Mineralogy

• 1. Introduction

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  Mineralogy asks a deceptively simple question: what, exactly, is a rock made of? The field began long before anyone had a name for it, with ancient Babylonian scribes cataloguing gemstones and Pliny the Elder filling his Natural History with descriptions of minerals and their properties. Today it sits at the intersection of chemistry, physics, and Earth science, and it has revealed that the planet beneath your feet follows mathematical laws as precise as any in physics. What gives a diamond its hardness? Why do crystals grow the way they do? How does the arrangement of atoms a billionth of a meter across control the shaking you feel in an earthquake? Those are the questions mineralogy was built to answer.

• 2. Centuries of Discovery

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  Al-Biruni, the Persian scientist, wrote Kitab al Jawahir, or the Book of Precious Stones, setting down observations that would influence scholars for generations. Georgius Agricola, the German Renaissance specialist, went further still, producing De re metallica in 1556 and De Natura Fossilium in 1546, works that pushed the study of minerals toward systematic science rather than folklore.

  Nicholas Steno made the next decisive step in 1669, when he observed in quartz crystals a rule now called the law of constancy of interfacial angles: the angles between corresponding faces of any two crystals of the same mineral are always equal. Jean-Baptiste L. Romé de l'Islee generalized and confirmed this experimentally in 1783. René Just Haüy, later called the father of modern crystallography, then showed that crystals are periodic structures, and that the orientations of crystal faces can be expressed as rational numbers, a principle later encoded as the Miller indices.

  In 1814, Jöns Jacob Berzelius proposed sorting minerals by their chemistry rather than by their crystal form. William Nicol developed the Nicol prism, which polarizes light, in 1827-1828, a tool that would transform optical mineralogy. Henry Clifton Sorby then demonstrated that thin slices of minerals could be identified through their optical properties under a polarizing microscope. James D. Dana published his first edition of A System of Mineralogy in 1837, and a later edition introduced a chemical classification that remains standard today.

  The final breakthrough of the classical era came in 1912, when Max von Laue demonstrated X-ray diffraction. William Henry Bragg and his son William Lawrence Bragg then developed that discovery into a practical method for determining the crystal structure of minerals, opening a new era in the science.

• 3. Reading a Mineral by Hand

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  Friedrich Mohs devised the scale that still carries his name: ten reference minerals arranged from 1, assigned to talc, up to 10, assigned to diamond. An unknown specimen can be placed on that scale simply by scratching it against the reference minerals and noting which scratch which. A few minerals, calcite and kyanite among them, show a hardness that actually varies depending on the direction in which you scratch them. For a more rigorous measurement, a sclerometer can give absolute hardness values; on that absolute scale, the Mohs intervals turn out to be uneven.

  Beyond hardness, a mineralogist examines tenacity, which describes how a mineral behaves when broken, bent, or crushed. A mineral may be brittle, malleable, sectile, ductile, flexible, or elastic, and which of those it is depends in large part on the type of chemical bond holding it together. Cleavage, another property, describes a mineral's tendency to split cleanly along specific crystallographic planes, and it is recorded both by quality and by the orientation of those planes.

  Where cleavage and parting are absent, fracture takes over. A fracture may be conchoidal, producing smooth curves resembling the inside of a shell; it may be fibrous, splintery, hackly with jagged sharp edges, or simply uneven. Luster, color, streak, luminescence, and transparency round out the suite of visual properties a mineralogist can assess without any instrument at all, and radioactivity and solubility in hydrochloric acid complete the list of measurable characteristics for a hand sample.

• 4. Crystal Architecture

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  Every crystal is a pattern that repeats. The repeating unit is called a unit cell, and the whole crystal is built by stacking that cell in three dimensions to form a lattice. The geometry of the lattice can be described by three Miller indices representing its dimensions, and the lattice's symmetry falls into one of exactly 32 possible crystal classes, known as crystallographic point groups. When translation, screw axes, and glide planes are added to those point symmetries, the total number of possible arrangements, called space groups, comes to exactly 230.

