Apply the criteria of a mineral and identify unknown minerals by hardness, streak, cleavage, luster, specific gravity and other tested properties

Learning goal: Apply the four criteria of a mineral (crystalline solid, occurs in nature, inorganic, defined chemical composition) to decide what is and is not a mineral; explain the physical basis of hardness, luster, cleavage and fracture, streak, crystal shape, fluorescence, flammability, density and specific gravity, malleability and magnetism; plan and conduct a safe investigation that tests hardness on the Mohs scale, streak, cleavage, specific gravity and malleability; identify and classify a mineral from tested properties, including the minerals present in a rock; distinguish pyrite from gold with a prospector's tools; determine the best use of a mineral from its observable properties and identify the minerals in a consumer product; and explain how GIS is used to target mineral exploration and evaluate mining conditions with minimal environmental impact.

The four criteria. A mineral must be (1) a crystalline solid, with atoms in a repeating three-dimensional pattern; (2) naturally occurring; (3) inorganic, not produced by living things as part of their bodies; and (4) of a defined chemical composition, fixed or variable within limits. Apply the four in order. Liquid water fails (1) because it is not solid, while ice passes all four, so ice in a glacier is a mineral and water is not. Window glass fails (1) because it is amorphous and (2) because it is manufactured. A synthetic diamond fails (2) although it is identical in every other way. Coal fails (3) and (1): it is altered plant matter with no single structure, so it is a rock, not a mineral. Amber, fossilized tree resin, also fails (3) and (1), but it is an organic mineraloid rather than a rock. A pearl is calcium carbonate laid down by an oyster, and by the definition used in this course it fails (3). Obsidian and opal fail (1) and are called mineraloids. Granite fails (4): it is a mixture of several minerals with no fixed formula, which makes it a rock. A rock is an aggregate of one or more minerals or mineraloids, and everything about identifying rocks begins with identifying the minerals in them.

Properties and the physics behind them. Each testable property is a consequence of composition, bonding and structure, which the previous lessons developed, and a good identification names the property and what it implies.

Hardness is resistance to scratching. The Mohs scale ranks ten reference minerals: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. It is an ordering, not a proportional measurement: the step from corundum to diamond is far larger in absolute terms than the step from talc to gypsum. Everyday tools give reference points: a fingernail about 2.5, a copper coin or strip about 3, a steel nail or a glass plate about 5.5, an unglazed porcelain streak plate about 6.5 to 7. A true scratch is a groove that survives wiping; a streak of powder from the softer tool is not a scratch.

Luster is how a fresh surface reflects light: metallic (opaque, shining like polished metal: galena, pyrite, gold) or non-metallic, which is subdivided into glassy (quartz, feldspar), pearly (mica, talc), greasy, silky, resinous, adamantine (diamond) and dull or earthy (kaolinite, earthy hematite). Luster is set mainly by bonding: mobile metallic electrons reflect light strongly.

Cleavage is breaking along flat planes of weak bonding; it is described by the number of directions and the angles between them. Fracture is any break that does not follow a cleavage plane: conchoidal (curved, shell-like: quartz, obsidian), irregular, splintery, or hackly (jagged, in native metals). A crystal face and a cleavage face can both be flat; a cleavage face repeats every time the mineral breaks, while a crystal face does not.

Streak is the color of the mineral's powder, obtained by rubbing it on an unglazed porcelain plate. It is more reliable than color because it removes the effects of grain size and surface tarnish: hematite specimens range from silver-gray to red, yet the streak is always reddish-brown; pyrite is brass-colored but streaks greenish-black; gold streaks golden yellow. Only minerals softer than the plate leave a streak; a harder mineral scratches the plate, and that result is recorded as "harder than the plate", not as a white streak.

Crystal shape, or habit, is the form the mineral takes when it grows freely: cubes (halite, pyrite, galena, fluorite), octahedra (magnetite; fluorite cleaves into octahedra), six-sided prisms (quartz, beryl), tabular plates (barite), needles, fibers (asbestos minerals, which are never handled), rounded botryoidal masses (hematite) or simply massive with no visible form.

