Read a mineral's chemical formula, classify minerals by composition and predict properties from ionic, covalent and metallic bonding

Learning goal: Explain what a mineral's chemical formula represents in terms of atoms, elements and ions; classify minerals into groups by chemical composition; distinguish ionic, covalent and metallic bonding and identify which bond types hold each mineral group together; use the bond type to predict hardness, cleavage, solubility and electrical behavior and to design a safe test that tells ionically and covalently bonded materials apart; distinguish a mineral from an ore; and evaluate the extraction and recycling of mineral resources using that chemistry.

Atoms, elements and the crust. Every mineral is built from atoms, and an atom's identity is fixed by the number of protons in its nucleus: that number defines the element. Electrons surround the nucleus, and it is the outermost electrons that decide how one atom joins another. Only a handful of elements make up almost all of Earth's crust by mass: oxygen (about 46%) and silicon (about 28%) dominate, followed by aluminum, iron, calcium, sodium, potassium and magnesium; together those eight account for roughly 98% of the crust. That is why most minerals are silicates, compounds built around silicon and oxygen, and why the uncommon elements that technology depends on, such as copper, gold, lithium or the rare earths, have to be concentrated by geological processes before they can be mined.

Atoms in minerals are usually treated as ions, atoms that have lost or gained electrons. Sodium gives up one electron and becomes Na⁺, and chlorine takes one and becomes Cl⁻, almost completely. For silicon and oxygen the charges written Si⁴⁺ and O²⁻ are formal charges used for bookkeeping: silicon and oxygen share electrons about as much as they transfer them (see the bonding section below), so the real charge on each atom is smaller than the label, but counting silicon as +4 and oxygen as −2 still gives the correct formula. The formal charges must balance in any mineral, and the sizes of the ions decide how they pack. Ions of similar size and charge can substitute for one another in a crystal, which is why the composition of some minerals is "defined within limits" rather than fixed: magnesium (Mg²⁺) and iron (Fe²⁺) are close enough in size and identical in charge to replace each other freely in olivine.

Reading a chemical formula. A formula states which elements a mineral contains and in what ratio. Halite is NaCl: one sodium ion for every chloride ion. Quartz is SiO₂: two oxygen atoms for every silicon atom, arranged in an endless framework rather than as separate molecules, so the formula gives a ratio, not a molecule. Calcite is CaCO₃: a calcium ion bonded to a carbonate group, CO₃²⁻, in which one carbon is bonded to three oxygens. Orthoclase feldspar is KAlSi₃O₈. Parentheses show a site where either of two ions may sit: olivine is written (Mg,Fe)₂SiO₄, meaning two ions of magnesium or iron, in any proportion, for every silicate tetrahedron. A subscript applies to the element or bracketed group it follows, so the 2 after (Mg,Fe) applies to the whole site; a number written in front of a whole formula, as in 2 NaCl, is a coefficient and multiplies the whole formula. When you draw or build a model of a formula, every ion or group named in the formula should appear in the stated ratio, and the charges should add to zero.

Here is the model worked out for galena, PbS, lead sulfide. The formula says one lead ion (Pb²⁺) for every sulfide ion (S²⁻), charges balancing at zero. Lead is one of the heaviest common metals, so a mineral made half of lead by count, and about 87% lead by mass, must be dense: galena's specific gravity is about 7.5, nearly three times that of quartz. In galena, as in many (not all) sulfides, the bonding has a partly metallic character, which is why galena has a bright metallic luster, is opaque and leaves a gray streak. Its ions pack in the same cubic arrangement as halite, so it cleaves into cubes. Composition, bonding and structure together explain nearly every property you can test.

