About Rocks
Rocks, to begin with, are made of minerals. What is a mineral?
The definition may sound difficult--a mineral is a chemical element
or compound (combination of elements) occurring naturally as the
result of inorganic processes. But don't be discouraged. Things
will clear up soon.
The world contains more than 1,100 kinds of minerals. These can
be grouped in three general classes.
1. METALLIC MINERALS. These include things most of us would think
of if we were asked to name some minerals. Familiar examples are
copper, silver, mercury, iron, nickel and cobalt. Most of them are
found in combination with other things--as ores. We get lead from
galena, or lead sulfide. Tin comes from the ore cassiterite; zinc
from sphalerite and zincblende, or blackjack. Chromium that makes
the family car flashy comes from chromite. Many minerals yield
aluminum. Uranium occurs in about 50 minerals, nearly all rare.
Twenty-four carat gold is a metallic mineral. A 14 carat gold ring
contains 14/24 or 58% gold.
An average sample of earth contains 9% aluminum, 5.5% iron, .01% zinc,
.008% copper, .004% tin, .002% lead, .0005% uranium, and .0000006% gold
or platinum. It would be hopelessly expensive to recover such metals
from an average ton of earth. That's why metallic minerals are taken
from concentrated deposits in mines.
Many valuable minerals are found in veins running through rock. Veins
can be formed when: (a) mineral-laden ground water seeps into cracks,
evaporates, and leaves mineral grains that build up into a vein;
(b) hot water from deep within the earth fills cracks, then cools
and deposits much of the material in solution as minerals in a
vein--sometimes including metals such as gold and silver; (c) molten
gaseous material squeezes into cracks near the earth's surface,
then slowly hardens into a vein.
2. NONMETALLIC MINERALS. These are of great importance to certain
industries. You will find them in insulation and filters. They are
used extensively in the ceramic and chemical industries. They include
sulfur, graphite (the "lead" in pencils), gypsum, halite (rock salt),
borax, talc, asbestos and quartz. Undoubtedly, you'll have some
nonmetallic minerals in your collection. Rocks containing asbestos
are especially handsome and varied.
3. ROCK-FORMING MINERALS. These are the building materials of the
earth. They make mountains and valleys. They furnish the ingredients
of soil and the salt of the sea. They are largely silicates--that is,
they contain silicon and oxygen. (Silicon is a nonmetallic element,
always found in combination with something else. It is second only
to oxygen as the chief elementary constituent of the earth's crust.)
Other rock-forming minerals are the large family of micas, with names
like muscovite and phlogopite. There are the feldspars, including
albite and orthoclase. Others are amphiboles, pyroxenes, zeolites,
garnets and many others you may never find or hear about unless
you become a true mineralogist.
A rock may be made almost entirely of one mineral or of more than
one mineral. Rocks containing different combinations of the same
minerals are different. Even two things made of the same single
mineral can be quite different. Carbon may turn up as a lump of
coal or a diamond.
How Minerals Got Their Names
Names of most minerals end in "ite"--apatite, calcite, dolomite,
fluorite. But many do not: amphibole, copper (the most common pure
metal in rocks), feldspar, galena, gypsum, hornblende, mica, quartz.
Many minerals take their names from a Greek word referring to some
outstanding property of the mineral. For example, hematite,
an oxide of iron, was named about 325 B.C. from the Greek HAIMA,
or blood, because of the color of its powder.
Some minerals are named for the locality in which they were first
discovered. Coloradoite was first found in Colorado. Benitoite
turned up in San Benito County, California. And so with labradorite
and brazilite.
Other minerals got their names from famous people. Willemite was
named in honor of Willem I, King of the Netherlands. The great
German poet-philosopher, Goethe, could turn up in your collection
as goethite. And there's smithsonite, named for James Smithson,
founder of the Smithsonian Institution.
Out Of This World
Some minerals come from outer space. They're meteorites, which
are rock fragments. Every day, hundreds of millions of them
enter the earth's atmosphere. Most of them, however, are burned up
by the heat from air friction and never reach the ground. Meteors
large enough to reach the earth are called meteorites. Most minerals
found in meteorites are the same as those we have on earth. But,
there are some rare minerals known only in meteorites. Two of them
are cohenite and schreibersite.
