Mineral Reading Handout
Using Resources
Geologists study minerals and rocks. Many geologists work for companies that find and
extract Earth’s fuel or mineral resources. If you look around, you will see products from
Earth almost everywhere. Most of the metal in furniture, vehicles, and building is iron or
steel. These metals originally come from ores. An ore is a mineral from which a metal or
nonmetal can be extracted profitably. Copper and aluminum ores are also mined from the
Earth and made into pots pans, pipes, and other useful products. Coal and petroleum
products are used as fuels as well as in manufacture of plastics, medicines, paints, and
cosmetics. Glass is made from sand. We also use sand and gravel to make roads, and we
combine them with limestone to make concrete. Our most basic construction materials as
well as most of our most advanced products, such as computer chips, are made from Earth
materials.
When Resources Run Out
The amount of mineral resources is limited. When these resources are depleted, people
will be faced with
1. Certain technologies and their products could be abandoned.
2. Resources can be conserved through restricted use and recycling.
3. Substitutes can be found for dwindling resources.
We have solved these problems in the past. Early in the industrial revolution, the
demand for energy severely depleted the forest of Britain. As trees become scarce, first coal
then oil was used to supply energy. In many technologically advanced countries, wood is
seldom used to supply energy. Instead, it is used for buildings and furniture.
Minerals
Minerals are natural, inorganic, uniform substances. Some minerals are chemical
elements or compounds. Graphite and diamond are different forms of elemental carbon.
Quartz (SiO2), also called silica, and calcite (CaCO3) are chemical compounds composed of
elements in set ratios. A third group of minerals are mixtures, for which chemical formulas
cannot be written. For example, minerals in the feldspar family contain potassium, calcium,
and/or sodium in varying proportions along with silicon, oxygen and aluminum. Rocks are
formed from one or more minerals.
Although thousands of minerals have been identified and classified, less than a dozen are
commonly found. Clay, feldspar, quartz, and calcite make up the bulk of minerals found near
Earth’s surface. Pyroxene, amphibole, and olivine are common deep underground, but less
common at or near the surface. Oxygen and silicon make up more than half of the mass of
these minerals. With just another dozen elements, you can account for about 98 percent of
the mass of rocks.
Identifying Minerals
Geologists usually identify minerals on the basis of their physical and chemical
properties. For the rare or uncommon minerals, geologists use charts, or a more
comprehensive information contained in a mineralogy resource.
Color
Many minerals have a characteristic color. Pyrite (fools gold) is usually a brassy yellow
color. Amandine garnet is valued for its deep red color. But certain colorless or white
minerals are often discolored by impurities. Quartz and calcite are usually colorless,
although they may be found in almost any color, depending on the impurities they contain.
Dark-colored minerals such as the amphibole and pyroxene families are easier to identify
because impurities are unlikely to cause color variations.
Luster
Luster describes the way light is reflected from the freshly cut surface of a mineral.
Minerals with metallic luster have a hard, shiny look, like polished metal. This is because
light is unable to penetrate the surface and almost all the light is reflected. Minerals with
nonmetallic luster can also be shiny, but nonmetallic luster differs from a metallic luster
because some of the light is transmitted into and through the mineral while some is reflected.
Nonmetallic luster include glassy, waxy, pearly, and earthy, which is dull.
Streak
Rubbing a fresh corner of the minerals across a white unglazed streak plate performs the
test for streak. The streak is the powdered form of the mineral. Some metallic minerals
leave behind a powder that is not the same color as the mineral.
Crystal Structure
A crystal is a regularly shaped solid formed by an ordered pattern of atoms. Minerals
form characteristically shaped crystals. Calcite and quartz can look very similar. Both are
fairly common and are often white or colorless with a glassy luster. However, their crystals
are very different. Quartz crystals are hexagonal (six-sided) in cross section, while crystals of
calcite from rhombohedra solids. (A rhombohedra solid resembles a rectangular solid that
has been pushed over to one side so that all the faces have parallel edges, but none meet at
right angles) Pyrite and galena form cubic crystals. The crystals of mica grow in “books” of
thin flexible sheets. Many minerals form distinctive and beautiful crystals. For that reason,
crystals are called the “flowers” of the mineral world.
Hardness
Calcite and quartz also differ in hardness. You test for hardness by scratching the
unknown mineral with the edge or point of other minerals of known hardness. Quartz is able
to scratch most other minerals. Because calcite is softer than quartz, calcite cannot scratch
quartz, but quartz can scratch calcite. The geologist Friedrich Mohs, created a scale of
hardness that uses relatively common minerals. On the Moh’s scale talc is the softest
mineral. The hardness of talc is 1. The hardest natural substance is diamond. Its hardness is
10. The hardness of a fingernail is 2.5, calcite is 3, window glass is 5.5 and quartz has a
hardness of 7.
Cleavage
If a mineral breaks readily along flat surfaces it shows cleavage. Cleavage comes from a
Latin word meaning to cut, or cleave. Many minerals break along flat surface called cleavage
planes. These cleavage planes are sometimes but not always parallel to the sides of crystals.
For example, halite (rock salt) breaks into small cubes and rectangular solids the same shape
as it’s crystals. However, quartz, which forms hexagonal crystals with flat faces, does not
break along parallel planes. Quartz breaks along curved surfaces: a property known as
concoidal fracture.
Density
Minerals also vary in density and specific gravity. Specific gravity is a ratio of the density
of a substance to the density of water. Water’s density is 1 g/cm 3. Therefore, a mineral with
a density of 4 g/cm3 has a specific gravity of 4. Gold is the densest substance you commonly
see, it has a density of about 10 g/cm3, and the density of lead is about 12 g/cm3. You can
determine density in two ways: dividing the mass of a sample by its volume or by floatation.
A substance will sink in a liquid that is less dense than it is and float in a liquid that is denser
than it is.