Classifying minerals by chemical composition. Mineralogists group minerals by the negatively charged unit in the formula. Silicates contain the silicon-oxygen tetrahedron SiO₄⁴⁻ (quartz, feldspar, mica, olivine, pyroxene, amphibole, clay minerals) and make up the great majority of the crust. Carbonates contain CO₃²⁻ (calcite, dolomite, aragonite). Oxides are metal-oxygen compounds (hematite Fe₂O₃, magnetite Fe₃O₄, corundum Al₂O₃). Sulfides are metal-sulfur compounds (pyrite FeS₂, galena PbS, chalcopyrite CuFeS₂, sphalerite ZnS). Sulfates contain SO₄²⁻ (gypsum CaSO₄·2H₂O, barite BaSO₄). Halides contain a halogen ion (halite NaCl, fluorite CaF₂). Native elements occur uncombined (gold, copper, silver, sulfur, diamond and graphite, both pure carbon). Phosphates contain PO₄³⁻ (apatite, the mineral of bones and teeth and the source of most phosphorus fertilizer). A classification chart therefore has two levels: first the compositional group, then, within it, the crystal structure, which the next lesson takes up.

Three kinds of chemical bonding. Ionic bonding happens when one atom transfers electrons to another and the resulting positive and negative ions attract electrostatically. The attraction acts equally in all directions, so ions pack in whatever geometry fits their sizes. Ionic minerals such as halite and fluorite are moderately hard and brittle. Some, such as halite, dissolve readily in water, because water molecules are polar and can pull ions off the crystal one at a time; others, such as fluorite, hold their ions so strongly that they barely dissolve at all, so solubility is a clue to bonding, not a rule. They do not conduct electricity as solids, because the charges cannot move, but their solutions do conduct. Covalent bonding happens when atoms share pairs of electrons. Shared pairs point in fixed directions, so covalent structures are rigid networks. Diamond, in which every carbon shares electrons with four neighbors in all directions, is the hardest natural substance and does not dissolve in water or acids; the silicon-oxygen bonds inside a silicate tetrahedron are roughly half covalent and half ionic in character, strong and directional, which is why quartz is hard, insoluble and an electrical insulator. Metallic bonding happens when metal atoms pool their outer electrons into a shared "sea" that moves freely through the whole crystal. Native gold, copper and silver are metallically bonded: the mobile electrons conduct electricity and heat, reflect light to give metallic luster, and let layers of atoms slide without breaking, so the metals are malleable rather than brittle. A fourth, much weaker attraction, the intermolecular force between neutral sheets or molecules, holds the carbon sheets of graphite together and explains why graphite, the same element as diamond, is soft enough to write with.

Which bonds hold each group together. Few minerals have a single bond type. Halides and most oxides are essentially ionic. Native metals are metallic. Carbonates are mixed: strong covalent bonds inside the CO₃ group, weaker ionic bonds between the group and the calcium ion, so calcite is only hardness 3 and breaks along planes that cut the weaker bonds. Silicates are mixed in the same way: strong bonds inside each tetrahedron, weaker ionic bonds from the tetrahedra to the metal ions that link them. Sulfates and phosphates are built like carbonates: strongly bonded SO₄ or PO₄ groups held to metal ions by weaker ionic bonds, which is why gypsum and apatite are soft to moderate. Sulfides are mostly covalent, and their bonding varies from one sulfide to another: in galena and pyrite it has a metallic contribution, which makes them opaque with a metallic luster, while sphalerite has little, so it is commonly translucent with a resinous to adamantine luster; several sulfides are useful semiconductors. Predicting a property therefore means asking which bond is weakest along the direction being tested.

