MINERALOGIA. — The science of minerals. Originally, its principal task was to describe and identify them; today it is chiefly concerned with gaining a better understanding of their physico-chemical structure, properties, and genesis.
I. MINERALS
From the Celtic meine, raw metal. A designation indicating the natural unitary constituent parts of the lithosphere. Minerals are considered to include simple substances, solid inorganic compounds (crystallized) or liquids (mercury), isomorphic mixtures of two or more elements or compounds, certain solid organic substances (amber, succinite, fichtelite, mellite, etc.), and certain amorphous substances (opal in part, collophane).Fossil coals and petroleum as such are mixtures and therefore are to be numbered among the rocks. Volcanic glasses, since, once equilibrium has been reached (devitrification, crystallization), they normally give rise to aggregates of different minerals, are customarily regarded as rocks. Meteorites may be considered aggregates of minerals from other celestial bodies.
It is well to observe that the chemical composition of certain minerals is not always definite and constant. This is due to the phenomenon of isomorphism (v. CRISTALLO), whereby the atoms or ions of certain elements in the crystal lattice may be replaced, to a variable extent and sometimes at will, by atoms or ions having a similar ionic radius and equal polarizing power; alternatively, spaces normally vacant in the crystal lattice may be occupied through interstitial addition. Thus, for example, the term olivine does not designate a mineral of definite composition, but a series of rhombic isomorphic mixtures whose composition may vary between the two extreme members: forsterite Mg₂ SiO₄ and fayalite Fe₂ SiO₄, through the replacement in the crystal lattice of Fe⁺⁺⁺ ions by Mg⁺⁺⁺ ions.
Crystallized minerals may occur in the form of single individuals; more often, the individuals are found associated in groupings or concretions. The latter are due to the successive, sometimes rhythmic, deposition from solutions. Groupings and concretions may display a characteristic form: thus one speaks of iron-rose groupings (hematite), discoidal and spheroidal groupings; and of concretions in mammillated form and with a fibrous-radiate and concentrically zoned structure (alabaster, agate, onyx), in stalactitic or coralloid form, or in the form of pisolites and oolites.
In many cases, depending on the speed of formation and growth of the crystal nuclei, crystals cannot develop completely in polyhedral form and mutually restrict one another. The result is crystalline aggregates of individuals of the same or of different species, which may be called granular, saccharoidal, bacillary, fibrous (often with a radiate structure), lamellar, scaly, squamous, sheaf-like aggregates, and so forth.
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 occurrence of minerals.
Indeed, most of these owe their origin to the processes of rock formation (v.). Thus one distinguishes minerals of magmatic origin in the broad sense (including, that is, 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 from processes connected with it; the second are formed through the evaporation of saline or colloidal solutions; the last arise following transformations of phases or modifications of chemical equilibrium caused by changed conditions of pressure and temperature.
Minerals that have formed under conditions different from those of their surroundings tend to reach equilibrium with the environment. Structural phase changes or chemical reactions involving the formation of new compounds may consequently occur. In the first case there are polymorphic transformations; in the second, the so-called chemical alteration of minerals. This may be due to exogenous agents (water, atmosphere, etc.) or endogenous agents (temperature, magmatic gases, thermal magmatic solutions). Typical examples of polymorphic transformations are the transformation of aragonite into calcite and of tridymite into quartz; equally common examples of chemical alteration are the oxidation of pyrite into limonite or hematite, the transformation of feldspars into clay minerals, and of olivine into serpentine or magnesite. In processes of polymorphic transformation and alteration, the newly formed product often replaces the mineral while leaving its primitive form intact. The respective phenomena are thus paramorphism and pseudomorphism.
In many cases it is possible to reproduce minerals artificially in the laboratory by means of operations known as mineral synthesis: in some cases the synthesis consists of simple chemical preparations or crystallizations; in others, complex apparatus and a special technique are required, as in pneumatolytic and hydrothermal synthesis. Mineral synthesis provides important data in research into the origin and conditions of formation and stability of minerals; but the most rigorous deductions concerning their genesis can be obtained from the study of multicomponent systems and the equilibria of aqueous solutions.
