MINERALOGY. - The science of minerals. Originally its principal task was to describe and identify them; today it aims above all to better understand their chemical-physical structure, properties, and genesis.
I. MINERALS
From the Celtic *meine*, raw metal. This term denotes the natural, unitary constituent parts of the lithosphere. Minerals are considered to include simple substances, solid (crystalline) or liquid (mercury) inorganic compounds, isomorphous 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, being mixtures, are therefore to be counted among rocks. Volcanic glasses, since upon reaching equilibrium (devitrification, crystallization) they normally give rise to aggregates of different minerals, are customarily regarded as rocks. Meteorites may be considered as aggregates of minerals from other celestial bodies.
It should be noted that the chemical composition of some minerals is not always definite and constant. This is due to the phenomenon of isomorphism (q.V. CRISTALLO), whereby the atoms or ions of certain elements within the crystal lattice may be replaced, to varying degrees and sometimes at will, by atoms or ions with similar ionic radii and equal polarizing power, or normally vacant spaces in the crystal lattice may be filled by interstitial addition. Thus, for example, the term olivine does not refer to a mineral of fixed composition but to a series of orthorhombic isomorphous mixtures whose composition can vary between the two end-members: forsterite Mg₂SiO₄ and fayalite Fe₂SiO₄, through substitution in the crystal lattice of Fe²⁺ ions with Mg²⁺ ions.
Crystalline minerals may occur as single individuals; more often, individuals are found associated in aggregates or concretions. The latter arise from successive deposition, sometimes rhythmic, from solutions. Aggregates and concretions may exhibit characteristic forms: thus one speaks of iron-rose (hematite) aggregates, discoidal, spheroidal; of concretions with mammillary form and fibrous-radiate and concentric zoned structure (alabasters, agate, onyx), stalactitic, coralloidal, pisolitic, oolitic forms.
In many cases, crystals, depending on the rate of formation and growth of the crystalline nuclei, cannot develop fully into polyhedral forms and mutually limit one another. Thus arise crystalline aggregates of individuals of the same species or of different species, which may be termed granular, saccharoidal, bacillary, fibrous (often with radiate structure), lamellar, scaly, squamose, sheaf-like, and so on.
Mineral aggregates often extend to the scale of a geological unit and are then called rocks; in this sense, a rock represents the normal occurrence of minerals.
Most minerals owe their origin to rock-forming processes (q.v.). Thus one distinguishes magmatic minerals in the broad sense (including pneumatolytic and hydrothermal minerals), sedimentary minerals, and metamorphic minerals. The first originate from the consolidation of natural molten masses (magmas) or in processes connected with them; the second form through the evaporation of saline or colloidal solutions; the last arise from transformations of phases or modifications of chemical equilibrium due to changes in pressure and temperature.
Minerals formed under conditions different from those of their environment tend to reach equilibrium with their surroundings. Thus structural phase changes or chemical reactions with the formation of new compounds may occur. In the first case, one has polymorphic transformations; in the second, the so-called chemical alteration of minerals. This may 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 conversion of aragonite to calcite and of tridymite to quartz; equally common examples of chemical alteration are the oxidation of pyrite to limonite or hematite, the transformation of feldspars into clay minerals, and of olivine into serpentine or magnesite. In polymorphic transformation and alteration processes, the newly formed product often replaces the mineral while preserving its original form. Thus one has, respectively, the phenomena of paramorphism and pseudomorphism.
In many cases, it is possible to reproduce minerals artificially in the laboratory through operations known as minerogenesis: in some instances, synthesis amounts to simple chemical preparations or crystallizations, while in others, complex apparatus and special techniques are required, as in pneumatolytic and hydrothermal synthesis. Minerogenesis provides important insights into the origin and conditions of formation and stability of minerals; but the most rigorous deductions about their genesis can be obtained from the study of multi-component systems and equilibria of aqueous solutions.
Many classifications have been proposed for minerals. Unlike the classifications of animals and plants, Linnaeus’s classification met with little success, given the limited knowledge at the time of the chemical composition of minerals. The classification most commonly accepted today is that of J. D. Dana, according to which minerals are grouped into eight classes based on their chemical composition.
