Conchoidalfracture.
Conchoidalfracture.
Thespecific gravityis 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 mineralquartz, 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.
This is a property that depends on the chemical composition of the mineral. Carbonate minerals, which contain (in addition to at least one otherelement) 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 ascalcite(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 aslimestoneandmarble, that contain carbonate minerals.
Beautiful mineral deposits occur in some natural caves. Deposits that look like icicles, calledstalactites, are found hanging from the ceiling of a cave. Other deposits,stalagmites, are like the stalactites except that they jut upward from the floor.Columnsare 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 calledflowstone.Calciteis one of the minerals that commonly form cave deposits.
Just a few of the caves in Texas contain these deposits. They occur mostly in thelimestonerocks that are south and southwest of theLlano upliftarea of central Texas. Some of the commercial caves that contain good examples ofcalcitedeposits 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 dripsover 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 ofcalcite.
Calcitestalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas. Photograph courtesy of the Travel and Information Division of the Texas Highway Department.
Calcitestalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas. Photograph courtesy of the Travel and Information Division of the Texas Highway Department.
Limestone,shale, and othersedimentaryrocks commonly have scattered throughout them masses of other rocks and minerals, such aslimonite,chert, andpyrite. These masses are calledconcretions. 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 wordnoduleis 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 ofsand, to form a concretion.
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 aslarge 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 ofcalcite,celestite, orquartzthat point inward toward the hollow center.
Calcitegeode found in LowerCretaceousstrata of western Travis County, Texas.
Calcitegeode found in LowerCretaceousstrata 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 LowerCretaceouslimestonerocks of Travis, Williamson, and Lampasas counties containcalciteandcelestitegeodes. Celestite geodes have also been found inPermianrocks in parts of Coke, Fisher, and Nolan counties.
Petrified wood from Texas Gulf Coastal Plain.
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 replacedby a mineral. It is formed by underground water carrying dissolved mineral matter. As this water seeps throughsedimentsin which the plants are buried, it gradually depositsagate,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 theTertiary,Permian, and LowerCretaceousrocks of Texas. (SeeOpal,Quartz,Copper Minerals, pp.78,84,52).
Perhaps you would like to start your own collection of rocks and minerals. For this purpose you will need ahammer(a prospector’s hammer with a pick on one end of it is a good tool), somenewspapersto wrap around the specimens to keep them from breaking, and acloth bagin which to carry the specimens.
Prospector’s hammer.
Prospector’s hammer.
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. Apocket knife, acopper penny, a piece ofwindow glass, asteel file, and a piece ofquartzto test the hardness. If you prefer to use a group of minerals of known hardness, such as those of Mohs scale described on pages16-17, you can either collect your own or buy a prepared set from a mineral supply house.2. Astreakplateto test the color of the mineral’s streak. Mineral streak plates can be purchased, or a piece of unglazed tile can be used.3. Amagnifying glassto examine smallcleavagesurfaces, 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 smallmagnetto test whether or not a mineral is magnetic.5.Dilute(10%)hydrochloric acid(HCl), also known asmuriatic 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 aPOISON. If you spill any on yourself, it will burn your skin and eat holes in your clothes.
1. Apocket knife, acopper penny, a piece ofwindow glass, asteel file, and a piece ofquartzto test the hardness. If you prefer to use a group of minerals of known hardness, such as those of Mohs scale described on pages16-17, you can either collect your own or buy a prepared set from a mineral supply house.
2. Astreakplateto test the color of the mineral’s streak. Mineral streak plates can be purchased, or a piece of unglazed tile can be used.
3. Amagnifying glassto examine smallcleavagesurfaces, 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 smallmagnetto test whether or not a mineral is magnetic.
5.Dilute(10%)hydrochloric acid(HCl), also known asmuriatic 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 aPOISON. If you spill any on yourself, it will burn your skin and eat holes in your clothes.
Hand lens.
Hand lens.
The rock and mineral identification charts on pages24-41 will help you to make the simple identification tests in a methodical way.
It is a good idea to have somesystemof 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.
To help you identify them, various Texas rocks and minerals are listed together in the following charts according to properties that they have in common. Although useful, the identification charts may not always give you perfect results. For example, hardness, which is used as a guide, is not to be completely relied upon in the identification of rocks.
The charts on the following pages pertain only to the rocks and minerals that are described in this book. It is quite possible that you will find rocks and minerals in Texas that are not included in these charts.
If you find a rock or a mineral that you are unable to identify, you can check your local library for reference books that may aid you (several such references are noted on pages100-101). If you need further help, possibly the science teacher at a nearby public school will be able to identify the specimen for you. Or if a college or university is located in your area (especially one that has a department of geology), you can obtain help there. In Texas, the Bureau of Economic Geology is a mineral information center. Most other states have similar geological research and public-service organizations. Other sources of information might be the gem and mineral societies that are found in a number of communities. Many of the members of these organizations are experts in the identification of rocks and minerals.
In the mineral identification charts (pp.26-38), the minerals have been grouped, first of all, on the basis ofluster: the first group includes the minerals that appearmetallicandalmost metallic(submetallic); the second group includes those that appearnonmetallic. Next, the minerals have been arranged within the two groups according tocolor.
After you have determined the luster and the color of an unknown mineral, turn to theKey to Mineral Identification Chartsonpage 25. It will direct you to the proper mineral chart.
Mineral Charts1through5, which include the minerals of various colors withmetallicandsubmetalliclusters, are subdivided according to thehardnessof the minerals. To determine the hardness of a mineral that has one of these lusters, you can make the following tests:
1. Will the mineral readily leave a mark on paper?2. If it will not readily leave a mark on paper, will an ordinary pocket knife scratch it?3. Is it too hard to be scratched by an ordinary pocket knife?
1. Will the mineral readily leave a mark on paper?
2. If it will not readily leave a mark on paper, will an ordinary pocket knife scratch it?
3. Is it too hard to be scratched by an ordinary pocket knife?
Mineral Charts6through15are for thenonmetallicminerals of various colors. They, too, are subdivided according to thehardnessof the minerals, as follows:
1. Can the mineral be scratched by a fingernail?2. If it cannot be scratched by a fingernail, can it be scratched by a copper penny?3. If it cannot be scratched by a copper penny, can it be scratched by an ordinary pocket knife?4. If it cannot be scratched by an ordinary pocket knife, can it be scratched by a piece ofquartz?5. Is it too hard to be scratched byquartz?
1. Can the mineral be scratched by a fingernail?
2. If it cannot be scratched by a fingernail, can it be scratched by a copper penny?
3. If it cannot be scratched by a copper penny, can it be scratched by an ordinary pocket knife?
4. If it cannot be scratched by an ordinary pocket knife, can it be scratched by a piece ofquartz?
5. Is it too hard to be scratched byquartz?
When the luster, color, and hardness of a mineral have been determined, you may find that several minerals on the charts fit the description. To narrow your choice, you can then test other properties of the mineral. Notice the “remarks” column on the charts. In it, is mentioned anything that is distinctive about the mineral.
For more complete mineral identification lists and tables, you can use textbooks, such asDana’s Manual of Mineralogy, revised by C. S. Hurlbut, Jr., orMineralogy, by E. H. Kraus, W. F. Hunt, and L. S. Ramsdell.
If the mineral has ametallicorsubmetallicluster,
If the mineral has anonmetallicluster,