Part 4

HOW MINERALS OCCUR

Rocks are made up of minerals. In addition, minerals are associated with rocks in other ways. For example, minerals fill or coat cracks and cavities that have developed in some of the rocks. Minerals are either crystalline or amorphous.

Crystalline Minerals

Most minerals are crystalline. In crystalline minerals, combinations of atoms are arranged in ordered patterns, which are repeated over and over. This orderly internal structure of atoms is a characteristic of each crystalline mineral, as mineralogists are able to determine by using X-rays and special microscopes.

Crystals.—

When a mineral occurs as a well-formed individual crystal, it has a definite, precise shape. The kind of crystal shape it has depends on its own type of crystalline internal structure. A well-formed crystal has smooth, flat, outer surfaces called crystal faces, which are arranged together to form prisms, cubes, pyramids, and many other geometric shapes. For example, quartz, a common Texas mineral, is commonly found as a six-sided, prism-shaped crystal that is topped by pyramid-like forms. Pyrite, another common mineral, occurs as cube-shaped crystals. We can identify some minerals more readily by learning to recognize their crystal shapes.

Imperfect crystals.—

A crystalline mineral commonly forms under conditions that do not permit it to become a well-shaped crystal. Although the mineral may show a few crystal faces, it does not have a complete crystal shape and so is described as massive, or is said to occur in masses. Some of the minerals that make up rocks occur as crystalline masses. For example, calcite is a crystalline mineral that occurs in the metamorphic rock marble without its normal crystal shape.

Many crystalline minerals occur as incomplete and imperfect crystals that are grouped together in various arrangements. If these incomplete crystals are arranged around a common center like the spokes of a wheel, they are said to be radial or radiated. If the groups of incomplete crystals look like bundles of strings or fibers, they are described as fibrous. If they are in rounded masses that resemble bunches of grapes, they are called botryoidal. If they look like fish scales, they are described as scaly. Some crystalline minerals are made up of tiny grains that are grouped together like the grains in a lump of sugar. A mineral occurring in this way is described as granular. More descriptions of crystalline minerals are found in the section on Texas rocks and minerals (pp. 43-98).

Amorphous Minerals

An amorphous mineral, unlike a crystalline mineral, does not have a definite, orderly arrangement of its atoms. Because of this lack of internal structure, the mineral occurs in masses that have no regular geometric shapes, and it has no crystal form of its own. Only a few minerals are amorphous.

SOME DISTINGUISHING PROPERTIES OF MINERALS

We use our senses of sight, hearing, smell, touch, and taste to become aware of the world around us. For example, we recognize a flower by noting its color, its fragrance, and the texture, shape, and arrangement of its petals. These are some of its characteristic properties. A mineral also has distinguishing properties, among them color, luster, and hardness, which help us identify it. Some minerals have a single outstanding property, such as the magnetism of magnetite, that makes them easier to recognize. But to identify most minerals, we need to determine not just one, but several properties.

Color

Color is one of the properties we notice first. The color of some minerals is always the same, and it helps us to identify them. But it is not a dependable property to use in identifying all minerals, because some contain impurities that change or hide the real color.

Luster

The luster is the way the surface of a mineral reflects light. The luster of a mineral may be nonmetallic, submetallic, or metallic. Mineral metals such as gold, silver, galena, and pyrite have a metallic luster. A few minerals have a luster that is almost, but not quite metallic—their luster is submetallic. A mineral with a nonmetallic luster may look vitreous (glassy), silky, resinous (like resin), greasy, earthy (dull), pearly, or adamantine (brilliant).

Transmission of Light

Some minerals allow light to pass through them; others do not. A mineral is transparent if you can see both light and objects through it, as through clear glass. If you can see only light, but no objects, as through frosted glass, the mineral is translucent. When you hold an opaque mineral up to the light, it looks dark. No light at all comes through it, even through the thin edges.

Hardness

Some minerals are soft and can be scratched easily. Others, which are harder, are resistant to scratching. To measure a mineral’s hardness, we try to find out which substances will scratch it and which substances will not scratch it. To do this in a general way, several ordinary objects—such as a fingernail, a copper penny, a pocket knife, a piece of window glass, and a steel file—can be used. For a more exact way of testing hardness, we can use ten minerals that make up what is known as Mohs scale. Each mineral in this scale has a different hardness, and each one has been given a number that represents its hardness. For example, talc, the softest mineral in this scale, is given a hardness of 1. Gypsum, the next softest mineral in the scale, has a hardness of 2. Diamond, the hardest mineral known, is given the top hardness of 10 in this scale. These ten minerals are listed below. Alongside them are five common objects with their hardnesses.

1—Talc 2—Gypsum Fingernail—slightly over 2 3—Calcite Copper penny—about 3 4—Fluorite 5—Apatite Pocket knife—slightly over 5 6—Orthoclase Window glass—5½ 7—Quartz Steel file—about 6½ 8—Topaz 9—Corundum 10—Diamond

Suppose, for example, that a mineral can be scratched by fluorite, which has a hardness of 4 on Mohs scale, but cannot be scratched by calcite, which has a hardness of 3. We then know that this mineral is softer than fluorite, but harder than calcite; therefore, it has a hardness of about 3½. In the same way, if a mineral can be scratched by a pocket knife, which is slightly more than 5 in hardness, but not by a copper penny, which has a hardness of about 3, we know then that its hardness is between 3 and 5.

Streak or Powder

The streak is the mark, made of fine powder, that a mineral leaves as you rub it across a streak plate. A streak plate is a flat piece of white tile or porcelain that has a dull, unglazed surface. The streak plate is about as hard as quartz, which is 7 on Mohs scale, and you will not be able to use it for minerals that have a greater hardness. For these, you can obtain the powder by scratching the mineral or by crushing a small piece of it.

The color of the streak or powder is extremely helpful in identifying some minerals. For example, hematite is a mineral that may be any one of several different colors, but its streak or powder is always reddish brown.

Cleavage

As they break, some crystalline minerals always split along a smooth, flat surface. This property is known as cleavage. Some cleavages are smooth and perfect; others are not so perfect. The cleavage surfaces, because of the mineral’s crystalline internal structure, are parallel to possible crystal faces, even though the mineral itself may occur as a crystalline mass without a perfect crystal shape.