Handbook of Nature-Study · Lesson CCXV · Topic 33: Crystals, minerals and rocks
215. Feldspar
Goal: Your child can tell feldspar from quartz by its smooth, flat break and slightly lower hardness.
You'll need
- Feldspar pieces (pink or white; from a rock shop, or found in coarse granite)
- Quartz to compare
- 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, a method listing three kinds of feldspar, and nine observations on its crystal, colors, luster, hardness, streak and cleavage, how to tell it from quartz, and why sand holds so much more quartz.
The lesson, step by step
The leading thought
Feldspar is about five times as common as quartz. The crystal is obliquely brick-shaped, and when broken splits in two directions at right angles to each other. It is next in hardness to quartz, and will scratch glass but will not cut it.
Look at it
What colors are your specimens of feldspar? How many kinds have you?
What is the luster of feldspar?
Hardness
Can you scratch feldspar with the point of a knife? Can you scratch it with quartz?
Use the steel nail, then a quartz point.
How it breaks
If feldspar is broken, does it break along certain lines, leaving smooth faces? At what angles do these smooth faces stand to each other?
Tilt it in the light: the smooth faces flash like little mirrors. Quartz never does this.
Compare
How can you tell feldspar from quartz? Write a comparison of feldspar and quartz, giving clearly the characteristics of both.
A thinking question
When there is so much more feldspar than quartz in the earth's crust, why is there so much more quartz than feldspar in sand?
Nature notebook
Make a two-column chart, Quartz and Feldspar, comparing shape, color, luster, hardness and how it breaks. Add a feldspar row to your mineral chart.
Answers to the book's problems
Feldspar is often pink, white or gray, with a glassy to pearly luster. A nail scratches it only with difficulty or not at all, and quartz scratches it. It breaks along smooth flat faces nearly at right angles, while quartz breaks unevenly like glass. Sand holds more quartz because feldspar slowly weathers into clay, while hard quartz does not break down chemically and survives as grains.
Worked out for this site from the scan. If a number in your copy differs, trust the book.