Fluorescence is the emission of visible light while a mineral is under ultraviolet light, caused by trace impurities called activators. Some fluorite glows blue-violet, some calcite red or pink, willemite green, scheelite blue-white. Because it depends on impurities, one specimen of a mineral may fluoresce and another not, so a negative result proves nothing and a positive result is supporting rather than decisive evidence. Only a long-wave ultraviolet lamp is used in class, it is never looked into, and skin is not exposed for long.

Flammability is rare among minerals and is never tested in a classroom. Native sulfur burns with a blue flame and releases toxic sulfur dioxide; almost every other mineral does not burn. Flammable natural materials such as coal, oil shale and amber are organic and fail the mineral criteria, so flammability mostly serves to exclude a sample from being a mineral at all.

Density and specific gravity. Density is mass per unit volume. Specific gravity (SG) is the ratio of a mineral's density to that of water, a pure number. Most common silicates and carbonates fall between about 2.5 and 2.8 (quartz 2.65, orthoclase about 2.56, calcite 2.71); metallic sulfides and oxides run from about 5 to 7.5 (pyrite 5.0, magnetite 5.2, galena 7.5); native gold is about 19.3 when pure. SG is measured by weighing the specimen in air (mass A) and then suspended in water (apparent mass W): SG = A ÷ (A − W), because the loss of apparent mass equals the mass of the displaced water. Alternatively, measure mass and find volume by displacement in a graduated cylinder.

Malleability is the ability to be flattened without breaking; only metallically bonded minerals such as gold, copper and silver are malleable. Most minerals are brittle and shatter. A few, such as gypsum and talc, are sectile: they can be shaved with a knife. Magnetism singles out magnetite, which a hand magnet lifts; reaction with dilute acid singles out calcite, which fizzes in household vinegar as the acid reacts with the carbonate and releases bubbles of carbon dioxide, while dolomite fizzes only when powdered. Color is recorded last because it is the least reliable property: quartz alone occurs clear, white, purple, pink, yellow, brown and black.

Here is a model identification worked through. An unknown is white, glassy, and shows three flat cleavage faces meeting at angles that are clearly not 90°. A fingernail does not scratch it; a copper strip does not; a steel nail scratches it easily. A drop of vinegar makes it fizz. Weighing gives 27.1 g in air and 17.1 g in water, so SG = 27.1 ÷ (27.1 − 17.1) = 27.1 ÷ 10.0 = 2.71. Reasoning: hardness between about 3 and 5.5 excludes gypsum (2), halite (2 to 2.5) and quartz (7); rhombohedral cleavage excludes halite and fluorite (which cleave in cubes and octahedra); the acid reaction and an SG of 2.71 both fit calcite and nothing else in the classroom set. Conclusion: calcite, on four independent lines of evidence. The claim would be falsified if the sample scratched glass, failed to fizz, or cleaved at right angles. Naming what would falsify the conclusion is part of the conclusion.

Pyrite or gold? Early prospectors had no laboratory, and the properties settle it anyway. Gold is malleable: a hammer or a knife point dents and flattens it, while pyrite is brittle and shatters or crumbles. Gold is soft, about 2.5 to 3, so a knife scratches it; pyrite is 6 to 6.5, and a knife slides off. Rubbed on a piece of unglazed pottery, gold leaves a golden streak, pyrite a greenish-black one. Panning helps but does not decide. Gold's specific gravity of about 19 is almost four times pyrite's 5 and about seven times that of quartz sand, so gold sinks first and is the last grain to wash out; but pyrite is also heavier than sand, and coarse pyrite can stay in the pan beside fine gold. Panning therefore concentrates the heavy grains, and the tests above decide which of them are gold. Pyrite grows cubes with fine parallel lines on the faces and tarnishes; gold occurs as rounded grains, wires or flakes and never tarnishes. Any one test raises a suspicion; two or three settle the question.