Numerous classifications of minerals have been proposed. Unlike the classifications of animals and plants, Linnaeus’s classification met with little success, given the scant knowledge then available concerning the chemical composition of minerals. The classification 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.
(Fides photograph)
II. HISTORY OF M
The earliest knowledge of minerals, of a practical and empirical nature, dates back to very ancient times. While there are no precise elements for judging 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 stibnite; many other minerals were employed as ornaments, amulets, medicines, and for the extraction of metals.The Phoenicians knew and traded in minerals, while the Etruscans worked mines and were acquainted with metallurgy.
The first mineralogical writing to which 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, may be regarded not only as the oldest but also, for several centuries, as the most perfect. Numerous pieces of information on minerals are contained in Pliny’s Historia naturalis (A.D. 23–79).
The Middle Ages saw the origins of experimental m. in the Muslim East: al-Kindī (d. 873) and al-Bīrūnī (973–1048) left writings on m.; the latter was the first to determine, with astonishing accuracy by means of the planimeter, the density of metals, minerals, and precious stones. Ibn Sīnā, known as Avicenna (980–1037), provided the first classification of minerals. Later, Albertus Magnus (1193–1280) wrote five books on minerals and four on meteorites. Vannuccio Biringuccio (1480–1539?), a Sienese metallurgist, dealt with metallurgical questions in his Pirotechnia; his work is still alchemical, but he may perhaps be credited with the first intuition of the constancy of angles in crystals. Georg Bauer (Agricola, 1490–1555) may be considered the founder of mining; his fundamental works are De natura fossilum and De re metallica. Steno (1637–1696) observed the constancy of the dihedral angle in quartz crystals; the Bolognese Domenico Guglielmini (1655–1710) clearly expressed the law of constancy of the dihedral angle for salts. This law bears the name of Romé de l’Isle (1736–90), who deserves credit for having explicitly stated and generalized II. With the beginning of crystallographic studies, due to Romé de l’Isle and Abbé Haüy, the morphological knowledge of minerals began to make great advances.
From this point onward, the history of m. is partly identified with that of crystallography; most 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 approximately 1750 to 1850, a very rapid and fruitful development of mineralogical studies took place. While physical crystallography attained a high degree of perfection in the study of crystallographic optics, m. thereby acquired 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), and Berzelius (1779–1848), the discovery of isomorphism and polymorphism by Mitscherlich (1794–1863) was of great importance.
Numerous mineralogical species were discovered and described during this period, their chemical constitution and their crystallographic, physical, and especially optical properties being established. Mention may be made here of the work of Giovanni Strüver (1842–1915), who, together with Quintino Sella (1827–84) and Arcangelo Scacchi (1810–93), may be regarded as one of the founders of m. in Italy.
MINERVA – Relief of M. in the homonymous temple (Triassic age). Rome.
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 experimentally and successfully addressed since the last century by A. Michel-Lévy and F. Fouqué in France and by G. Spezia (1842–1911) in Italy. During this period, research in mineralogical chemistry continued: it will suffice to recall P. Groth, G. Tschermak, C. Doelter, F. J. Boecke, and F. Zambonini. At the same time, A. Michel-Lévy, L. Duparc, and E. Von Fedoroff brought microscopic mineralogy to a high degree of perfection, particularly through the use of the universal stage.
Modern research on the structure of crystals, which developed rapidly after Laue’s discovery (1912), has, on the one hand, led to the determination of the crystal structure of very numerous minerals and, on the other, made it possible to clarify complex relationships between chemical composition and crystal structure. Many questions concerning chemical bonding were resolved through the study of the crystal structure of minerals, in which all their physical properties find their natural interpretation, although such relationships are not always explicit. Mineralogy thus now possesses a new method of investigation, and physics and chemistry are continually offering it new means. Nevertheless, numerous problems remain open, concerning both the constitution of individual minerals and their relations with the environment in which they were formed.
