Slow cooling, big crystals: from basalt to iron meteorites

Years 7 to 960 minutesScience, National Curriculum for England

Students cool molten salol at two different rates, explain the crystal sizes with the particle model, and apply the rule to basalt and gabbro, to chondrules that cooled in hours, and to iron meteorites whose crystals grew over millions of years.

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National Curriculum links

• Key Stage 3 chemistry, Earth and atmosphere: the rock cycle and the formation of igneous, sedimentary and metamorphic rocks; the composition and structure of the Earth.
• Key Stage 3 chemistry, the particulate nature of matter: the properties of the different states of matter in terms of the particle model; changes of state in terms of the particle model.
• Key Stage 3, working scientifically: make predictions using scientific knowledge and understanding; make and record observations; interpret observations and data, identifying patterns and drawing conclusions.

Key vocabulary

• igneous
• crystal
• cooling rate
• particle model
• basalt
• gabbro
• obsidian
• chondrule
• kamacite
• Widmanstätten pattern

Resources

• Phenyl salicylate (salol) in a labelled container, and a spatula for each group.
• Three microscope slides per group, a small test tube of salol, and a beaker of hot water at about 60 °C prepared by the teacher from a kettle.
• A beaker of iced water, tongs, a dropper, and a hand lens or low-power microscope.
• Eye protection for everyone.
• Hand specimens of basalt and gabbro (granite will do), and obsidian if available.
• Pictures on a screen of chondrules and an etched iron meteorite, from the site's glossary and famous meteorite pages.
• The worksheet, one per student.

Background for the teacher

Igneous rocks form when molten rock cools and crystallises. In the liquid, particles move past each other; as it cools they slow down and lock into a regular arrangement, the crystal. When cooling is slow, few crystals start growing and particles have time to join them, so the crystals grow large. When cooling is fast, many crystals start at once and none has time to grow, so they stay small. Cooled fast enough, the liquid freezes before particles can arrange themselves at all, and it becomes glass, like obsidian.

Basalt and gabbro are made of much the same minerals. Basalt cooled quickly from lava at or near the surface, so its crystals are too small to see easily; gabbro cooled slowly deep underground, so its crystals are easy to see.

Meteorites record the same rule. Chondrules, the millimetre-sized beads in most stony meteorites, were molten droplets in space that cooled within hours, so they contain small crystals and sometimes glass. The eucrites from the asteroid Vesta are basalts, while the diogenites, which cooled slowly deep inside Vesta, have much coarser crystals; the howardites are broken mixtures of both. In iron meteorites, the metal was already solid as it cooled inside an asteroid's core, and plates of the nickel-poor metal kamacite slowly grew out of the nickel-rich metal taenite. The cooling was extraordinarily slow, typically between a few degrees and a few hundred degrees per million years, which is why the crystals that make the Widmanstätten pattern can be centimetres long. That is solid-state growth rather than freezing from a liquid, but the principle is the same: the slower the cooling, the larger the crystals.

Salol melts at about 42 °C, so it can be melted in hot water without a flame. On a warm slide it crystallises slowly into long crystals; on a chilled slide it crystallises quickly into a mass of tiny ones.

Lesson plan

Differentiation

Safety and risk assessment

Check these points against your employer's risk assessment and your school's policies before the lesson.

Assessment questions with answers

1. Which slide grew larger salol crystals, and why?Answer: The warm slide. The salol cooled slowly, so fewer crystals started and particles had time to join them in an orderly way, making them grow large.
2. Basalt and gabbro contain much the same minerals. Why are gabbro's crystals larger?Answer: Gabbro cooled slowly deep underground; basalt cooled quickly from lava at or near the surface.
3. Obsidian has no crystals at all. What does that tell you about how it formed?Answer: It cooled so quickly, for example lava quenched in air or water, that the particles had no time to arrange themselves into crystals, so it formed glass.
4. Chondrules contain small crystals and sometimes glass. What does that suggest?Answer: They were small molten droplets that cooled quickly, within hours, in space.
5. An asteroid core cooled at 50 °C per million years. How long did it take to cool from 700 °C to 450 °C?Answer: It cooled by 250 °C. 250 ÷ 50 = 5 million years.

Homework

Use the site's pages on eucrites, diogenites and howardites to make a fact card about one of them. Include where it comes from, whether its crystals are fine or coarse, and what that tells us about how it cooled.

Answer guidance: Eucrites: basalts from Vesta's crust, fine to medium crystals, cooled fairly quickly near the surface (cumulate eucrites cooled more slowly). Diogenites: coarse crystals of pyroxene that cooled slowly deep inside Vesta. Howardites: broken mixtures of eucrite and diogenite fragments from Vesta's surface.

Use with these pages

• Igneous rock
• Basalt
• Chondrule
• Widmanstätten pattern
• Kamacite
• Taenite
• Eucrite
• Diogenite
• Howardite
• Meteorites and the rock cycle