Minerals & Their Properties · Grade 9 Science

Minerals & Their Properties

Learning goals

• I can list the five traits that define a mineral.
• I can identify a mineral using color, streak, luster, and hardness.
• I can use the Mohs hardness scale to predict which minerals can scratch each other.

Readings for this unit

• What happens to rain after it reaches the ground?

Not every hard, shiny rock you pick up is a mineral — to earn that name, a substance has to pass a strict five-part test. It must be (1) naturally occurring, (2) inorganic (not made by living things), (3) a solid, (4) of a definite chemical composition, and (5) built from atoms locked in an orderly, repeating pattern called a crystal structure. That last requirement is the secret behind almost everything else about a mineral: because the atoms line up the same way all through it, they give each mineral its characteristic hardness, its shape, and the way it breaks. Quartz and feldspar are minerals; granite is *not*, because it's a mixture of several minerals — quartz, feldspar, and mica — cemented together, more like a fruitcake than a single ingredient.

Because two different minerals can look almost identical at a glance, geologists don't trust their eyes alone — they run quick physical tests. Color is actually the least reliable clue of all, because tiny impurities can paint the very same mineral wildly different shades: pure quartz is clear, but a trace of iron can make it purple (the gem amethyst), and other impurities turn it pink or smoky, all still quartz. So color can start a guess, but it rarely settles one.

A far more honest test is streak — the color of a mineral's powder, which you reveal by scraping it across a rough, unglazed porcelain tile. Even when a mineral's outer surface is stained, tarnished, or weathered, its powdered streak stays true, which is why two shiny silver-gray minerals can be told apart the instant one leaves a reddish-brown streak and the other a gray-black one. Luster describes the way a mineral's surface reflects light — is it metallic, gleaming like a polished spoon, or non-metallic, looking glassy, pearly, waxy, dull, or earthy? And watch how a mineral breaks: some split along smooth, flat cleavage planes that follow the neat rows of their crystal structure, while others shatter along rough, uneven fracture surfaces — yet another clue to the hidden order inside.

One of the most useful tests of all is hardness — how well a mineral resists being scratched — measured on the Mohs hardness scale, which runs from 1 (softest) to 10 (hardest). Ten reference minerals anchor it: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10), the hardest natural material on Earth. The rule behind the scale is beautifully simple: a mineral will scratch anything softer than itself and be scratched by anything harder.

You don't need a fancy kit to use it, either — the objects in your pocket work fine as testing tools. Your fingernail has a hardness of about 2.5, a copper penny about 3.5, a piece of glass or a steel knife blade about 5.5, and a steel file about 6.5. If a mystery mineral scratches glass but not a steel file, you've already pinned its hardness between 5.5 and 6.5 without any lab at all. That's the real power of these properties: with nothing more than a porcelain tile, a nail, and a coin, you can identify minerals right there in the field — the same detective work professional geologists rely on every day.

A mineral is identified using several properties, such as hardness, streak, cleavage and crystal form. A rock is an aggregate of one or more minerals or other materials. Color alone can vary with impurities and is usually a weak identification test.

The Mohs scale ranks scratch resistance. Its numbered steps are not equal physical intervals, so hardness 8 is not twice hardness 4. Test which sample scratches which using a specified safe procedure rather than averaging the scale as if it were linear.

Record uncertainty when a property is ambiguous. A weathered surface may hide cleavage, and mixtures can show several streaks or grains. A defensible identification names the observed evidence and a next test instead of guessing from one familiar color.

Worked examples

Avoid these mistakes

• Mohs hardness is an ordered scale, not a linear measurement of strength.

Words to know

mineral

A naturally occurring, inorganic solid with a definite chemical composition and an orderly, repeating crystal structure.

luster

How a mineral's surface reflects light — described as metallic (shiny like metal) or non-metallic (glassy, pearly, dull, or earthy).

streak

The color of a mineral's powder, revealed by rubbing it across an unglazed porcelain plate; more reliable than color for identification.

hardness

A mineral's resistance to being scratched, ranked on the 1–10 Mohs scale.

Same atoms, totally different mineral: the polymorph surprise

Graphite (the 'lead' in your pencil) and diamond (the hardest natural material on Earth) are made of exactly the same element — pure carbon — and nothing else. The dramatic difference comes entirely from how the carbon atoms are arranged. In graphite, carbon atoms bond into flat, weakly-stacked sheets that slide past each other easily, which is why it feels soft and smudges onto paper. In diamond, each carbon atom bonds tightly to four neighbors in a rigid three-dimensional lattice, making it extraordinarily hard. Minerals like this, with identical chemistry but different crystal structures, are called polymorphs.

Cleavage is really about weak spots in the atomic lattice

A mineral doesn't cleave along smooth planes by accident — it's breaking along the weakest bonds in its atomic structure, the same way a piece of paper tears more easily along its grain. Mica is the classic example: its atoms are arranged in strong, tightly bonded sheets with much weaker bonds between the sheets, so mica peels apart into thin, flat, almost transparent layers with almost no effort, a property called perfect cleavage in one direction. Other minerals cleave in two, three, or more directions depending on how many weak planes their crystal structure contains — which is exactly why cleavage is such a reliable identification clue.

The Mohs scale's hidden trap: it isn't a ruler

When Austrian mineralogist Friedrich Mohs created his hardness scale in 1812, he ranked ten minerals in order of which could scratch which — but the scale is ordinal, not evenly spaced like a ruler. The actual jump in absolute hardness between each step gets bigger near the top: diamond (10) is roughly four times harder in absolute terms than corundum (9), even though they're only one step apart on the scale. So while the Mohs scale is a fantastic quick field test, it would be a mistake to assume a mineral rated '8' is exactly 'twice as hard' as one rated '4.'

New York's own claims to mineral fame

New York doesn't just teach about minerals — it produces some famous ones. The Adirondack Mountains are home to some of the largest garnet mines on Earth, including operations near North Creek that have supplied industrial garnet abrasive for over a century (garnet is New York's official state gemstone). Farther south, quarries near Herkimer, NY produce 'Herkimer diamonds' — not true diamonds at all, but unusually clear, double-terminated quartz crystals that form naturally with points on both ends, prized by collectors for their exceptional sparkle and clarity.

In the real world

Geologists and jewelers use the same streak, hardness, and luster tests to tell a genuine diamond from cheap glass or to identify valuable ore minerals in rock samples brought up from a mine.

Exit check

A specimen scratches calcite (3) but is scratched by apatite (5). What can you conclude about Mohs hardness?