Minerals: Properties, Patterns & Identification · Grade 6 Science
Chapter: Earth Systems, Plates & Earth MaterialsMinerals: Properties, Patterns & Identification
Learning goals
- I can state the four conditions something must meet to be a mineral.
- I can explain how a mineral's internal structure produces the way it breaks.
- I can rank identification tests by how much they actually discriminate.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
A mineral has to meet four conditions: it occurs naturally, it is not made by living things in the ordinary sense, it is solid, and it has both a definite chemical composition and an orderly internal arrangement of atoms. Quartz qualifies. Glass does not, because its atoms are disordered; steel does not, because it is manufactured.
Ice is a mineral, which surprises people: it occurs naturally, it is solid, it is water in a definite composition, and its atoms sit in the ordered six-sided arrangement that chapter 2 used to explain why it floats at 0.92 g/cm3.
Rocks are different again. A rock is a mixture of minerals, usually several, and granite is the standard example: visible grains of quartz, feldspar and mica, each a distinct mineral, locked together in one lump.
Build your understanding
Where this fits
The last two lessons went inside the planet and across its moving plates. This one picks up a piece of it and asks what it is made of, using the identification skills from chapter 2 where gold and pyrite were separated by density and hardness. The next lesson follows these minerals through the rock cycle.
Use three flexible 40-45-minute sessions: what counts as a mineral; how internal structure produces visible properties; the identification tests and which ones actually discriminate, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · Four conditions, and the awkward cases
Work through the four conditions on a few test cases and the definition starts doing real work. Quartz is natural, inorganic, solid, has the composition silicon dioxide and an ordered atomic structure, so it is a mineral. Window glass has the right chemistry and fails on structure, because its atoms are frozen in a disordered jumble rather than a repeating pattern, which is also why it fractures in smooth curved shells rather than along flat faces. Coal fails because it is the compressed remains of plants, so it is organic in origin and has no fixed composition.
The awkward cases are the useful ones. Ice passes every test and is a mineral, while liquid water is not, because it is not solid. A pearl is calcium carbonate with an ordered structure but is made by a living animal, so by the strict definition it is excluded. Opal has the right chemistry and origin but no long-range order, so it is usually called a mineraloid instead. Pushing a definition against its edges like this is how you find out what it actually says, and it is the same habit the first chapter applied to models. The definition also has to cope with scale. Around 6000 minerals have been formally described, and yet fewer than 20 make up almost all of the rock in Earth's crust, with feldspars alone accounting for roughly 60 per cent of it. Most of the 6000 are rare, found at a handful of sites in quantities of a few grams. A definition has to admit all of them while still excluding glass and coal, which is why it is written in terms of structure and composition rather than in terms of what is common.
Pause and explain
Glass and quartz can have almost the same chemical composition. Why is only one of them a mineral?
Check your reasoning after answering
Because a mineral needs an ordered internal arrangement of atoms as well as a definite composition. Quartz has a repeating crystal structure; the atoms in glass are frozen in a disordered jumble. That difference is visible in how they break: quartz can form flat crystal faces while glass fractures in smooth curved shells.
Part 2 · Structure you can see without a microscope
How a mineral breaks is a direct readout of how its atoms are bonded, and it is the most satisfying evidence in this lesson because the atomic scale becomes visible in your hand. Mica peels into transparent sheets thin enough to see through, because its atoms are bonded strongly within flat layers and weakly between them, so it parts along those planes and nowhere else. Halite, common salt, breaks into little cubes, because its atoms sit on a cubic grid with equally weak planes in three directions at right angles.
