Howardites

Worksheet for the lesson Slow cooling, big crystals: from basalt to iron meteorites (Years 7 to 9). Printing gives the pupil copy; open the answers below first to print them as a teacher copy on a separate page.

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Slow cooling, big crystals

Name:Date:

Put two drops of molten salol on a warm slide and two on a cold slide. Watch with a hand lens.

SlideTime for crystals to cover the drop (s)Size and shape of crystals
Warm slide
Cold slide
Liquid salol: particles
Slowly cooled solid
Quickly cooled solid
  1. Conclusion: how does the rate of cooling affect crystal size? Use the word particles.
  2. Put these in order from fastest to slowest cooling: gabbro, obsidian, iron meteorite crystals, basalt, chondrules.
  3. An asteroid core cooled by 100 °C per million years. How long did it take to cool from 700 °C to 400 °C? Show your working.
  4. Why are the crystals in an iron meteorite so much larger than those in steel made in a factory?
Answers for the teacher
  1. Typical results: the cold slide crystallises within seconds into many tiny crystals; the warm slide takes much longer, often a minute or more, and grows long needle-shaped crystals.
  2. Particle diagrams: liquid, particles touching but irregular; slowly cooled, a few large regular blocks; quickly cooled, many small regular patches at different angles.
  3. The slower the cooling, the larger the crystals, because particles have more time to move into place on a few growing crystals.
  4. Fastest to slowest: obsidian, chondrules, basalt, gabbro, iron meteorite crystals. (Chondrules and obsidian may be argued either way; both cooled very fast.)
  5. 300 °C ÷ 100 °C per million years = 3 million years.
  6. The iron meteorite cooled over millions of years, a few degrees to a few hundred degrees per million years, giving the metal crystals time to grow centimetres long. Factory steel cools in hours or days.

Howardites · free to copy for use in your classroom