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Minerals

Do you know what minerals are? The typical answer is, "Crystalline inorganic compounds". "That form through geological processes", many would add. Yes indeed, the overwhelming majority of minerals (like orpiment or tourmalines) are inorganic, crystalline, and restricted in occurrence to rocks.
Orpiment Tourmaline
Orpiment Tourmaline (dravite)

However, there are also organic minerals - for instance, the calcium oxalate whewellite (CaC2O4H2O) found in coal seams. Then, there are things like hydroxylapatite crystals in teeth, and tiny plates of aragonite secreted by the mollusk mantle to form the shiny lining of a seashell or a nobly pearl.
Pearls Bioapatite
Pearls Bioapatite

What would we call these wonderful creations of Nature? It turns out, biologically formed hydroxylapatite and aragonite are also minerals - it's just that some of their characteristics do not really comply with the conservative definition cited above. Some scientists prefer to use the term "biominerals" when referring to organically produced crystalline substances. Biomineralogy is a young, but rapidly evolving science of great importance to the human race. Just one example: When our organism is malfunctioning, some chemical elements fall out of the normal metabolic cycle and accumulate as crystalline buildup inside the bladder, kidneys and other organs. In kidney stones, the buildup consists of various oxalates and phosphates similar to inorganically produced minerals in sedimentary and other rocks (for instance, whewellite). It is impossible to pinpoint the source of such pathologies or find ways to prevent them, unless we know exactly what the stones are composed of and under what conditions they form. Studying minerals making up rocks is important for ascertaining the origin of biominerals.

Crystallinity is common to most, but not all, minerals. The symmetric internal arrangement of atoms in a mineral's structure is reflected in the external shape of its crystals and its physical properties (click here for some examples). However, minerals more commonly form irregularly shaped grains, crystals that are too small or shapeless masses than large crystals with a recognizable symmetry. Whether or not a specific mineral is crystalline can be readily detected with X-ray and electron diffraction techniques.
Zircon Minerals containing uranium and thorium are crystalline initially, but may eventually lose a long-range ordered arrangement of atoms in their structure because of progressive damage from radioactive decay and alpha-particle emission. In effect, these minerals are destroying themselves from within! Zircon (ZrSiO4), for instance, commonly contains both U and Th, and, consequently, may not show any peaks on X-ray patterns. A few other minerals do not contain any radioactive elements, but are nevertheless amorphous (non-crystalline). Such are allophane (hydrous aluminum silicate) and opal, consisting of tightly-packed spheres of amorphous silica (SiO2). These "unconventional" minerals have been known for so long and are so important for certain geological environments that we mineralogists have decided to keep them in our professional lingo.
Metamict Zircon
Keeping the above exceptions in mind, one restriction is upheld by mineralogists around the globe: "Antropogenic substances ... are not regarded as minerals" Nickel & Grice, 1998, Canadian Mineralogist, 36, 913-926). To rephrase, anything that looks like a mineral, but formed as a result of people's interaction with the environment, is unworthy of bearing the noble title of mineral. The reasons for this restriction are obvious: The products of human labor (buildings, mechanisms, etc.) and various types of waste, when exposed to the elements, undergo physical and chemical transformations that generate new chemical compounds - like green patina on a bronze statue, or carbonate scales inside a water pipe. These substances are too numerous and diversified to be easily systematized, while their spatial distribution is too restricted (to the Earth's surface or slightly below it) to have any significant lasting effect on the life of our planet (so far, at least). Having said that, I must admit, it is usually people with a degree in mineralogy who study these substances. Some of them are so fond of the subjects of their research that they refer to them affectionately as technogenic minerals. Such "minerals" are all around us and, however insignificant on the geological scale of things, are of tremendous practical importance. For instance, leakage of uranium from a buried container with radioactive waste can be a serious environmental problem. However, some of that uranium may become bound and immobilized in technogenic "minerals" deposited right next to the container. The understanding of what "minerals" are likely to form is essential for prognosticating the future of radioactive-waste repositories. Similar to "biomineralogists", technologists and engineers facing these kinds of problems rely on the mineralogical wisdom acquired by decades and decades of meticulous observation, accurate lab research and interpretative work.
The distinction between technogenic compounds and true minerals is not always straightforward. To illustrate that, we have picked two photos: One shows a ladder from an old mine incased in a thick "jacket" of aragonite (CaCO3), another a spray of gypsum crystals (CaSO42H2O) perched on a rusty bolt.Clearly, the aragonite and gypsum would not have existed, were it not for the ladder and bolt (i.e. had there been no human intervention). In these examples, manmade objects served as a mere "construction site" for growing crystals. There are also more complex cases where the same crystalline compound may form in both geological and anthropogenic environments. Actually, quite a few minerals were first described as technogenic substances, and only later found in the geological environment. Srebrodolskite (Ca2Fe2O5) was initially described in burning coal dumps near Chelyabinsk (Russia), but a detailed study of lavas from the extinct Bellerberg volcano in Germany revealed that this mineral also forms naturally, by high-temperature metamorphism of xenoliths (fragments of country rock) trapped in the lava, i.e. in much the same way as it does in the tailings of the Chelyabinsk coal mines. Aragonite

