Handbook of Nature-Study · Lesson CCXVII · Topic 33: Crystals, minerals and rocks
217. Granite
Goal: Your child can find quartz, feldspar and mica in a piece of granite and explain how granite weathers into sand.
You'll need
- A piece of coarse granite (a landscaping or rock-shop piece)
- A steel nail
- A spoonful of sand
- 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, a method, and six observations on the minerals in granite, granite buildings and monuments, weathering, sand, and Cleopatra's Needle in Central Park.
The lesson, step by step
The leading thought
Granite is composed of feldspar, quartz and mica, and often contains hornblende.
Find the minerals
What minerals do you find in granite? How can you tell what these minerals are? Look at the granite with a lens. How can you tell the quartz from feldspar?
Glassy gray grains are quartz; pink or white blocky grains with flat shiny faces are feldspar; tiny flakes like mirrors are mica; black, stubby grains are often hornblende.
Scratch test
Take a knife and scratch the two. Can you tell them apart in that way?
Use the steel nail.
Granite in your town
What buildings have you seen made of granite? What monuments have you seen made from it?
Look at curbstones, steps, bridges, monuments and old gravestones on your next walk.
Weathering
Which mineral in granite is especially affected by water? Which remains unharmed the longest?
Examine some sand with a lens. What mineral do you find present in it in the greatest quantity?
Nature notebook
Draw a magnified circle of your granite, coloring each mineral grain and labeling it. Add a granite row to your chart.
Answers to the book's problems
Feldspar is the mineral most affected by water: it slowly turns to clay, loosening the grains. Quartz lasts longest, so sand is mostly quartz. Rough granite weathers sooner than polished granite because water and frost can get into its many tiny cracks.
Worked out for this site from the scan. If a number in your copy differs, trust the book.