Mineral Science. Descriptive Mineralogy

Mineral science, also referred to as mineralogy, is the title as well as the subject of this text. It concerns itself mainly with the study of naturally occurring inorganic solids called minerals; these may be of terrestrial or extraterrestrial origin. The subject is most closely related to the discipline of inorganic chemistry, but in mineral science the focus is specifically on naturally occurring solid substances.

The subject of mineral science encompasses five subdisciplines (see Fig. 1.2). Here we will briefly introduce these categories, beginning with descriptive mineralogy and followed by the next subject in a clockwise order of Fig. 1.2.

FIG. 1.2. Diagram showing the major subdisciplines that constitute mineral science. Arrows imply major contribution of one subfield to another

Descriptive Mineralogy. This involves the measurement and recording of physical properties (parameters) that help identify and describe a specific mineral. Some gross features of a mineral such as crystal form, hardness, color, and specific gravity ("heft") can be evaluated in hand specimen (i.e., using the five senses and some basic testing tools). Other, more objective criteria such as optical properties (e.g., refractive indices) and X-ray diffraction data (e.g., dimensions of the smallest building block of the atomic structure, the size of the unit cell) require specialized techniques and equipment such as a petrographic microscope and an X-ray diffraction system, respectively. Other measurable physical properties of minerals include their magnetic behavior, electrical properties, radioactivity, and their mechanical response to applied compressive or tensile loading.

Crystallography. This branch of science is very broad. Prior to the discovery of X-radiation by Wilhelm Conrad Roentgen in 1895 and the subsequent X-ray diffraction experiment by Max von Laue (observing the regular pattern of X-ray diffraction effects when a crystalline substance is properly positioned in an X-ray beam), crystallography concerned itself mainly with the geometric form, external symmetry, and optical properties of crystals. Since 1912 its main emphasis has been, and still is, the investigation of the internal structure of crystalline materials, whether organic or inorganic in origin.

Most commonly an X-ray beam is used as the energy source for X-ray diffraction experiments. However, electron beams are also used for producing electron diffraction patterns. The ultimate aim of modern crystallographic techniques is the determination of the crystal structure. It provides information on the location of all the atoms, bond positions and bond types, internal' symmetry (space group symmetry), and the chemical content of the unit cell. All such information is fundamental to concepts of crystal chemistry, which concerns itself with the interrelation of variability in chemistry and structure.

Crystal Chemistry. The field of crystal chemistry relates the chemical composition, the internal structure, and the physical properties of crystalline materials. A specific mineral is defined on the basis of its crystal structure, chemical composition, and related physical properties. In many mineral groups the overall pattern of the structure is relatively constant, whereas the chemical composition of such a group may be highly variable. This is commonly called a specific structure type, showing extensive chemical substitution (i.e., solid solution). The assessment of structure type, of the atomic bonding arrangement, of the variability in overall chemistry, and of related changes in physical properties of a crystalline subtance are the domain of crystal chemistry.

Classification. There are approximately 3800 mineral species, and each has a distinctive name. To make sense of the divergent chemistries and structures represented by these minerals, it is customary to classify them according to a rational crystallochemical scheme. This means that minerals are first classified by their anionic group. Such classifications would include elements, sulfides, oxides, carbonates, silicates, and so on. Second, in groups with many species and complex structures, as in the silicate group, further subclassifications are made, mainly on the basis of the structural (atomic) arrangements of the silicate tetrahedra.

Geologic Occurrence. A common synonymous term is paragenesis. Geologic occurrence refers to the characteristic association or occurrence of a mineral or a mineral assemblage in a well-defined geologic setting. For example, a common occurrence (or paragenesis) for the relatively common sulfide sphalerite, ZnS, would be reported as "in ore deposits of hydrothermal origin." Garnet, a chemically complex silicate, is especially characteristic of metamorphic rocks. Its occurrence would be reported as "mainly in Al-rich rock types that are the product of regional metamorphism."