  X-ray diffraction is the primary instrument for reading that architecture. X-rays have wavelengths roughly the same size as the spaces between atoms, so they scatter off atoms in a crystal and interfere with one another constructively and destructively, producing distinctive patterns of intensity. When a mineral is ground to a powder, the X-rays sample every possible crystal orientation at once, and the resulting pattern of lines identifies the mineral unambiguously. This technique separates quartz from its polymorphs tridymite and cristobalite, minerals that look identical in a hand sample but differ in internal structure. The halite structure, designated space group Fm3m, is shared by sylvite, periclase, bunsenite, galena, alabandite, chlorargyrite, and osbornite, a fact that powder diffraction makes immediately legible.

• 5. Chemistry and Light

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  A few minerals are pure chemical elements, sulfur, copper, silver, and gold among them, but the vast majority are compounds. The traditional method for identifying their composition is wet chemical analysis, dissolving a mineral in an acid such as hydrochloric acid and then identifying the elements in solution through colorimetry, volumetric analysis, or gravimetric analysis. Since 1960, most chemical work has moved to instruments. Atomic absorption spectroscopy vaporizes the dissolved sample and measures its absorption spectrum in visible and ultraviolet light. X-ray fluorescence, electron microprobe analysis, atom probe tomography, and optical emission spectrography are among the other techniques now standard in the field.

  For optical properties, a polarizing microscope carries two filters set perpendicular to each other. With no sample, one filter blocks all the light that passed through the other. An anisotropic mineral, meaning one whose crystal symmetry falls outside the cubic system, changes the polarization of passing light so that some of it slips through the analyzer and becomes visible. An isotropic mineral, by contrast, appears dark under crossed polarizers. When an isotropic crystal is immersed in a calibrated liquid with a lower refractive index and the microscope is defocused, a bright ring called a Becke line appears at the crystal's edge. By testing the crystal in liquids with different indices, a mineralogist can pin down the refractive index to within a narrow margin.

• 6. Naming and Classifying

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  In 1959, the International Mineralogical Association established the Commission of New Minerals and Mineral Names to bring order to a field that had accumulated centuries of overlapping and sometimes contradictory names. In July 2006, that body merged with the Commission on Classification of Minerals to become the Commission on New Minerals, Nomenclature, and Classification. Over 6,000 minerals now carry names, with about 100 new species added each year. The Manual of Mineralogy groups them into thirteen classes, from native elements and sulfides through silicates.

  Biomineralogy has opened a separate branch of the field, bridging mineralogy with paleontology and biology. It studies how living plants and animals control the growth of minerals within their bodies and how those minerals are replaced after the organism dies. Isotopic techniques drawn from chemical mineralogy let researchers reconstruct growth patterns in living organisms and recover the original mineral content of fossils.

  Mineral evolution, a newer approach, examines how the diversity of minerals on Earth changed through geological time, tracing the co-evolution of the geosphere and the biosphere and probing the role minerals may have played in the origin of life, including processes such as mineral-catalyzed organic synthesis and the selective attachment of organic molecules to mineral surfaces.

• 7. Mineral Ecology and Big Data

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  In 2011, researchers began building the Mineral Evolution Database, combining the crowd-sourced site Mindat.org, which holds over 690,000 mineral-locality pairs, with the official International Mineralogical Association list of approved minerals and age data drawn from geological publications. The database made it possible to apply statistical methods to questions that had never before been quantifiable.

  In a 2015 paper, Robert Hazen and colleagues analyzed how many minerals involve each chemical element as a function of that element's abundance. Earth, with over 4,800 known minerals incorporating 72 elements, shows a power law relationship between abundance and mineral count. The Moon, with only 63 minerals and 24 elements, based on a much smaller sample, shows essentially the same relationship. That parallel suggests the more common minerals on any planet might be predicted simply from its chemical composition.

  The distribution has a long tail: 34 percent of known minerals have been found at only one or two locations worldwide. The model therefore predicts that thousands of mineral species either remain undiscovered or once existed and were lost to erosion, burial, or other processes, pointing to a genuine element of chance in the formation of rare minerals.

  Network theory applied to datasets of carbon minerals has revealed new patterns in how minerals are distributed and which ones tend to coexist, along with the geological, physical, chemical, and biological conditions associated with each pairing. That kind of analysis now points investigators toward locations where new deposits, or even new species, are most likely to be waiting.