Best use from properties, and the minerals in a product. Use follows property. Quartz is hard, chemically inert and vibrates at a precise frequency under an electric field, so it goes into glass, abrasives and watch oscillators, and its element silicon into computer chips. Halite dissolves, so it seasons food and melts road ice. Gypsum loses water when heated and takes it back when wetted, hardening in place, so it becomes plaster and wallboard. Graphite's sliding sheets make pencil "lead" and dry lubricant; talc's make body powder. Corundum (9) and garnet (about 7) are abrasives; diamond tips drill bits and saw blades. Mica's thin, heat-resistant, insulating sheets once served as stove windows and now line electronic components. Magnetite and hematite are smelted for iron; chalcopyrite for the copper in every wire. A smartphone alone contains silicon from quartz, copper from chalcopyrite, gold on its contacts because gold does not tarnish, tantalum in its capacitors from the ore called coltan, tungsten from wolframite as the dense weight in its vibration motor, lithium and cobalt in its battery, and rare-earth elements in its magnets and speakers. Tin, tantalum, tungsten and gold are called conflict minerals because in some regions their mining has financed armed groups; a 2010 United States law led to a 2012 Securities and Exchange Commission rule, with first reports covering 2013, under which companies that report to the SEC and make products needing these four minerals must disclose whether they came from the Democratic Republic of the Congo or its neighbors. Alternative sources exist, including mines in Australia, Brazil and Canada, recycled electronics, and substitutes such as ceramic capacitors in place of tantalum ones, and evaluating a product means asking where each mineral came from as well as why it is there.

GIS in exploration and mining. A geographic information system stacks map layers on a common grid: the geologic map, stream-sediment and soil chemistry, airborne magnetic surveys (magnetite-bearing bodies show as anomalies), gravity surveys (dense sulfide bodies show as highs), satellite imagery that picks out the clay and iron-oxide minerals produced where hot fluids altered rock, existing mines and drill holes, and, on the other side of the ledger, aquifers, streams, wetlands, towns, roads and protected land. A target is a place where several favorable layers overlap and no exclusion layer applies. The same overlay evaluates mining conditions: slope, rock strength, groundwater depth and distance to people. A minimal-impact site model then sets buffer distances from water, lines the tailings storage, treats all discharge, controls dust, and reclaims the ground progressively rather than at the end.

The investigation, done safely. Materials: a set of numbered classroom minerals; a hardness kit (fingernail, copper strip, steel nail, glass plate, streak plate, and reference minerals if available); hand magnet; dropper of household vinegar; balance and a beaker of water with thread for SG; long-wave UV lamp; goggles. Steps: (1) record color, luster and habit; (2) test hardness by trying to scratch the sample with each tool from softest to hardest, on a small inconspicuous surface, wiping after each try to check for a true groove; (3) streak on the plate; (4) examine, but do not create, cleavage and fracture surfaces with a hand lens, counting directions and estimating angles; (5) test with the magnet and with one drop of vinegar; (6) weigh in air and in water and compute SG; (7) view under the UV lamp; (8) compare the full record with an identification key restricted to the classroom set. Record every result in a table before looking for a name. Safety: goggles throughout; the glass plate lies flat on the bench and is never held in the hand; the streak plate's edges are taped; dust is wiped up damp, never blown; no sample is tasted, struck with a hammer or heated; the UV lamp is never looked into; hands are washed at the end. Flammability is not tested. (9) Malleability, with the teacher: place a known copper strip and a small chip of halite (not galena, which is a lead mineral) each inside a sealed zip bag and press firmly with the back of a spoon; the copper flattens and the brittle mineral crumbles. The key resolves among the minerals it contains, so a sample that matches nothing is reported as "not resolved by this key", never forced to a name.

Worked example, identifying an unknown: Record for a hypothetical unknown: brassy yellow, metallic luster, cubes with fine striations, no cleavage, brittle (a corner crumbles rather than dents), not scratched by a steel nail, greenish-black streak, not magnetic, no acid reaction, 40.0 g in air and 32.0 g in water. SG = 40.0 ÷ 8.0 = 5.0. Reasoning: metallic luster and brass color suggest pyrite, chalcopyrite or gold; hardness above 5.5 excludes gold (2.5 to 3) and chalcopyrite (3.5 to 4); greenish-black streak and SG 5.0 both match pyrite; brittleness confirms it is not a native metal. Conclusion: pyrite. Falsifier: a golden streak or a dent from the nail.