Calcite does something stranger and more revealing: it breaks into rhombs, leaning boxes whose faces are not at right angles, and a crushed crystal produces a pile of smaller rhombs of exactly the same shape. That shape is the atomic arrangement repeated at a scale you can see. Minerals without planes of weakness, such as quartz, do not cleave at all; they fracture along irregular curved surfaces instead, which is itself a diagnostic property. In each case the hidden structure is inferred from the visible break, which is the indirect reasoning from the Earth-interior lesson applied on a scale that fits in a pocket. Crystal faces tell the same story when a mineral has had room to grow. A quartz crystal that formed in an open cavity ends in six faces meeting at a point, and the angle between any two adjacent faces is 120 degrees in every quartz crystal ever measured, whether it is 2 mm long or 2 m. That constancy was noticed in 1669, well over a century before anyone knew atoms existed, and it was one of the first real clues that matter has an ordered structure underneath.
Pause and explain
A crushed calcite crystal gives a pile of small rhombs, all the same shape. What does this show?
Check your reasoning after answering
That the atomic arrangement repeats throughout the crystal with planes of weakness in three fixed directions. Breaking it anywhere separates the atoms along those same planes, so every fragment reproduces the same leaning-box shape at a smaller size. The shape visible in the hand is the shape of the atomic structure repeated.
Part 3 · Which tests are worth doing
The tests are not equally useful and it pays to know the order. Streak, the colour of the powder left when a mineral is rubbed on unglazed white tile, is highly reliable because the powder's colour does not vary with impurities the way the lump's colour does: hematite looks black, silver or red but always streaks reddish-brown. Hardness on the Mohs scale is next, with talc at 1, gypsum 2, calcite 3, feldspar 6, quartz 7 and diamond 10, and a fingernail at about 2.5, a copper coin 3.5 and a steel knife 5.5 giving you three dividing lines. Cleavage is next, since sheets, cubes and rhombs point to particular minerals immediately.
Colour is last and nearly useless on its own, which students find hard to accept because it is the most obvious thing about a specimen. Quartz can be clear, white, pink, purple, smoky grey or black depending on traces of other elements, and plenty of unrelated minerals are also white. Gold and pyrite from chapter 2 share colour and differ in density, 19.3 against 5.0 g/cm3, and in hardness. One specific chemical test is worth adding here: a drop of dilute acid fizzes vigorously on calcite and does nothing on quartz or feldspar, which separates limestone from sandstone in seconds.
Pause and explain
Why is streak more reliable for identification than the colour of the specimen?
Check your reasoning after answering
Because the powdered colour is far less affected by impurities. Traces of other elements can make the same mineral appear in several colours, so quartz may be clear, pink, purple or black, and hematite may look black, silver or red. Ground to powder, hematite always streaks reddish-brown, so the test gives the same answer whatever the lump looks like.
Part 4 · From minerals to rocks
Most of what you pick up outdoors is a rock rather than a mineral, meaning a mixture of several minerals grown or cemented together. Granite is the clearest example: look closely and you can see glassy grey quartz, pink or white blocky feldspar, and small black or silvery flakes of mica, three different minerals with different hardnesses and different ways of breaking, all in one piece. The proportions vary from one granite to another, which is exactly why a rock has no fixed composition and a mineral does.
This matters for how rock behaves. Granite weathers unevenly because its minerals do not weather at the same rate, with feldspar breaking down to clay long before the quartz is touched, which is why old granite buildings develop a rough gritty surface of loosened quartz grains. Limestone, made almost entirely of calcite, dissolves in slightly acidic rainwater and produces caves and sinkholes, while quartz-rich sandstone beside it stands unaffected. The next lesson is about those processes. The practical consequences run well beyond buildings. Soil forms where feldspar and other minerals break down into clay, so the fertility of a region traces back to the minerals in its underlying rock. Limestone regions supply hard water because dissolved calcite carries calcium into the supply, at concentrations often above 200 mg per litre, while granite regions give soft water. Which minerals lie beneath a place shapes its soil, its water and what will grow there.
Pause and explain
Why does a granite building develop a rough, gritty surface over many decades?
Check your reasoning after answering
Because its minerals weather at different rates. Granite contains quartz, feldspar and mica, and the feldspar breaks down chemically into clay far faster than the quartz does. As the feldspar between the grains decays, the hard quartz grains are loosened and left standing proud, producing the gritty surface.