MAIN KINDS OF ROCKS
Rocks are the building blocks of the earth's crust. They may be
massive, as in granite ledges, or tiny. Soil, gravel, sand and clay
are rocks. THERE ARE THREE MAIN TYPES OF ROCKS.
1. IGNEOUS rocks are those formed at very high temperatures or from
molten materials. They come from magmas--molten mixtures of minerals,
often containing gases. They come from deep below the surface of the
earth. If they cool off while below the surface, they form intrusive
rocks, which may later be revealed by erosion. When magmas reach the
surface red hot, they form extrusive rocks, such as volcanic rocks.
Thus, granite is an igneous, intrusive rock; lava is an igneous,
extrusive rock. (Notice how the type of rock tells its past
history--if you know what to look for.)
2. SEDIMENTARY rocks are formed by the action of wind, water, or
organisms. They cover about three quarters of the Earth's surface.
Most are laid down--as sediments--on the bottom of rivers, lakes
and seas. Many have been moved by water, wind, waves, currents,
ice or gravity. The most common sedimentary rocks are sandstones,
limestones, conglomerates and shales. Oil is found in sedimentary
formations.
3. METAMORPHIC rocks are those that have been changed from what
they were at first into something else--by heat, pressure, or
chemical action. All kinds of rocks can be changed. The result
is a new crystalline structure, the formation of new minerals,
or a change in the rock's texture. Slate was once shale. Marble
came from limestone. Gneiss (pronounced "nice") is perhaps
reworked granite.
A Word On Fossils
Perhaps you'll find rocks containing fossils--or even fossils
by themselves. They should form a separate part of your collection.
Fossils are the remains--or the outlines--of former plant or
animal life buried in rock. The older the rock, the simpler the
plant and animal life it contains. Thus fossils can give a clue
to the age of the rock strata.
Fossils can teach history. They tell us about plants and animals
that are now extinct--the dinosaur, for example. They can also tell
of ancient climates. Coral found in rocks in Greenland suggests it
must have once been warm. Remains of fir and spruce trees have been
found in the tropics.
How are fossils formed? Teeth, bone and wood don't last long in
their original state. However, buried materials decompose, leaving
a film of carbon as a fossil. This results in a leaf tracery, or
the outlines of some simple animal. On a gigantic scale, this process
of forming carbon has resulted in our great coal deposits.
Sometimes the buried material is gradually replaced by silica or other
substances, making petrified objects. Wood can be replaced--cell by
cell--by agate or opal from silica-bearing water. The result is
petrified wood, the finest examples of which can be found in our
Petrified Forest National Park in Arizona. This can happen to
shells, too.
How about molds and casts of footprints of ancient animals? A
brontosaurus might have stomped along in soft, warm mud eons ago.
The mud hardened and later another layer of soft earth covered the
print, preserving it.
COLLECTING
If you want to collect rocks and minerals just for the sake of
having them, you can buy specimens. Many can be purchased for
25 cents to $1 each, while a rare specimen can cost hundreds of
dollars.
The true pleasure is in finding your own samples. Later, when you
have a good-sized collection, you can fill gaps by buying specimens
or swapping extras with other collectors. You'll be amazed at the
number of amateur collectors. Perhaps no branch of science owes more
to the work of amateurs than mineralogy. Our great collection of
minerals in the U.S. National Museum in Washington, D.C., was
gathered almost entirely by two amateurs who devoted many years
and much money to their hobby.
Where To Look
Look for pebbles by the roadside, in beds of streams and riverbanks.
Go out into the country for ledges on hillsides. Every road cut,
cliff, bank, excavation, or quarry shows rocks and minerals.
Railroad cuts, rock pits, dump piles around mines, building
sites--they'll all yield specimens. Some of the best mineral
specimens collected in New York City came from skyscraper and subway
excavations. Help a New England farmer clear his field and you'll
have more rocks than you know what to do with.
As for reference books, many states publish guides to mineral
deposits. Mineralogical magazines list mineral localities.
Tips For The Field
Don't try to collect too much at once. Work early in the day or
late in the afternoon. A hot sun on bare rock can make you
sizzle--especially if you're loaded with equipment and samples.
Here's the equipment to take: newspapers for wrapping samples,
notebook and pencil, geologist's pick, cold chisel, magnifying
glass, compass, heavy gloves, a knife, and a knapsack. Later on,
you may want a Geiger counter for spotting radioactive rocks.