Testing for bond type safely. Ionic and covalent materials can be told apart without heating or hazardous chemicals. First, try dissolving a small piece in room-temperature distilled water; ionic salts such as halite dissolve, while covalent networks such as quartz do not. Failing to dissolve does not prove a sample is covalent: fluorite and calcite are largely ionic yet almost insoluble in water, so an insoluble sample is classified by its hardness and the way it breaks instead. Second, test the solution with a low-voltage conductivity tester (a 9 V battery, a resistor and an LED, or a multimeter on the resistance range): a solution that lights the LED contains mobile ions. For the known classroom samples used here (salts, quartz, calcite, sugar) that means the solid was ionic, but it is not a general proof: some molecular substances, such as acids, react with water to form ions, so an unrestricted unknown needs more than one test. This second test matters, because table sugar, a covalently bonded molecular solid, dissolves readily and yet its solution does not conduct. Third, compare hardness and the way the sample breaks: covalent networks are typically very hard, and they break with curved fractures where no plane is weaker (quartz) or cleave where the structure has weaker planes (diamond cleaves along octahedral planes); ionic solids are moderately hard and cleave along flat planes; metals dent and flatten. Only known, non-toxic specimens are used for the solubility test, nothing is tasted, and hands are washed afterwards.

Minerals and ores. A mineral is defined by its chemistry and structure. An ore is an economic term: a rock or mineral deposit from which a metal or other useful substance can be extracted at a profit under current prices and technology. A deposit can stop being ore when prices fall or become ore when a cheaper process is invented, without any change in the rock. Hematite and magnetite are the main iron ores; bauxite, aluminum ore, is not a single mineral but a mixture of aluminum hydroxide minerals; chalcopyrite is the main copper ore; galena yields lead; sphalerite yields zinc; halite is the source of sodium and chlorine; apatite in phosphate rock supplies phosphorus; uraninite is the main uranium ore; platinum occurs as a native metal concentrated in certain layered mafic intrusions. Ores are rarely rich. Copper is commonly mined at grades under one percent, so a hypothetical mine working ore at 0.5% copper must process at least 200 metric tons of ore for every metric ton of copper it produces, leaving about 199 metric tons of tailings. That is an ideal minimum: it assumes every bit of copper in the ore is recovered and leaves out the overburden removed to reach the ore. With a recovery of 90%, the ore needed is 1 ÷ (0.005 × 0.90) ≈ 222 metric tons.

Extraction near people, and recycling. Whether it is appropriate to extract a mineral in a populated area depends on the chemistry of the ore as much as on its location. Sulfide ores (copper, lead, zinc) expose pyrite that reacts with oxygen and water to make sulfuric acid, so drainage from the pit and tailings must be collected and treated for decades. Uranium ore and phosphate rock carry naturally radioactive elements, so dust, radon and tailings need containment. Salt can be extracted by solution mining, dissolving it underground and pumping brine, with a small surface footprint but a brine-disposal problem. Iron oxide ores are chemically benign but are dug from very large open pits. Every case is judged on the same criteria: grade and tonnage, extraction method, dust, noise, traffic, water use, water contamination, subsidence, the reclamation plan and the jobs and taxes it brings. Recycling reduces all of this. Because metals are elements, they can be recovered and re-refined indefinitely: aluminum cans and foil, copper pipe and wire, and steel ("tin") cans are collected, separated by density and by magnets (steel is magnetic, aluminum and copper are not), melted and re-cast. Recycled aluminum skips the energy-hungry electrolysis that separating aluminum from its oxide requires, so it saves a large share of the energy of primary production. A recycling system for a product therefore starts by listing the elements in it, then designs a separation step that exploits a physical difference between them.

Worked example, reading a formula: Olivine is (Mg,Fe)₂SiO₄. Elemental composition: magnesium and/or iron, silicon and oxygen, in the ratio 2 : 1 : 4. Charge check: two divalent ions give +4, one silicon (formal charge +4) gives +4, four oxygens (formal charge −2) give −8, total zero. Property predictions: the tetrahedra are isolated and linked only by ionic bonds to Mg and Fe, so olivine has no good cleavage direction and breaks with a conchoidal fracture; the bonds are strong in all directions, so it is hard (about 6.5 to 7); the iron-rich variety is denser and darker than the magnesium-rich one, because iron atoms are heavier than magnesium atoms; and because it forms at very high temperatures and its ionic bonds are attacked by water and acids, olivine weathers quickly at the surface.