Worked example, specific gravity by displacement: A hypothetical specimen of mass 60.0 g is lowered into a graduated cylinder reading 50.0 mL; the level rises to 58.0 mL. Volume = 8.0 mL, density = 60.0 ÷ 8.0 = 7.5 g/cm³, SG = 7.5. Among classroom minerals only galena comes close. Cross-check: a gray metallic luster, cubic cleavage and a gray streak agree. A common error is to read the volume of water instead of the change in volume; the subtraction is the step that turns a reading into a measurement.

Worked example, minerals in a rock: A coarse-grained light-colored rock shows three kinds of grain. Gray, glassy grains with no flat faces and conchoidal fracture that scratch glass: quartz. Pink and white blocky grains that flash light from flat cleavage faces meeting near 90° and are not scratched by the steel nail: feldspar. Black shiny flakes that a needle pries apart into sheets: biotite mica. Three identified minerals, quartz plus feldspar plus mica, in interlocking coarse grains, identify the rock as granite. In a fine-grained rock such as basalt the grains are too small to test individually, so the mineral content is inferred from color, density and, in a laboratory, a thin section under a microscope; the limit of hand identification should be stated.

Worked example, applying the criteria: Is ice a mineral? Solid: yes, at the temperature in question. Crystalline: yes, hexagonal, which is why snowflakes have six-fold symmetry. Natural: yes in a glacier, no in a freezer. Inorganic: yes. Defined composition: H₂O. Glacier ice passes all four criteria and is a mineral; ice from a freezer fails only on origin. A seashell fails the inorganic criterion although it is crystalline aragonite or calcite; the mineral is the same, the object is not counted as one.

Representation task: Construct a two-level classification chart of at least eight classroom minerals. The first level groups them by chemical composition (silicate, carbonate, sulfide, oxide, halide, sulfate, native element) and the second level, within each group, by crystal system. Beside each mineral write its formula, hardness and specific gravity. Then add a second, separate chart in which the same minerals are sorted by the tested property most useful for telling them apart, and explain in two sentences why the two charts differ.

Investigation record: Carry out the investigation on three numbered unknowns from the restricted classroom set. For each, submit the complete data table (color, luster, habit, hardness bracket with the tools that did and did not scratch it, streak, cleavage directions and angles or fracture type, magnetism, acid reaction, SG with both masses shown, fluorescence), the identification, the lines of evidence that support it, and the observation that would have falsified it.

Atom-building game and site model: In pairs, make cards for ions labeled with their formal charges (Na⁺, Cl⁻, Ca²⁺, F⁻, Si⁴⁺, O²⁻, Fe³⁺, Pb²⁺, S²⁻) and score a point for each electrically neutral mineral formula you can build and name, such as NaCl (halite), CaF₂ (fluorite), SiO₂ (quartz), Fe₂O₃ (hematite) and PbS (galena). Then, for a hypothetical copper target chosen with the GIS layers described in this lesson, build a small tabletop model of a mine site with minimal environmental impact: pit, lined tailings pond, buffer from the stream, water-treatment step and the area to be reclaimed first.

Independent performance: Choose a consumer product (a bicycle, a phone, a pencil, a bag of wallboard, a battery). Success criteria: you name at least four minerals or mineral-derived materials in it, give each one's formula and compositional group, state the tested property that explains why it was chosen for that job, identify one mineral in the product that could be replaced by a recycled or alternative source and say where; for one conflict mineral used in electronics (tin, tantalum, tungsten or gold), whether or not your product contains it, summarize from two sources you cite the social issue linked to its mining and one alternative source or substitute; and, for one mineral, describe the GIS layers an exploration geologist would overlay to find a new deposit and two layers that would exclude a site to keep environmental impact minimal.