Be selective. Hand-sized specimens are best. If your sample is
too large, trim it to size, showing its most striking feature
to best advantage. When you wrap the sample in newspaper, include
a note telling when and where you found it. This information will
be transcribed to a filing card when you add the specimens to your
display, so make it as complete and accurate as you can.
When you get home, clean specimens with soapy, warm water, applied
with a soft brush. Soluble minerals like halite can't be washed,
but should be rinsed with alcohol. A coat of clear lacquer will
protect some samples against dirt.
Arranging Your Collection
Put a spot of enamel on the specimen. Write on the spot--in India
ink--a catalog number and have this number refer to a card in a
file drawer. The card should list date, place found, identification
of specimen, etc.
Group your samples: metallic minerals, semiprecious stones, nonmetallic
minerals. Display them on a shelf, or buy or build a mineral cabinet
with partitioned drawers. For smaller samples, use a Riker mount
with a glass top.
What Do I Have?
How do you identify specimens?
Get books and magazines on rocks and minerals. Many have colored
pictures that help.
But identification is best made by noting the physical characteristics
of the rock or mineral. For minerals, there's a hardness scale in
which a mineral of the higher number can scratch a mineral of the
lower number but not be scratched by it. The scale is: 1) talc;
2)gypsum; 3) calcite; 4) fluorite; 5) apatite; 6) orthoclase;
7) quartz; 8) topaz; 9) corundum; 10) diamond. Remember it by
this silly sentence: "The girls can flirt and other queer things
can do."
When on a trip, remember that a fingernail has a hardness of 2.5;
a penny, 3; a knife blade, 5.5; and a steel file, 6.5. Use these
to scratch your sample and you can get an approximate idea of its
hardness.
You can buy a set of hardness points. They're pointed pieces of
minerals set in brass tubes, each marked with its hardness scale.
The set costs about $30 (half that if you assemble your own).
Other tests for identifying minerals include specific gravity
(weight of mineral compared to the weight of an equal volume of
water), optical properties and crystal form, color and luster.
Minerals differ in cleavage and fracture (how they come apart
when cut). They leave distinctive streaks on unglazed porcelain.
Some are magnetic, some have electrical properties, some glow under
ultraviolet or black light, some are radioactive, some fuse under
a low flame while others are unaffected. Many studies with the
dissolved mineral can identify it beyond doubt.
But most of these are too complicated for the beginner. As you read,
look at pictures and samples, and talk with other rockhounds or
leaders of mineralogy clubs, you'll get better at identifying rocks.
Museum experts and your state's geologist can help, too.
GEMS FOR THE LUCKY FEW
If you're lucky, you'll find gems or semiprecious stones. Gems are
the finer, more crystalline forms of minerals which are ordinarily
less beautiful and spectacular. The true gems are diamonds, emeralds,
rubies and sapphires. All others are semiprecious and ornamental.
Diamonds are pure carbon, but did you know that rubies and sapphires
are corundum minerals--rare forms of alumina. In slightly different
form, they'd turn up on emery paper.
Other stones you might find are the quartz gems: rose quartz,
amethyst, rock crystal, agate, jasper, bloodstone. Or opaque gems
such as jade, moonstone, lapis-lazuli, obsidian, and turquoise.
You don't have to find them. You can buy gems in the rough or in
blanks, then cut and polish them to make your own jewelry or
decorations. This takes practice, plus a cutting and polishing
outfit, wood vise, maybe a diamond wheel. (Or you can join a
lapidary club that might already have the equipment).
First learn to make cabochons--stones with round or curved surfaces.
Then try cutting facets (or faces) in transparent gems. Learn by
reading, working with an expert, trial and error. Making jewelry
is fun, and collecting gems is as interesting as collecting rocks
and minerals; it brings the world into your home. From the West
come agates, jaspers, petrified woods; from the East, colorful
marbles, serpentines, granites. Alaska, Idaho, Connecticut or
Austria will yield dark red garnets. Fine moonstones come from
Ontario; quartz crystals from Hot Springs, Arkansas, can be compared
with similar ones from the Swiss Alps or Brazil.
Rock collecting is a hobby you can tailor to your taste. But
whether you concentrate on an area close to home or travel across
whole continents, you'll find that the pleasure and knowledge you
gain from your collection are matched by the fun and adventure of
the search.