At present, there are about 4,100 mineral species known. Three to four dozen new ones are discovered every year around the world. Each new species has to be submitted by its discoverer(s) to the Commission on New Minerals and Mineral Names for their approval. The Commission, working under the aegis of the International Mineralogical Association (IMA) and consisting of some 40 mineralogists from about 30 countries, votes on both the proposed mineral and its name. Approval requires a "yes" vote from two-thirds or more of the Commission membership. The greatest number of proposals is typically submitted by scientists from North America and Russia, which is reflected in the overall statistics on the geographic distribution of mineral type localities (i.e. places where one or more new mineral species have been found). Among type localities, the indisputable leader is the Lovozero Mountains in the Kola Peninsula (Russia).
Lovozero Lovozero comprises diverse alkaline rocks and truly is a mineralogist's paradise. Through the second half of the 1900s, the record numbers of new mineral species have been discovered by Pete J. Dunn of the Smithsonian Institution (Washington) and Alexander Khomyakov of the Institute for the Mineralogy of Rare Elements (Moscow). The American and Russian scientists also lead in terms of the number of mineral species that have been named after them.
Lovozero Mountains
Of course, some minerals are much more common and, in that sense, more important than others. Actually, the bulk of the Earth's crust and mantle is made up of about 20 minerals - that's all! The most common mineral in the crust is quartz (12% by weight). Quartz is truly ubiquitous in crustal rocks. It is a chemically stable and mechanically durable mineral that accumulates during weathering and erosion forming the bulk of clastic (i.e., broken-up) sedimentary material. Pure quartz sand is eventually lithified into arenite composed almost entirely of quartz. Sandstone buried into the crust is converted under pressure and heat to quartzite, a metamorphic quartz-rich rock. Metamorphic material in the lower crust can melt and then harden producing granite and other igneous rocks, in which quartz is the principal rock-forming mineral. Finally, hot water solutions percolating through the crust can precipitate quartz in association with various ore minerals. Such formations are called hydrothermal veins. Second in abundance are various feldspar-group minerals, which also occur in a wide spectrum of crustal rocks.
(Mg,Fe)SiO3 "perovskite", a mineral chemically equivalent to, but structurally different from, orthopyroxene, is believed to be the most volumetrically significant constituent of the Earth's mantle. It is restricted in occurrence to the lower mantle (between 670 and 2890 km of depth), where pressures are high enough to keep the silicon coordinated by six oxygen atoms. (Mg,Fe)SiO3 and structurally similar Ca-Si "perovskite" probably make up as much as 85% of the lower mantle by weight. I say "probably" because no one has really seen a sample of these lower-mantle minerals, even though the available scientific evidence leaves very little doubt about their existence. Perovskite structure
Structure of Perovskite
Many other minerals (calcite, beryl and pyrite, to name a few), although not as abundant as quartz, occur in a wide variety of rocks from around the world. A fair proportion of minerals are paragenesis-specific, i.e. even though they are not unique to a certain geographic locality, they can only be found in a certain type of rock. One example is dioptase; this hydrous copper-silicate mineral forms exclusively in the near-surface environment, where sulfides of Cu, Fe, Zn, Pb and other ore minerals react with oxygen and moisture in the atmosphere to produce the so-called oxidation zone.

