), minerals of sedimentary origin and minerals of metamorphic origin. The first originate from the consolidation of natural molten masses (magmas) or in the processes connected with it, the second are formed by evaporation of saline or colloidal solutions, the last following transformations of phases or modifications of chemical equilibrium due to changed conditions of pressure and temperature.
Minerals that have formed under conditions different from those of their environment tend to reach equilibrium with the environment. Thus changes of structural phase or chemical reactions with formation of new compounds can occur. In the first case one has polymorphic transformations, in the second the so-called chemical alteration of minerals; this can be due to exogenous agents (water, atmosphere, etc.), or to endogenous agents (temperature, magmatic gases, thermal magmatic solutions). Typical examples of polymorphic transformations are the transformation of aragonite into calcite, of tridymite into quartz, and equally common examples of chemical alteration are the oxidation of pyrite into limonite or into hematite, the transformation of feldspars into clay minerals, of olivine into serpentine or into magnesite. In the processes of polymorphic transformation and of alteration the newly formed product often replaces the mineral while leaving its primitive form intact. Thus one has respectively the phenomena of paramorphism and pseudomorphism.
In many cases it is possible to reproduce minerals artificially in the laboratory by means of operations called mineral synthesis: in some cases the synthesis amounts to simple chemical preparations or crystallizations, in other cases complex apparatus and a particular technique are required, as in pneumatolytic and hydrothermal synthesis. Mineral synthesis provides important elements in the investigation of the origin and the conditions of formation and stability of minerals; but the most rigorous deductions concerning their genesis can be obtained from the study of systems with several components and of the equilibria of aqueous solutions.
Numerous classifications have been proposed for minerals. Linnaeus's classification had little success, unlike the classifications of animals and plants, given the scant knowledge that was had at that time concerning the chemical composition of minerals. The classification that is most commonly accepted today is that of J. D. Dana, according to which minerals, on the basis of their chemical composition, are grouped into eight classes.
(phot. Fides)
Other classifications have recently been proposed, intended to take into account, besides chemical composition, the crystal structure of minerals, and Dana's own classification is currently in evolution.
II. HISTORY OF MINERALOGY
The first knowledge of minerals, of a practical and empirical kind, goes back to very ancient times. While there is no precise evidence by which to judge the state of the mineralogical knowledge of the Assyrians and Babylonians, it appears from the Ebers papyrus (16th century B.C.) that the Egyptians knew and used numerous minerals, such as malachite, hematite and antimonite; numerous other minerals were used as ornaments, amulets, medicines and for the extraction of metals.
The Phoenicians knew and traded in minerals, while the Etruscans worked mines and knew metallurgy.
The first writing on mineralogy to which a scientific value can be attributed is the fragment Περί λίβων by Theophrastus (371-300 B.C.), a pupil of Aristotle. His work, despite its errors and philosophical prejudices, can be considered not only the oldest but for several centuries also the most perfect. Numerous notices on minerals are contained in the Historia naturalis of Pliny (23-79 A.D.).
The Middle Ages sees in the Muslim East the origins of experimental m.: al-Kindī (d. 873) and al-Bīrūnī (973-1048) left writings on m.; the latter was the first to determine with the pycnometer, with surprising accuracy, the density of metals, minerals and precious stones. Ibn Sīnā, called Avicenna (980-1037), gives the first classification of minerals. Later Albertus Magnus (1193-1280) writes 5 books on minerals and 4 on meteorites. Vannuccio Biringuccio (1480-1539?), a Sienese metallurgist, in his Pirotechnia deals with metallurgical questions; his work still belongs to alchemy, but the first intuition of the constancy of angles in crystals is perhaps due to him. Georg Bauer (Agricola, 1490-1555) can be considered the founder of the mining art; his fundamental works are De natura fossilium and De re metallica. Steno (1637-1696) observes the constancy of the dihedral angle in quartz crystals: the Bolognese Domenico Guglielmini (1655-1710) clearly expresses the law of constancy of the dihedral angle for salts. This law bears the name of Romé de l'Isle (1736-90), to whom belongs the merit of having stated it explicitly and generalized it. With the beginning of crystallographic studies, due to Romé de l'Isle and the abbé Haüy, the morphological knowledge of minerals moves toward great progress.
From this point the history of m. is in part identified with that of crystallography; most of the research and discoveries in the field of physical and chemical crystallography are conducted on minerals: every advance in the science of crystals leads to a better knowledge of minerals. From about 1750 to 1850 there occurs a very rapid and fruitful development of mineralogical studies. While physical crystallography reached a high degree of perfection in the study of crystallographic optics, m. thus came into possession of a fundamental method of investigation and microscopic m. began to develop. In the chemical field, besides the fundamental analytical works of Cronstedt (1722-65), Bergmann (1735-84), Scheele (1742-86), Berzelius (1779-1848), the discovery of isomorphism and polymorphism by Mitscherlich (1794-1863) was of great importance.
Numerous mineralogical species are discovered and described in this period, defined in their chemical constitution, in their crystallographic, physical and particularly optical properties. Mention is made here of the work of Giovanni Strüver (1842-1915), who together with Quintino Sella (1827-84) and Arcangel-Scacchi (1810-93) can be considered one of the founders of m. in Italy.
Chemical-physical research on minerals developed predominantly in the second half of the 19th century and in the first decades of the 20th century, particularly through the work of H. W. Bakhuis Roozeboom (1874-1907), J. H. Van t'Hoff, G. Tammann, T. Vogt, P. Niggli and the researchers of the Geophysical Laboratory of Washington. They constitute the rational basis for the study of the problems of minerogenesis and mineral synthesis which had already been tackled experimentally and with success since the last century by A. Michel-Lévy and F. Fouqué in France and by G. Spezia (1842-1911) in Italy. In this period research in mineralogical chemistry continues: it will suffice to recall P. Groth, G. Tschermak, C. Doelter, F. J. Boecke, F. Zambonini. At the same time A. Michel-Lévy, L. Duparc, E. Von Fedoroff bring microscopic m. to a high degree of perfection, particularly with the use of the universal stage.
Modern research on the structure of crystals, which developed rapidly after Laue's discovery (1912), while on the one hand it has led to determining the crystal structure of very numerous minerals, has made it possible to clarify complex relations between chemical composition and crystal structure. Many questions relating to the chemical bond were resolved through the study of the crystal structure of minerals, in which all their physical properties find their natural interpretation, although such relations are not always explicit. M. thus today has at its disposal a new method of investigation, and continually new means are offered to it by physics and chemistry. However, numerous problems still remain open, concerning both the constitution of individual minerals and their relations with the environment in which they were generated.
Mineralogie im Altertum und im Mittelalter (Fortschr. der Min. Krist. allofr. u. Petr., 7), Jena 1922; P. Groth, Entwicklungsgeschichte der mineralogischen Wissenschaften, Berlin 1926.