Gateway to the Classics: Handbook of Nature Study: Earth and Sky by Anna Botsford Comstock
 
Handbook of Nature Study: Earth and Sky by  Anna Botsford Comstock


[Illustration]

Forms of quartz crystals.

Quartz

Teacher's Story

There is in the Cornell University Museum a great quartz crystal, a six-sided prism several inches in thickness. One-half of it is muddy and the other half clear, transparent and beautiful. The professor in charge, who has the imagination necessary to the expert crystallographer, said to his class: "This crystal was begun under conditions which made it cloudy; then something happened, perhaps some cataclysm that changed all the conditions around the half-grown crystal, and it may have lain a hundred or a thousand years unfinished, when, some other change occurring, there came about conditions which permitted it to resume growth, and the work began again exactly where it was left off, the shaft being perfected even to its six-sided pyramidal tip." And ever afterwards that crystal, half clouded and half clear, remained in the minds of his pupils as a witness of the eternal endurance of the laws which govern the growth of crystals.

Quartz is the least destructible and is one of the most abundant materials in the crust of the earth as we know it. It is made up of two elements chemically united—the solid silicon and the gas oxygen. It is the chief material of sand and sandstones, and it occurs, mixed with grains of other minerals, in granite, gneiss, and many lavas; it also occurs in thick masses or sheets, and sometimes in crystals ornamenting the walls of cavities in the rocks. Subterranean waters often contain a small amount of silica, the substance of quartz, in solution; from such solutions it may be deposited in fissures or cracks in the rock, thus forming bodies called "veins." Other materials are often deposited at the same time, and in this way the ores of the precious metals came to be associated with quartz. Sometimes quartz is deposited from hot springs or geysers, forming a spongy substance called sinter. In this case, some of the water is combined with the quartz, making what is called opal. Quartz crystal will cut glass.

Quartz occurs in many varieties: (a) In crystals like glass. If colorless and transparent it is called rock crystal; if smoky brown, it is called smoky quartz; if purple, amethyst. (b) In crystals, glassy but not transparent. If white, it is milky quartz; if pink, rose quartz. (c) As a compact crystalline structure without luster, waxy or dull, opaque or translucent, when polished. If bright red, it is carnelian; if brownish red, sard; if in various colors in bands, agate; if in horizontal layers, onyx; if dull red or brown, jasper; if green with red spots, bloodstone; if smoky or gray, breaking with small, shell-like or conchoidal fractures, flint.

Rock crystals are used in jewelry and especially are made to imitate diamonds. The amethyst is much prized as a semi-precious stone. Carnelian, bloodstone and agate are also used in jewelry; agate is used also in making many ornamental objects, and to make little mortars and pestles for grinding hard substances.

One of the marvels of the world is the petrified forest of Arizona, now set aside by the government as a national reserve. Great trees have been changed to agate and flint, the silica being substituted for the tissues of the wood so that the texture is preserved though the material is changed.

When our country was first settled, flint was used to start fires by striking it with steel and letting the sparks fly into dry, fine material, called tinder. It was also used in guns before the invention of cartridges, and the guns were called flintlocks. The Indians used flint to make hatchets and for tips to their arrows. The making of flint implements dates far back into prehistoric times; it was probably one of the first steps upward which man achieved in his long, hard climb from a level with the brute creation to the heights attained by our present civilization.

Quartz sand is used in making glass. It is melted with soda or potash or lead, and the glass varies in hardness according to the minerals added. Quartz is also used for sandpaper and glass paper; and ground to a fine powder, it is combined with Japans and oils and used as a finish for wood surfaces. Mineral wool is made from the slag refuse of furnaces where glass is made, and is used for rat-proof and fireproof padding for the walls of houses. Quartz combined with sodium or potassium and water, forms a liquid called water-glass, which is used for waterproof surfaces; it is also fireproof to a certain degree. Water-glass is the best substance in which to preserve eggs; one part of commercial water-glass to ten parts of water makes a proper solution for this purpose.

Lesson CCXIV

Quartz

Leading thought—Quartz is one of the most common of minerals. It occurs in many forms. As a crystal it is six-sided, and the ends terminate in a six-sided pyramid. It is very hard and will scratch and cut glass. When broken, it has a glassy luster and it does not break smoothly but shows an uneven surface.


Method—The pupils should have before them as many varieties of quartz as possible; at least they should have rock crystal, amethyst, rose and smoky quartz and flint.


Observations—

1. What is the shape of quartz crystals? Are the sides all of the same size? Has the pyramid-shaped end the same number of plane surfaces as the sides?

2. What is the luster of quartz? Is this luster the same in all the different colored kinds of quartz?

3. Can you scratch quartz with the point of a knife? Can you scratch glass with a corner or piece of the quartz? Can you cut glass with quartz?

4. Describe the following kinds of quartz and their uses: amethyst, agate, flint.

5. How many varieties of quartz do you know? What has quartz to do with the petrified forests of Arizona?


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