The Necessity of Nomenclature
The true purpose of nomenclature is to be a tool. Organizing large sets of data is often the first step in a scientific endeavor. It allows the scientist to easily manipulate data, and see the patterns in nature that lead to understanding. A good example of this is the taxonomic system into which animal and plant species are placed. From this organization, scientists were able to see the similarities among species, and from this devise the theory of evolution. James Dana recognized this when he said “Of such a system the science of Mineralogy is destitute.” He tried to rectify this by creating a robust methodical set of classifications for the realm of mineralogy. His efforts turned out to be very successful, and influenced the field exponentially as time has gone by.
So how did James Dana come to this conclusion and become such an influential geologist? Dana showed at interest in science at an early age, and took a traditional academic route. He went straight from high school to studying at Yale College in New Haven, Connecticut. There he studied under professor Benjamin Silliman, who was instrumental in stimulating Dana's interest in minerals and geology. After graduating in three years, Dana taught mathematics for two years before returning to Yale in 1835. He became an assistant to his former professor Benjamin Silliman, and worked in Yale's chemical laboratory. It was at this time that Dana began the preparation of his System of Mineralogy, a new mineral classification based on chemistry and crystallography. To aid in this development, he was able to make use of Silliman’s own collection of minerals as well as his own personal set. The end of his stint in the chemical laboratory coincided with the publishing of the first edition of A System of Mineralogy, published when Dana was just 24 years old. During his lifetime four editions of this work were published and several after his death. Now, Dana's schema for classifying minerals is considered to be the most thorough and commonly used classification system for minerals in geology today.
Dana was the first person to classify minerals into an arrangement by composition and structure. However: this system did not arise spontaneously, but developed over a long period of time over 8 editions.The first five editions of A System of Mineralogy, were published in 1837, 1844, 1850, 1854, and 1868, and were authored or revised by James Dwight Dana. Each edition varied somewhat, some editions having additional sections on crystallography. The specific chemical divisions were revised over time. The classification of silicate minerals in particular has changed significantly as the different structures of silicate minerals were understood.With each edition there was an enormous increase in knowledge due to extensive research in the field. The first edition described all 352 minerals known at that time, but current editions describe about 4000 species. There was a general trend toward mineral descriptions, rather than the methods and details of mineralogy and crystallography. These developments allowed mineralogists to systematically classify minerals by structure and composition in a way that was previously impossible. In early editions, Dana attempted to place minerals into a classification scheme similar to that of Lineaus’ taxonomy– the binomial nomenclature of genus and species used in botany and zoology. In the third edition, Dana abandoned his previous Latin based natural history system, and instead opted for a system which organizes mineral species for convenience for students and professionals, By the fourth edition, Dana decided to go back to a system which describes the true relationshps between mineral species, but this time chose to use the chemical classification system we use today. This was a bold move, considering that the exact composition and structure of many minerals were unknown at the time. The rest of the editions continued to add new scientific knowledge. The sixth edition was revised by his son Edward S. Dana and published in 1892. Three appendices were published in the American Journal of Science and these brought mineral descriptions of the sixth edition up to date as of 1915. The seventh revised edition was published in three editions by professors Charles Palache, Harry Berman, and Clifford Frondel. Volume one of the seventh edition was published in 1944, and the second volume was published in 1951. Volumes one and two covered all minerals except the silicates. The third volume, covering only silica minerals, was published in 1962. The eighth volume, Dana's New Mineralogy was published in 1997, which reformed the classification system into what we see today. However, the sixth and seventh editions are generally preferred due to errors in the eighth edition, as well as the detail that was sacrificed to condense the information into a single volume.
The modern System:
Now that we've covered the history of the inception of Dana's many editions of his influential work, we'll discuss what is actually covered within these many pages of information. Minerals are first separated into classes by anion.The main classes are Elements, Oxides, Halides, Sulfides, Carbonates, Phosphates, and Silicates. There are many subclasses still based on composition.For example, Sulfides is often grouped with Sulfolsalts, in which a metallic element is bonded to a sulfur and a semimetal, such as arsenic, antimony, etc. Smaller groups that are chemically similar are often grouped together. For example arsenates, and vanadates, and antimonates are generally grouped with phosphates. From there they are further divided by crystal symmetry. There are six groups: triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and isometric. The crystal forms for each group are also covered in Dana's system and a detailed mathematical treatise of crystallography is given in some editions. Then, things are further divided by exact structure and geochemical considerations. The Dana classification assigns a four digit number to a mineral species. First indicates its class, which is based on composition; the next gives the ratio of cations to anions in the mineral, and the last two numbers group minerals by structural similarity with a given type or class.
