Mineral

• 1. Introduction

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  A mineral is a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form. The International Mineralogical Association, the recognized standard body for naming these substances, currently counts 6,145 official species. That number sounds precise, yet the edges of the definition stay blurry. Native mercury crystallizes only below minus 39 degrees Celsius, and it is still called a mineral, grandfathered in before the modern rules existed. Water ice counts. Liquid water does not. Quartz and stishovite share one compound, silicon dioxide, and yet they are two separate minerals. So what exactly makes a substance a mineral, and who gets to decide? Why do geologists argue over whether a seashell or a quasicrystal belongs in the kingdom at all? And how can eight elements alone account for almost everything beneath your feet?

• 2. Drawing The Boundary

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  Tungsten carbide, urinary calculi, and seashells are all excluded by the International Mineralogical Association, because they form through human or living activity rather than geological process. The rules demand a naturally occurring substance, formed by natural geology, on Earth or other bodies in space. Yet origin alone does not settle the matter. Evenkite is derived from plant material, taranakite comes from bat guano, and alpersite forms from mine tailings, and all three qualify because geological processes shaped their genesis.

  Meridianiite, a hydrate of magnesium sulfate, is recognized even though it forms and stays stable only below 2 degrees Celsius. The association does not insist a mineral survive at room temperature. It asks only that the substance be stable enough for its structure and composition to be well-determined. Hypothetical substances predicted in the Earth's core get no such grace, since they cannot be reached and described.

  Amorphous materials sit outside the line entirely. Ozokerite, limonite, and obsidian lack a well-defined crystallographic structure, so the rules push them away, and proposals to admit amorphous substances have not been accepted. The association is also wary of substances that occur only as nanoparticles a few hundred atoms across, though it has set no minimum crystal size. A 2011 paper named icosahedrite, an aluminium-iron-copper alloy, after its natural icosahedral symmetry. It is a quasicrystal, ordered but not periodic, stretching the meaning of crystal in a new direction.

• 3. The Argument Over Life

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  Lowenstam wrote in 1981 that organisms can form a diverse array of minerals, some of which cannot be made inorganically in the biosphere. His point fed a long contention among geologists over the decision to exclude biogenic crystalline substances. Calcite is often biogenic, and living organisms synthesize hydroxylapatite that also turns up in rocks, so the wall between life and geology never stood clean.

  Skinner offered a wider door in 2005, treating all solids as potential minerals and folding biominerals into the mineral kingdom. He redefined a mineral as an element or compound, amorphous or crystalline, formed through biogeochemical processes. More than 60 biominerals had been discovered, named, and published before the association's listing, and many of them scatter across the 78 mineral classes of the Dana classification scheme.

  Mineral-forming microorganisms live on nearly every rock, soil, and particle surface, reaching at least 1600 metres below the sea floor and 70 kilometres up into the stratosphere. These organisms can precipitate metals from solution, helping build ore deposits, and they can also catalyze the dissolution of minerals. The association's Working Group on Environmental Mineralogy and Geochemistry studies exactly these relationships across the hydrosphere, atmosphere, and biosphere.

• 4. An Architecture Of Eight Elements

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  Eight elements make up over 98 percent of the Earth's crust by weight, and they govern which minerals can even exist. In order of decreasing abundance they are oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium. Oxygen alone composes 47 percent of the crust, and silicon accounts for 28 percent, which is why their compounds dominate everything that forms.

  The silica tetrahedron is the base unit, one silicon ion surrounded by four oxygens, giving silicon a coordination number of 4. Aluminium frequently substitutes for silicon because the two ions sit close in charge, size, and abundance. That single swap, Al3+ for Si4+, opens the door to a vast variety of minerals, since each substitution forces a rebalancing of charge elsewhere in the structure.

  The distinctive minerals of most elements are rare, appearing only where geology has concentrated them past the point where common minerals can hold them. Hydrothermal circulation is one such concentrator. At the extreme pressures of the mantle, many silicates such as olivine and garnet shift to a perovskite structure, where silicon abandons its usual four-fold coordination for six-fold octahedral coordination with oxygen.

• 5. Polymorphs And Twins

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  Pyrite and marcasite share the formula FeS2, yet the first is isometric and the second is orthorhombic. They are polymorphs, minerals with one chemical formula and different structures, and the same split runs through other sulfides with the generic AX2 form. The aluminosilicates kyanite, andalusite, and sillimanite all carry the formula Al2SiO5, differing only in how aluminium is coordinated. In kyanite the second aluminium sits in six-fold coordination, in andalusite five-fold, in sillimanite four-fold.

  Diamond and graphite are both pure carbon, and their properties could hardly differ more. Diamond is the hardest natural substance, with an adamantine lustre, crystallizing in the isometric family. Graphite is very soft, greasy in lustre, and hexagonal. In diamond each carbon bonds covalently to four neighbours in a tetrahedral framework, while graphite stacks sheets held together by much weaker van der Waals forces.

  Twinning is the intergrowth of two or more crystals of a single species, its geometry set by the mineral's symmetry. Contact twins, common in spinel, join two crystals at a plane. Penetration twins grow into each other, producing the cross-shaped staurolite twins and the Carlsbad twinning seen in orthoclase. Cyclic twins repeat around a rotation axis to form threelings, fourlings, fivelings, sixlings, and eightlings, with sixlings common in aragonite.

• 6. Reading A Stone

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  The Mohs hardness scale runs from talc at 1 to diamond at 10, each mineral scratching everything beneath it. Hardness need not stay constant in every direction. Kyanite measures 5 parallel to one axis and 7 parallel to another, a weakness written into its crystallography.

  Streak, the colour of a mineral in powdered form, often betrays what body colour hides. Hematite looks black, silver, or red in hand sample, but always leaves a cherry-red to reddish-brown streak. Chalcopyrite shines brassy gold yet streaks black. The test fails for minerals harder than 7, which powder the streak plate instead of marking it.

  Cleavage follows planes of weakness in the atomic arrangement. Micas show perfect basal cleavage in one direction, fluorite and diamond cleave octahedrally in four, and sphalerite breaks six ways. The angle between planes can identify a mineral. Pyroxenes cleave in two directions at roughly 90 degrees, while amphiboles cleave at roughly 120 and 60 degrees. A drop of dilute hydrochloric acid separates carbonates from the rest, since calcite effervesces at once and powdered dolomite must be scratched before it reacts.

• 7. Naming And Classifying

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  Theophrastus sorted minerals into stones, earths, or metals in 315 BCE, in his treatise On Stones, guided by his teachers Plato and Aristotle. In 1546 Georgius Agricola, in De Natura Fossilium, split minerals into simple, compound, and composite substances. Carl Linnaeus tried again in his 1735 Systema Naturae, placing minerals beside plants and animals under the same hierarchy of Phylum down to Species, though mineralogists never warmed to his scheme.

  James Dwight Dana first published his System of Mineralogy in 1837, now in its eighth edition, assigning each species a four-part number built from composition and structure. The less common Strunz classification, named for German mineralogist Karl Hugo Strunz, grew out of Dana's system but folds in the distribution of chemical bonds. Most mineral names end in -ite, with old exceptions like galena and diamond surviving from before mineralogy was organized as a discipline.

  Silicate minerals make up about 90 percent of the Earth's crust, and quartz alone forms 12 percent, the single most abundant species. Feldspars are the most abundant group at about 50 percent, with 22 species split into alkali and plagioclase subgroups. Among the gems, roughly 20 mineral species supply about 35 of the most common gemstones, and a single species can yield several, since ruby and sapphire are both corundum.