For many years the science of mineralogy was a science of classification. As was the case with most sciences, before the era of sophisticated analytical chemistry and physics, it was a game of collecting, sorting, and trying to understand basic relationships. The godfather of mineral classification was James Dwight Dana (1813–1895), American mineralogist, geologist, and zoologist, who for much of his career was a professor at Yale College (now Yale University) in New Haven, Connecticut. He studied there and took up assistance for the man who became his father in law, distinguished chemist Benjamin Silliman (1779–1864). Dana’s son, Edward Salisbury Dana (1849–1935) also became an accomplished mineralogist.
The cornerstone moment in mineralogical classification occurred in 1837, when Dana published his revolutionary System of Mineralogy. This work quickly became the bible of mineral scientists. The work has gone through 8 editions, the most recent useful printing occurring in 1944. In 1997 the 8th edition appeared but unfortunately was marred by a large number of errors, making it a curiosity for those in the mineral field but no longer a must-have reference. Moreover, just about the time of the last good edition of Dana, a German mineralogist came onto the scene whose work would reclassify and fine-tune the categorizations of Dana into a more useful, more complex system.
Karl Hugo Strunz (1910–2006) introduced a scheme of classifying minerals in his 1941 publication Mineralogische Tabellun. Strunz, who went by the name Hugo, was a noted German mineralogist who taught at Friedrich-Wilhelm University in Berlin (now Humboldt University) and subsequently at the Technical University in Berlin, where he established the Mineralogical Institute. Expansions of the 1941 publication appeared in 1966 and 1982, and by 2001 the work was updated and revised under the guidance and refinement of the Canadian mineralogist Ernest Henry Nickel (1925–2009). In what is now known as the Nickel-Strunz Classification, mineralogists have the most detailed, most reliable system of classifying minerals, and the 10th edition is now being prepared and will be finalized in 2012.
In similar fashion to the original Dana scheme, the Nickel-Strunz Classification breaks up mineral species into 10 basic groups of relatives, and then subdivides them precisely in linear relationships and in groups, subgroups, and series of similarity. The basic classes follow:
These are made of just one element, such as arsenic, antimony, bismuth, copper, diamond, gold, graphite, iron, lead, silver, platinum, and sulfur.
2. Sulfides and sulfosalts
These form when a metal or semimetal element combines with sulfur. Sulfides include acanthite, chalcopyrite, cinnabar, galena, molybdenite, pyrite, sphalerite, and stibnite. Sulfosalts are chemically complex sulfides that include andorite, bournonite, enargite, proustite, pyrargyrite, and teallite.
Halides form when a halogen (bromine, chlorine, fluorine, or iodine) combines with a metal or semimetal. These include atacamite, boleite, cumengeite, diaboleite, fluorite, halite, salammoniac, sylvite, and villaumite.
Oxides combine at least one oxygen atom with other elements. They include brucite, carnotite, cassiterite, chrysoberyl, corundum, cuprite, diaspore, goethite, hematite, opal, quartz, rutile, spinel, uraninite, and zincite.
Carbonates couple a carbonate radical, consisting of carbon and oxygen, with a metal or semimetal. Examples include aragonite, aurichalcite, azurite, bastnäsite, calcite, cerussite, dolomite, kutnohorite, magnesite, malachite, natron, rhodochrosite, siderite, smithsonite, strontianite, and trona.
These minerals contain radicals of boron and oxygen. They include boracite, borax, colemanite, gaudefroyite, hambergite, henmilite, jeremejevite, kernite, ludwigite, rhodizite, and ulexite.
Sulfates pair sulfur and oxygen in a radical that combines with a metal or semimetal in these minerals. Also in this class are chromates (chromium and oxygen radical), wolframates (tungsten and oxygen radical), and molybdates (molybdenum and oxygen radical). Sulfates include anglesite, anhydrite, barite, botryogen, brochantite, celestine, chalcanthite, cyanotrichite, devilline, epsomite, gypsum, hanksite, jarosite, langite, linarite, and thenardite. Crocoite is a celebrated chromate. Molybdates and wolframates include powellite, scheelite, stolzite, and wulfenite.
8. Phosphates, arsenates, and vanadates
Phosphates form when a radical of oxygen and phosphorus combines with a metal or semimetal; arsenates arsenic and oxygen in a radical; and vanadates vanadium and oxygen in a radical. Examples include adamite, apatite, autunite, bayldonite, beryllonite, brazilianite, clinoclase, duftite, lazulite, legrandite, libethenite, mimetite, monazite, mottramite, olivenite, purpurite, pyromorphite, torbernite, turquoise, uranocircite, vanadinite, vauxite, and xenotime.
Silicates represent by far the largest class of mineral species, simply because silicon and oxygen form the majority of Earth’s crust. These minerals are formed when silicon and oxygen form a tetrahedral silica radical that combines with metals or semimetals. Several subclasses of silicates each produce many examples, as follows:
Nesosilicates: almandine, andradite, boltwoodite, braunite, cuprosklodowskite, datolite, dumortierite, euclase, forsterite, grossular, kyanite, phenakite, pyrope, sillimanite, spessartine, staurolite, titanite, topaz, uranophane, uvarovite, willemite, and zircon.
Sorosilicates: allanite, axinite, bertrandite, clinozoisite, epidote, hemimorphite, hubeite, ilvaite, kinoite, kornerupine, murmanite, piemontite, pumpellyite, vesuvianite, and zoisite.
Cyclosilicates: benitoite, beryl, cordierite, dioptase, dravite, elbaite, eudialyte, liddicoatite, milarite, papagoite, pezzottaite, schorl, steenstrupine, sugilite, and uvite.
Inosilicates: actinolite, aegirine, arfvedsonite, astrophyllite, augite, bavenite, bustamite, charoite, cummingtonite, diopside, enstatite, pargasite, pectolite, riebeckite, serandite, shattuckite, spodumene, tremolite, and wollastonite.
Phyllosilicates: ajoite, amesite, antigorite, apophyllite, carletonite, cavansite, chrysocolla, clinochlore, dickite, kaolinite, margarite, neptunite, okenite, pentagonite, petalite, phlogopite, stilpnomelane, talc, and zinnwaldite.
Tectosilicates: adularia, afghanite, albite, analcime, cancrinite, chabazite, clinoptilolite, danburite, gmelinite, goosecreekite, harmotome, haüyne, heulandite, labradorite, lazurite, leucite, mordenite, natrolite, nepheline, orthoclase, pollucite, sanidine, scolecite, sodalite, stellerite, stilbite, and tugtupite.
10. Organic compounds
The rare and odd organic compounds classified as minerals include salts of organic acids (examples: mellite and whewellite), hydrocarbons (example: karpatite), and several miscellaneous organic compounds.
All specimens from the David J. Eicher Mineral Collection; images © David J. Eicher