What is included in Dana's descriptions?
Dana describes the many physical and chemical properties which can be used to identify minerals. Here Dana heavily draws from the work of previous mineralogists such as Mohs.
Crystal Form:The system in which the mineral crystallizes is listed first, then the exact observed forms and dimensions.
Color: The external color. This is not a reliable indicator for most minerals, but for a few it is a diagnostic feature.
Luster:An obvious feature which indicates which type of chemical bonding dominates in a mineral. Metallic luster indicates metallic bonding, and transparent indicates ionic or covalent bonding. The different qualities of nonmetallic luster are controlled by indices of retraction (ratio of the speed that light travels through the material to the speed in a vacuum).
Streak:The color of finely ground particles of a mineral.It is a more reliable test than mineral color, and can indicate the bonding type and sometimes the dominant elements.
Cleavage: The property of crystalline materials to fracture along crystallographic planes. This is controlled be the structure of chemical bonds. A well known example is mica which have one perfect cleavage in one direction. This is at the boundary of tetrahedral layers and cation layers in the mineral.
Fracture: How the crystal breaks when there is no obvious crystallographic control.
Twinning:A symmetric intergrowth of crystal segments of the same mineral.
Hardness: The resistance to scratching is controlled by the mechanical cohesion of chemical bonds. Each mineral has a distinctive range of hardness ,which is given in the Mohs Scale.
Reaction to HCl: Reaction to Hydrochloric acid is a diognostic indicator for minerals containing the carbonate anion.
Tenacity: The behavior of minerals when deformed or broken. For example, thin sheets of muscovite (mica) are elastic, but talc sheets are inelastic.
Substitutions: In many minerals, elements can substitute into the crystal structure of a mineral because of similar chemical properties such as ion charge and cation radius. In many cases minerals form a solid solution series in which elements swap freely for each other. A good example of this is olivine. The general formula is (Mg,Fe)2SiO4. The end-member of the iron series (fayalite) is entirely composed of Fe2SiO4 while the magnesium end-member (forsterite) is composed entirely of Mg2SiO4. Thus, a given specimen can fall anywhere on the range of compositions. A specimen of intermediate composition is just called olivine. If the physical properties of end members vary significantly, the composition can be determined be careful examination. Dana described these chemical relationships between minerals.
Specific Gravity:Most people know this property as density. Basically, specific gravity is just density divided by the density of water, to give a dimensionless number.
Blowpipe Reaction: Many minerals have a distinctive chemical reaction when subjected to flame. Often a certain color will be visible, or sometimes the specimen will give off fumes. This technique is now outdated and rarely used.
Magnetic Properties: There are a number of magnetic properties a mineral can exhibit. Usuall it is only mention if the mineral is ferrimagnetic ( produces a magnetic field because of unequel phase coupling in adjacent sites.) , or ferromagnetic (magnetic domains can become aligned and thus is attracted to magnets and can become magnetized).
Electrical properties: There are many electrical properties which can be used to identify minerals. Strong electrical conductivity can be used to identify minerals with metallic bonding, while ionically or covalently bonded minerals tend to be insulators. Some minerals will produce a voltage when deformed, which is known as piezoelectricity.This occurs only in minerals with no center of symmetry. Pyroelecricity is when a change in temperature causes a voltage. This is only possible in minerals with a single polar direction( ie. low symmetry).
Taste, odor: A few minerals have a distinctive taste or smell, such as sylvite, which has a bitter taste, or clays, which have an earthy smell.
Geologic Occurrence: The environment in which a mineral forms. For example feldspars form in igneous rocks, while kyanite forms in high grade metamorphic rocks.
Localities and associations: Common occurrences of the mineral and the minerals it occurs with at these locations.
Optical properties:Optical properties were not described in the earlier editions, but were quickly added with the rise in importance of thin sections for petrology. When light passes through a mineral the wave is split into two orthogonally vibrating waves. In Anisotropic (different in different directions) minerals these waves behave differently and reveal the structure of the mineral through various methods.