Other classifications have recently been proposed, intended to take into account, in addition to chemical composition, the crystal structure of minerals; Dana’s classification itself is currently evolving.
II. HISTORY OF MINERALOGY
The earliest knowledge of minerals, of a practical and empirical nature, dates back to very ancient times. While no precise evidence exists to judge the state of mineralogical knowledge among the Assyrians and Babylonians, the Ebers Papyrus (16th century B.C.) shows that the Egyptians knew and used numerous minerals, such as malachite, hematite, and antimonite; many other minerals were employed as ornaments, amulets, medicines, and for metal extraction.The Phoenicians knew and traded minerals, while the Etruscans worked mines and understood metallurgy.
The first mineralogical writing to which scientific value can be attributed is the fragment *De Lapidibus* by Theophrastus (371–300 B.C.), a pupil of Aristotle. Despite errors and philosophical prejudices, his work may be considered not only the oldest but for many centuries also the most complete. Numerous notices on minerals are contained in the *Naturalis Historia* of Pliny (23–79 A.D.).
The physician in the Muslim East: the origins of experimental mineralogy. Al-Kindī (d. 873) and al-Birūnī (973–1048) left writings on mineralogy; the latter first determined, with remarkable accuracy using the pycnometer, the densities 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. Vannoccio Biringuccio (1480–1539?), a Sienese metallurgist, in his *Pirotechnia* addressed metallurgical questions; his work still belongs to alchemy, but it may have been he who first intuited the constancy of angles in crystals. Georg Bauer (Agricola, 1490–1555) may be regarded as the founder of mining science; his fundamental works are *De natura fossilium* and *De re metallica*. Steno (1637–1696) observed the constancy of the dihedral angle in quartz crystals; the Bolognese Domenico Guglielmini (1655–1710) clearly formulated the law of constancy of the dihedral angle for salts. This law is named after Romé de l’Isle (1736–90), who deserves credit for explicitly stating and generalizing II. With the beginning of crystallographic studies, due to Romé de l’Isle and Abbé Haüy, the morphological knowledge of minerals advanced rapidly.
From this point onward, the history of mineralogy partly coincides with that of crystallography; most research and discoveries in physical and chemical crystallography have been conducted on minerals, and every advance in crystal science has led to a better understanding of minerals. Between roughly 1750 and 1850, mineralogical studies developed with remarkable speed and fertility. While physical crystallography attained a high degree of perfection in the study of crystallographic optics, mineralogy thereby acquired a fundamental investigative method and began to develop microscopic mineralogy. In the chemical field, in addition to the foundational analytical work of Cronstedt (1722–65), Bergman (1735–84), Scheele (1742–86) and Berzelius (1779–1848), of great importance was the discovery of isomorphism and polymorphism by Mitscherlich (1794–1863).
Numerous mineral species were discovered and described in this period, their chemical constitution, crystallographic, physical and especially optical properties defined. Among the figures cited here is Giovanni Strüver (1842–1915), who, together with Quintino Sella (1827–84) and Arcangelo Scacchi (1810–93), may be considered one of the founders of mineralogy in Italy.
Chemico-physical research on minerals developed especially in the second half of the 19th century and the early decades of the 20th, notably 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 in Washington. They laid the rational basis for studying problems of minerogenesis and mineral synthesis, which had already been tackled experimentally and successfully in the previous 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 mention F. Groth, G. Tschermak, C. Doelter, F. J. Boeke and F. Zambonini. At the same time, A. Michel-Lévy, L. Duparc and E. von Fedorov brought microscopic mineralogy to a high degree of perfection, particularly through the use of the universal stage.
Modern research on crystal structure, which developed rapidly after Laue’s discovery (1912), has on the one hand enabled the determination of the crystal structures of countless minerals and on the other hand clarified complex relationships between chemical composition and crystal structure. Many questions concerning chemical bonding were resolved through the study of the crystal structures 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 investigative method, and continually receives new tools from physics and chemistry. Yet many problems remain open, concerning both the constitution of individual minerals and their relations with the environments in which they were generated.