Then, there are minerals that have so far been reported from a single locality (e.g., charoite and kovdorskite), and those that are known at just a few places in the entire world (e.g., murmanite and tinaksite). The reasons for such selectivity are uncertain and, naturally, have something to do with extraordinary juxtaposition of geological factors conducive to the crystallization of these exotic minerals.
Charoite In much the same way, the many endemic species of animals living in Australia owe their existence to that continent's unique geography and its prolonged isolation from other large landmasses. One might think that these exceedingly rare minerals with unpronounceable names occur in minuscule grains discernible only with the electron microscope. Nope... charoite, for example, forms blocks and lenses many hundreds of pounds in weight, while murmanite is a major constituent of some alkaline rocks at Lovozero.
Charoite
Contrary to certain stereotypes, minerals are not immortal (and, alas, no diamond is really forever). Minerals come into this world, born out of bubbling lava, sediment-laden seawater, or chemical reactions deep in the Earth's interior, they live and then perish consumed by weathering or new chemical reactions that render the previously-formed assemblages of minerals unstable. Some minerals have existed for billions of years, approaching our planet in age. For example, the oldest terrestrial material known to date is grains of the mineral zircon found in Western Australia and dated at 4.4 billion years (Wilde et al., Nature, 2001, 409, 175-178). And, yes, you read the number right - this zircon is just a few hundred million years younger than the Earth itself! Other minerals exist only briefly. Water-soluble Na carbonates in lavas of the Oldoinyo Lengai volcano in Tanzania, for example, will have disappeared within a few days after their eruption. Removing such evanescent minerals from their natural "habitat" may be destructive if no precaution is taken to conserve them. One example is the mineral downeyite, named after Wayne Downey Jr. who found and preserved it for research. Downeyite is a selenium dioxide that sublimates from poisonous gases emanating from burning coal dumps in Pennsylvania. This mineral evaporates just as soon as it is removed from its fiery cradle and exposed to oxygen in the atmosphere. The Museums, where reference samples of downeyite were sent to, did not take the necessary precautions and, alas, the only surviving specimen of this mineral is in its discoverer's possession. As to the diamond, it is actually thermodynamically unstable under the ambient conditions, but the process of diamond conversion to graphite (form of crystalline carbon stable at low pressure) is so sluggish that the Oppenheimers have nothing to worry about on that front.

Halite Minerals grow at a various pace. The fastest-growing crystals in nature are probably halite (NaCl) and other soluble minerals precipitating in lagoons and seawater puddles on a sunny day. They form because evaporation of water increases the proportion of salts dissolved in a given volume of solution. When some critical level of salt saturation is reached, spontaneously forming clusters of atoms stop breaking apart and start assembling into larger clusters (nuclei) and, ultimately, into crystals. The biocrystals that make up our bones and teeth must be nucleating and growing fast, too. Well, at least, on the geological time scale ...
Minerals that compose igneous and metamorphic rocks generally have slow growth rates. One exception is crystals of plagioclase and mafic silicates found in the glassy matrix of basalts and other volcanic rocks. It has been estimated that plagioclase crystals in basaltic lava can grow at a rate of 3-5 cm annually. This is way faster than tiny plates of hydroxylapatite inside the human tooth! In other cases, millions of years and even eons will have elapsed before a crystal has grown large enough to be visible in the microscope. In some cases, mineral's development is not continuous and involves two or more "growth spurts" separated by intermissions. During such intermissions, the mineral can be partly dissolved, fractured, deformed, replaced by, or "iced" with crystals of, another mineral, and affected by geological processes in many other ways. An observant mineralogist will use any evidence of such transformations and changes to reconstruct the "life-story" of a mineral. We usually refer to it as the evolutionary history or genesis of a mineral. That is why the word "paragenesis" means to an assemblage of minerals formed nearly simultaneously by means of the same geological process or processes.

