Part 2

Some of the rock fragments carried by water are still fairly large when they reach their destinations. On the basis of size, they are called _boulders_, _cobbles_, _pebbles_, and _granules_. Loose deposits of these larger-size sediments make up what is known as _gravel_. Nature cements gravels together to form rocks such as _conglomerates_ (made up of rounded gravel) and _breccias_ (made up of sharp-cornered gravel).

The finer sediments are called _sand_, _silt_, _mud_, and _clay_. When cemented, the sand grains become _sandstones_, the silt particles become _siltstones_, and the mud and clay particles become _shale_. The sedimentary rocks that are made up of these rock fragments are called _clastic_ or _fragmental_ rocks.

Sedimentary Rock Materials in Solution

As they are weathered, some rocks dissolve and go into solution. For example, a number of the Texas creeks and rivers carry calcium carbonate in solution because they flow through areas where limestone rocks, which consist mostly of calcium carbonate, are being weathered. (Water that contains a large amount of dissolved rock material is called _hard_ water.)

_Cementing materials and chemical sediments._—

Some of the waters containing dissolved rock material seep through loose sediments where the dissolved material may come out of solution and form a _cement_, which binds the sediments together. For example, when loose sand sediments are cemented, they form sandstone. Three of the most common cements are iron oxide, calcium carbonate, and silicon dioxide, although a number of other materials also serve as cements.

Dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some sedimentary rocks as well. Sedimentary rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers. When the dissolved material comes out of solution, it is said to be _precipitated_ and the mineral sediments it forms are the _chemical_ sediments. Some limestones originate this way. You can see examples of precipitated materials by noting the crust-like deposits that form inside some water pipes and teakettles, as dissolved material in the water comes out of solution.

_Sedimentary rocks formed by plants and animals._—

The dissolved rock material can come out of solution in another way. Some plants and animals are able to take dissolved calcium carbonate out of the sea water and use it to build their shells and other structures. Some of these organisms, such as corals and algae, can grow upward from the sea floor in large groups to form reefs that later become reef limestones. Other limestones are made up of the remains of plants and animals that collect on the sea floor and become cemented together.

METAMORPHIC ROCKS

Metamorphic rocks come from earlier-formed rocks that have undergone a change or a _metamorphosis_. All igneous and sedimentary rocks, and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, granite, an igneous rock, can be changed into the metamorphic rock known as _gneiss_; limestone, a sedimentary rock, can be changed into _marble_; shale, a sedimentary rock, can be changed into _slate_. These changes occur because the earth is a big and complex chemical system. The agents that bring about these changes, which always occur below the surface of the earth, are heat, pressure, and fluids—both liquids and gases. Several different kinds of change or metamorphism can take place.

Static Metamorphism

Some of the changes occur because the rocks are at great depths. As more and more younger rocks are deposited on top of them, the older rocks become deeply buried. The great thicknesses of younger rocks are heavy, and they squeeze and press down on the rocks beneath them. The deeply buried rocks are also hotter than surface rocks. In general, the temperature increases about 1° Fahrenheit for each 50 feet of depth below the surface. The change of deeply buried rocks into new rocks by pressure and heat is known as _static metamorphism_.

Contact Metamorphism

Another method of change or metamorphism involves molten igneous rock material. When hot magma moves up through rocks, it not only heats and pushes them, but it also may soak them with liquids and gases, causing the nearby rocks to change into new rocks, by a process called _contact metamorphism_.

UNALTERED ROCK METAMORPHIC ROCK MAGMA

Dynamic Metamorphism

Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, fracture, or slide over each other. These forces cause great changes in the rocks in widespread areas. This process of change is known as _dynamic metamorphism_.

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.

_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.

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.

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.

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

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).

COLLECTING ROCKS AND MINERALS

Perhaps you would like to start your own collection of rocks and minerals. For this purpose you will need a _hammer_ (a prospector’s hammer with a pick on one end of it is a good tool), some _newspapers_ to wrap around the specimens to keep them from breaking, and a _cloth bag_ in which to carry the specimens.

Before you start to collect, be sure to ask the owner’s permission to go on his property. If he agrees to let you come on his land, be careful about closing gates, and do not leave holes into which his livestock might step and be injured. Look out for snakes. Plenty of rattlers, copperheads, and moccasins are still left in Texas. And, incidentally, collecting is not allowed in State or National parks.

To identify the rocks and minerals that you collect, you probably will need several articles with which to make simple tests. The following can be easily obtained:

1. A _pocket knife_, a _copper penny_, a piece of _window glass_, a _steel file_, and a piece of _quartz_ to test the hardness. If you prefer to use a group of minerals of known hardness, such as those of Mohs scale described on pages 16-17, you can either collect your own or buy a prepared set from a mineral supply house.

2. A _streak plate_ to test the color of the mineral’s streak. Mineral streak plates can be purchased, or a piece of unglazed tile can be used.

3. A _magnifying glass_ to examine small cleavage surfaces, crystals, and rock grains. A number of different kinds can be bought, from the simple reading glass to the precisely made hand lens. A lens with ten-power magnification is good for general use.

4. A small _magnet_ to test whether or not a mineral is magnetic.

5. _Dilute_ (10%) _hydrochloric acid_ (HCl), also known as _muriatic acid_, to test carbonate rocks and minerals. You can buy a small bottle at a drug store. Be extremely careful in handling this acid, and keep it away from small children—it is a _POISON_. If you spill any on yourself, it will burn your skin and eat holes in your clothes.

The rock and mineral identification charts on pages 24-41 will help you to make the simple identification tests in a methodical way.

It is a good idea to have some system of labeling your rock and mineral specimens. Some collectors carry note paper with them on field trips. Then they can write down the location and, if possible, the name of the rock or mineral. This information is either wrapped with the specimen or stuck to it with tape. One way to label large collections is to put a small spot of paint or fingernail polish on each of the rock and mineral specimens. When the paint has dried, a number can be written on it in black India ink. Then, on a file card, the name and the number of the specimen can be written, together with the place where it was found, the date of collection, and the name of the collector.

ROCK AND MINERAL IDENTIFICATION CHARTS