Part 5

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: