Occurrence and Properties of Minerals
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.
A scalenohedron, one of the many crystal forms of [calcite].
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).
[Barite] specimen showing radial form.
[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.
[Chalcedony] showing botryoidal form.
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.
[Transparent] mineral.
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.
A [streak] plate is used to determine the color of the streak or powder of a mineral.
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.
Some minerals will cleave in only one direction; some, in several directions. For example, [galena], a mineral found in Texas, has perfect cubic [cleavage]. It cleaves in three directions that are at right angles to each other. These cleavage directions are parallel to possible cubic crystal faces, and some of the [cleavage fragments] are [cubes].
[Parting]
A few minerals sometimes show a kind of false [cleavage] known as [parting]. Parting, unlike cleavage, is not constant and does not occur in every specimen of a particular mineral. For this reason, it is not a very dependable means of identification.
[Fracture]
Minerals also break in another way. When the break is in a different direction from that of the [cleavage] or [parting], it is known as the [fracture]. A fracture is called [conchoidal] if the mineral’s broken surface is curved like the inside of a spoon or shell. Thick pieces of glass break with this conchoidal fracture. A fracture is described as hackly if the broken surface has sharp, jagged edges; as even, if the surface is generally flat; and as uneven, if it is rough and not flat. If the mineral breaks into splinters, its fracture is called splintery.
[Specific Gravity]
The [specific gravity] is a measure of whether a mineral is heavy or light. It is a comparison of the weight of a piece of the mineral with the weight of an equal volume of water. The mineral [quartz], for example, has a specific gravity of 2.65. This means that a piece of quartz is a little more than 2½ times as heavy as an equal volume of water. Accurate measurements of specific gravity can be made in a laboratory. You can, however, learn to estimate specific gravities just by lifting various minerals and judging whether they are heavy or light.
Effervescence in Acid
This is a property that depends on the chemical composition of the mineral. Carbonate minerals, which contain (in addition to at least one other [element]) three parts of oxygen and one part of carbon, can be tested with dilute hydrochloric acid. When a drop or two of this acid is put on a carbonate mineral such as [calcite] (calcium carbonate, CaCO₃), the acid begins to bubble and fizz. The fizzing or effervescence is caused by the carbon dioxide gas that is formed when the acid and mineral come in contact with each other. This test is also helpful in identifying rocks, such as [limestone] and [marble], that contain carbonate minerals.
SOME SPECIAL OCCURRENCES OF MINERALS
Cave Deposits
Beautiful mineral deposits occur in some natural caves. Deposits that look like icicles, called stalactites, are found hanging from the ceiling of a cave. Other deposits, stalagmites, are like the stalactites except that they jut upward from the floor. Columns are formed from stalactites and stalagmites that have joined together. In addition, some caves contain sheet-like deposits that are spread along the ceiling, floor, and walls. These deposits are called flowstone. [Calcite] is one of the minerals that commonly form cave deposits.
Just a few of the caves in Texas contain these deposits. They occur mostly in the [limestone] rocks that are south and southwest of the [Llano uplift] area of central Texas. Some of the commercial caves that contain good examples of [calcite] deposits are located near Boerne in Kendall County and near Sonora in Sutton County. Calcite deposits also occur in Longhorn Cavern, a large cave located in the Longhorn Cavern State Park of Burnet County. These caves were formed by underground waters that moved through cracks and pores in the limestone rocks and dissolved passageways in them. After the cave passages were made, water containing dissolved calcium carbonate dripped into the cave. As it evaporated, this water left behind a deposit of calcium carbonate—the mineral calcite.
You can better understand how the cave deposits are formed by watching icicles grow in wet, freezing weather. First, small hanging drops of water freeze, and a small icicle forms. Then, as more water drips over it and freezes, the icicle grows longer and wider. Some of the water drips completely over the icicle and falls to the ground. There, it either freezes into a sheet of ice, or it begins to build upward to form an upside-down icicle. The water dripping down in the caves evaporates instead of freezing, and in doing so it leaves behind a deposit of [calcite].
[Calcite] stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas. Photograph courtesy of the Travel and Information Division of the Texas Highway Department.
Concretions
[Limestone], [shale], and other [sedimentary] rocks commonly have scattered throughout them masses of other rocks and minerals, such as [limonite], [chert], and [pyrite]. These masses are called concretions. Concretions may be round or oval, or they may have odd, irregular shapes. They—such as some of the limonite concretions of east Texas—even may look like gourds or sweet potatoes. Concretions generally are harder than the surrounding rocks. Some are smaller than peas, but others are several feet wide. (The word [nodule] is used to describe small, rounded concretions as well as other small, rounded mineral occurrences.)
It is believed that some concretions form at the same time as the rocks in which they occur. Other concretions develop after the rocks themselves have formed. These are deposited by underground water that contains dissolved mineral matter. The water seeps through the rocks and deposits mineral matter around an object in the rock, such as a fossil or a grain of [sand], to form a concretion.
Geodes
Geodes are rounded, generally hollow masses that occur mostly in limestones. They are scattered through the rocks and can be lifted or dug out. Some geodes are as small as walnuts, and some are as large as basketballs. Most of them have a rough, dull-looking outer surface. If you break geodes open, you will find that many are lined with beautiful crystals of [calcite], [celestite], or [quartz] that point inward toward the hollow center.
[Calcite] geode found in Lower [Cretaceous] strata of western Travis County, Texas.
It is thought that a geode forms when water, carrying dissolved mineral material, seeps into a cavity in the rock, then deposits the mineral material as a lining in the cavity. This lining becomes the outer part of the geode. Thus a geode—unlike a concretion, which grows from the center outward—forms from outside to inside.
Some of the Lower [Cretaceous] [limestone] rocks of Travis, Williamson, and Lampasas counties contain [calcite] and [celestite] geodes. Celestite geodes have also been found in [Permian] rocks in parts of Coke, Fisher, and Nolan counties.
Petrified Wood
Petrified wood from Texas Gulf Coastal Plain.
We often find some minerals occurring as petrified wood. (Petrified wood includes silicified wood, opalized wood, [agatized wood], and carbonized wood.) Petrified wood forms when plant material, such as a tree or a bush, is replaced by a mineral. It is formed by underground water carrying dissolved mineral matter. As this water seeps through [sediments] in which the plants are buried, it gradually deposits [agate], [chalcedony], [calcite], [opal], [chalcocite], or some other mineral in the place of each fiber of the wood. By this slow change from plant to mineral matter, the original shape and structure of the wood remain unchanged.
Petrified wood is commonly found in some of the [Tertiary], [Permian], and Lower [Cretaceous] rocks of Texas. (See [Opal], [Quartz], [Copper Minerals], pp. [78], [84], [52]).