Handbook of Nature-Study · Lesson CCXVIII · Topic 33: Crystals, minerals and rocks
218. Calcite, marble and limestone
Goal: Your child tests rocks and shells with vinegar and explains that calcite, limestone, marble, chalk and shells are all made of the same stuff.
You'll need
- Household vinegar and a dropper or spoon
- Pieces of limestone, marble (a tile scrap works), calcite if you have it, a seashell and some chalk
- A steel nail
- A hand lens (magnifying glass)
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
The lesson has a leading thought about calcite and its forms, a method calling for specimens and dilute hydrochloric acid (or strong vinegar), and eleven observations on calcite, marble, limestone, caves, lime, chalk and shells, followed by a passage by Thomas Huxley on chalk.
The lesson, step by step
Safety change
The book suggests dilute hydrochloric acid and says strong vinegar will do instead. We use only household vinegar. It works more slowly: look closely with the hand lens for tiny bubbles.
The leading thought
Limestone is a solid form of calcite. Marble is granular limestone which shows the broken crystals of calcite. Chalk is very fine, pulverized calcite.
Calcite
What is the form of the calcite crystal? What is the luster of the crystal? Is it the same as the inside of sea-shells? Will calcite scratch glass? Can you scratch it with a knife?
Use the steel nail. Then put a drop of vinegar on it and watch.
Marble and limestone
Is marble made up of crystals? Examine it with a lens to see.
Test a piece of limestone for hardness. Can you scratch it with a knife? Is it as soft as marble? Put on it a drop of acid. Does it effervesce?
"Effervesce" means fizz. Use vinegar for the acid.
Shells
Test a shell with acid; test a piece of coral with acid. How does it happen that these, which were once a part of living creatures, are now limestone?
Caves
Why are limestone regions likely to have caves within the rocks? How are stalactites and stalagmites formed in caves?
A line from Huxley
For older children, read the start of the quotation printed after the lesson:
A great chapter in the history of the world is written in the chalk.
Test your chalk with vinegar. Some modern chalk is made of gypsum and hardly fizzes at all, which makes a good puzzle.
Nature notebook
Make a fizz chart: each sample, "fizzes" or "no fizz," and whether the nail scratches it. Add these to your mineral chart.
Answers to the book's problems
Calcite can be scratched by a knife or nail and will not scratch glass; it fizzes in acid (slowly in vinegar). Marble shows sparkling crystals under the lens; limestone and marble fizz too. Shells and coral fizz because sea creatures build them of the same mineral, and their remains pile up on the sea floor and become limestone. Rainwater, a weak acid, slowly dissolves limestone and hollows out caves; dripping water leaves the mineral behind as stalactites (hanging) and stalagmites (standing).
Worked out for this site from the scan. If a number in your copy differs, trust the book.