Other things, that help us decipher how minerals form, are the shape of their crystals and crystal groups (called aggregates), their chemical composition and crystal structure. For example, a round cluster of elongate crystals radiating from its center (called spherulite) usually indicates fast growth and heterogeneous nucleation, i.e. simultaneous nucleation of many crystals on a "seed" that eventually becomes concealed in the center of the spherulite. Hollow-sided hopper crystals also form when growth rates are high (e.g., from oversaturated solutions). The Museum exhibit in the hallway of the Wallace Building features a display dedicated to the symmetry, shapes and patterns of natural crystals and their aggregates.
Display on Crystals and Aggregates
Display on Crystals and Aggregates
The chemistry of a mineral is a sensitive indicator of the conditions in which that mineral crystallized. In that respect, particularly informative are minerals that form solid solutions (continuous or partially incomplete series of intermediate compositions) with other minerals. For example, metamorphic amphiboles crystallizing under low temperatures and medium pressures will contain less sodium and aluminum than amphiboles crystallizing under relatively higher temperatures in the same type of protolith (i.e. precursor rock). That is why greenschists and lower-grade metamorphic rocks typically contain green actinolite, whereas higher-grade amphibolites and gneisses comprise black hornblende.
Zoning in Amphibole By doing high-temperature and high-pressure experimental studies, petrologists (scientists who study rocks) have been able to constrain the conditions at which actinolite is no longer stable and gives way to hornblende. In our example, these conditions will correspond to the boundary between the greenschist and amphibolite facies of metamorphism. In many cases, physical and chemical conditions change while a mineral is growing, triggering compositional changes across the mineral grain. We refer to these variations as zoning (or zonation) and use them for interpretation of growth conditions. In a similar fashion, the structural characteristics of a mineral also respond to changes in its crystallization environment and that, too, can be used to "read" rocks and their evolutionary history.
Zoning in Amphibole

So, why exactly do we need to study minerals? Gosh, we really don't know where to begin. Most of what we find at or below the Earth's surface (except groundwater and aqueous fluids, melts, oil, natural gas and glassy volcanic rocks), much of the particulate matter in the atmosphere and oceans, and solid materials that make up other planets consist of minerals. Be it methane ice in the Neptunian atmosphere, olivine in the Earth's upper mantle, or tiny ferrihydrite crystals in the core of iron-storage proteins, minerals are integral components of our world. There is increasing evidence that they played a key role in the origin of life on our planet (see Elements, 2005, 1, no. 3). Need we say that the progress of technology, science and art would not have been possible without mineral resources? Did you know that an average North American needs about 9.9 tons (20,000 pounds) of mineral resources annually, including 220 kg of clay, 160 kg of salt, 25 kg of aluminum, 10 kg of copper, 6 kg of lead and 20 g of uranium?
Lead, for example, is extracted from the mineral galena (PbS), whereas uranium from uraninite (UO2). In some cases, a whole assemblage of minerals (rock) is utilized for practical purposes. Aluminum, for example, is extracted from the sedimentary rock bauxite, which is composed of various Al hydroxides and silicates. Granite, used in construction, and blue lapis lazuli, used as an ornamental stone, are also rocks consisting of several different minerals. Truly, what can't be grown has to be mined! Granite pillars
St. Isaac's Cathedral, St. Petersburg, Russia
Last, but not least, minerals have been used as prototypes in the development of many advanced materials. For example, the unique structural characteristics of zeolites result in a wide spectrum of potentially useful physical and chemical properties, which had already been known in the first half of the 1900s. Low-cost synthesis of zeolites, pioneered by Robert M. Milton and Donald W. Breck (late 1940s), and the discovery of the strong catalytic properties of zeolites by Jule A. Rabo (1957) revolutionized the petroleum industry. The use of synthetic zeolites (like zeolite Y equivalent to the mineral faujasite) in catalytic petroleum cracking since the early 1960s has drastically improved the yield of high-quality fuels, while reducing the cost of fuel production and its environmental impact (Sherman, 1999, Proceedings of the National Academy of Sciences, 96, 3471-3478). Today, zeolites are used in a wide spectrum of industrial applications, from oil refining and synthesis of fiber for permanent-press fabrics to the production of environmentally friendly detergents and control of nitrogen-oxide emission by diesel engines.

Although one of the oldest Earth-science disciplines, mineralogy is, at the same time, one of the most progressive. Much of the contemporary mineralogical research relies on cutting-edge analytical and computational techniques, bridging the microcosm of atoms and electron transitions with the macrocosm of tectonic plates and planetary surfaces. Working with minerals is not just a science career, it is a great way to explore the natural world around us and the forces that have been tirelessly shaping and reshaping it over billions of years.