Worked example, predicting properties from bonding: Three specimens: halite, quartz and native copper. Halite is ionic, so it should dissolve in water, give a conducting solution, cleave into cubes and have a low-to-moderate hardness (it is 2.5). Quartz is a covalent-ionic network, so it should be insoluble, non-conducting, hard (7) and break with curved fractures. Copper is metallic, so it should be insoluble in water, conduct electricity as a solid, have metallic luster and flatten under a hammer instead of shattering. Each prediction is testable, and one failed prediction would send the classification back for revision.

Worked example, ore grade and recycling: A hypothetical copper deposit averages 0.5% copper. Assume, as an ideal minimum, that all of the copper is recovered. Producing 1 metric ton of copper then needs 1 ÷ 0.005 = 200 metric tons of ore, and leaves 199 metric tons of tailings. If the mine plans 20,000 metric tons of copper per year, it must mine and process at least 4,000,000 metric tons of ore per year and store 3,980,000 metric tons of tailings. Real mills recover less than all of the metal: at 90% recovery the same output needs 4,000,000 ÷ 0.90 ≈ 4,440,000 metric tons of ore, and the rock stripped to reach the ore is extra. Compare a city recycling program collecting 2,000 metric tons of scrap copper wire per year: that scrap is nearly pure copper, so it replaces at least 400,000 metric tons of mined ore. The arithmetic shows why scrap metal is the highest-grade "ore" there is.

Worked example, the conductivity test: A student dissolves samples A and B in distilled water; both disappear. The LED tester lights for solution A and stays dark for B. Conclusion, for samples drawn from the classroom set: A released mobile ions, so it was ionically bonded (halite fits); B dissolved as whole neutral molecules, so it was a covalent molecular solid (sugar fits). A sample C that will not dissolve at all and scratches glass is a covalent network solid (quartz fits); insolubility alone would not show that, because calcite also fails to dissolve, but calcite is soft (3) and cleaves into rhombs. Note what each test cannot do: solubility alone does not separate A from B, and a non-conducting result for an undissolved solid says nothing, because there is nothing in solution to carry charge.

Representation task: Build or draw two models. (1) A halite unit cube: the eight corner positions, the face and edge positions and the body center, with sodium and chloride ions alternating in every direction; label the ions and their charges and write the formula the model represents. (2) A silicon-oxygen tetrahedron: one silicon at the center, four oxygens at the corners; label the charge of the whole unit (−4) and explain in one sentence how sharing corner oxygens between tetrahedra lets quartz reach the ratio SiO₂. Add a third, 3-D model of metallic bonding: a layer of same-size balls (metal ions) with small beads or loose rice scattered through the gaps to stand for the shared electron sea, and show how a layer can slide without breaking. Under each model write the bond type that holds it together and one property that follows.

Test design task: Using only distilled water, small labeled samples (halite, quartz, calcite, sugar), a low-voltage LED conductivity tester and a hardness kit, write a procedure that classifies each sample as ionic, covalent network or covalent molecular. Calcite will not dissolve, so your procedure must say how it recognizes an insoluble ionic mineral rather than calling it covalent. State the safety rules (goggles, no tasting, wash hands, low voltage only) and the expected result for each sample before you run it.

Independent performance: A hypothetical company proposes an open-pit copper mine, ore grade 0.6%, 3 km from a town of 20,000 people, in a valley drained by a river that supplies the town's water. Write an evaluation of at least 300 words. Success criteria: it identifies the ore mineral and its group (chalcopyrite, a sulfide); it explains the acid-drainage chemistry the sulfide creates and what would be required to manage it; it computes the metric tons of ore and waste per metric ton of copper, stating the recovery assumed; it lists at least four criteria for judging the site, including water, dust, traffic and reclamation; it proposes one design change that would reduce impact and one recycling measure that would reduce demand; and it reaches a stated recommendation supported by that evidence.