Apply the whole unit · Identify three specimens
Specimen 1 is white, cannot be scratched by a steel knife, and shows no cleavage. Specimen 2 is white, is scratched by a copper coin, and fizzes when dilute acid is applied. Specimen 3 is black and silvery, peels into thin flexible sheets, and is scratched easily by a fingernail.
Your task
Identify each specimen and state which test was decisive.
What your response should show
- Identifies all three specimens correctly
- Names one decisive test per specimen rather than restating all the evidence
- Says what each decisive test ruled out
- Explains the unreliability of colour with a specific example
Response guidance — after your attempt
Identify each specimen from the properties given.
Name the single decisive test in each case and say what it ruled out.
Explain why colour was of no help with specimens 1 and 2.
Specimen 1 is quartz. The decisive test is hardness: a steel knife is about 5.5 on the Mohs scale and cannot scratch it, so the mineral must be harder than that, which with no cleavage points to quartz at 7. Specimen 2 is calcite. The decisive test is the acid, since fizzing identifies a carbonate immediately and rules out quartz and feldspar completely; the hardness is consistent, because calcite at 3 is scratched by a copper coin at about 3.5. Specimen 3 is mica. The decisive test is the cleavage into thin flexible sheets, which almost nothing else does, and the very low hardness confirms it. Colour was useless for the first two because both are white and so are a great many minerals: quartz can be clear, pink, purple or black depending on impurities, and calcite is commonly white, yellow or colourless, so the shared colour of specimens 1 and 2 carries no information at all about which is which.
Scientific references for this unit
- US Geological Survey: mineral properties, streak and the Mohs hardness scale
- Smithsonian National Museum of Natural History: the mineral collection and identification
- US Geological Survey: granite, limestone and how rocks weather
- Mineralogical Society of America: what defines a mineral
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
A mineral's properties are not a list of coincidences; they come from how its atoms are arranged. That is why mica peels into sheets and halite breaks into cubes every time, and why a handful of simple tests can identify a specimen reliably when colour cannot.
Worked examples
Example 1 · Use a property key
- Compare the same three criteria.
- Eliminate incompatible records.
- State the candidate-list and method limits.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Use properties with their actual meaning · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: Surface color alone always uniquely identifies a mineral.
- Misleading claim: Hardness measures how difficult a sample is to break.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Why does a rock’s name not identify every grain?
Discuss the answer
The rock classification concerns the whole material; individual constituents can be different minerals.
Why can streak add evidence beyond color?
Discuss the answer
It records a different defined property that may remain more informative despite surface variation.
Can a hardness rank of 4 be called twice a rank of 2?
Discuss the answer
No. The scale ranks scratch resistance; its numbers do not represent equal linear steps or a simple doubling ratio.
Why call the result a provisional match?
Discuss the answer
It is limited by the candidate list, test quality, and incomplete properties; it is not a universal unique identification.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Minerals: Properties, Patterns & Identification · Model and explain
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Minerals: Properties, Patterns & Identification, study Parts 3–4 and attempt the worked case: Identify the best match within this key and explain why color alone is unnecessary and the match is limited. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
S matches R within the supplied three-candidate key. P differs in luster and Q in streak and scratch relation. The result combines defined properties rather than rely on surface color. It remains a match within this illustrative key, not a verified identification of an actual specimen or every possible mineral.
Activity 2 · Minerals: Properties, Patterns & Identification · Practice, revise and exit
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Minerals: Properties, Patterns & Identification, complete practice questions 1–6 and both choice checks, then answer independently: A sample breaks easily but scratches another material. Does that make its hardness test impossible? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
No. Brittleness or toughness and resistance to scratching are different properties. Use the stated scratch record without equating it with ease of breaking.
Words to know
- hardness
- Resistance to scratching under a stated comparative test.
- streak
- The color of a mineral’s powder under a defined observation method.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Exit check
A sample breaks easily but scratches another material. Does that make its hardness test impossible?
Check your explanation after answering
No. Brittleness or toughness and resistance to scratching are different properties. Use the stated scratch record without equating it with ease of breaking.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.