Texas Rocks and Minerals: An Amateur's Guide :: ROCK AND MINERAL IDENTIFICATION CHARTS

ROCK AND MINERAL IDENTIFICATION CHARTS

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 pages 100-101). 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.

How To Use the Mineral Identification Charts

In the mineral identification charts (pp. 26-38), the minerals have been grouped, first of all, on the basis of luster: the first group includes the minerals that appear metallic and almost metallic (submetallic); the second group includes those that appear nonmetallic. Next, the minerals have been arranged within the two groups according to color.

After you have determined the luster and the color of an unknown mineral, turn to the Key to Mineral Identification Charts on page 25. It will direct you to the proper mineral chart.

Mineral Charts 1 through 5, which include the minerals of various colors with metallic and submetallic lusters, are subdivided according to the hardness of 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?

Mineral Charts 6 through 15 are for the nonmetallic minerals of various colors. They, too, are subdivided according to the hardness of 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 of quartz?
5. Is it too hard to be scratched by quartz?

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 as Dana’s Manual of Mineralogy, revised by C. S. Hurlbut, Jr., or Mineralogy, by E. H. Kraus, W. F. Hunt, and L. S. Ramsdell.

Key to Mineral Identification Charts

If the mineral has a metallic or submetallic luster,

and is:	Consult Mineral Chart
white	1
gray	2
yellow	3
brown	4
black	5

If the mineral has a nonmetallic luster,

and is:	Consult Mineral Chart
white	6
gray	7
yellow	8
brown	9
black	10
green	11
blue	12
red or pink	13
purple or violet	14
colorless	15

Mineral Identification Charts

ithout breaking

2½-3

   Calcite White Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments 3 

   Halite White Salty taste; dissolves in water; cube-shaped cleavage fragments 2½ 

   Muscovite (white mica) White Light colored; splits into thin, flat, transparent sheets that will bend without breaking 2-2½ 

   Sulfur White or pale yellow Will burn with a blue flame; commonly found as crystals, crusts, or grains 1½-2½ 

  C. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife 

   Barite White Rather heavy; cleavage fragments may be flat and slab-like 3-3½ 

   Dolomite White Commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite 3½-4 

   Fluorite White Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cube-shaped 4 

  D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz 

   Cassiterite Pale brown, pale yellow, or white Brown, reddish brown, or yellowish brown; heavy; dull to brilliant luster 6-7 

   Feldspar White Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks 6 

   Garnet White Commonly occurs as crystals 6½-7 

   Quartz White Curved conchoidal fracture; brown smoky quartz crystals commonly 6-sided prisms with pyramid-like ends; chalcedony, chert, and jasper may be a shade brown, too 7 

   Tourmaline White Dark brown variety is dravite; notice hardness, striations on crystals, and triangular cross section of some crystals 7-7½ 

  E. Cannot be scratched by quartz 

   Tourmaline White Dark brown variety is dravite; notice hardness, striations on crystals, and triangular cross section of some crystals 7-7½ 

10.

NONMETALLIC luster, BLACK color

  A. Cannot be scratched by a fingernail but can be scratched by a copper penny 

   Biotite (black mica) White Splits into thin, flat sheets that will bend without breaking 2½-3 

  B. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz 

   Garnet White Commonly occurs as crystals 6½-7 

   Quartz White Curved conchoidal fracture; brownish-black smoky quartz crystals commonly 6-sided prisms with pyramid-like ends; chalcedony and chert may be black, too 7 

   Tourmaline White Black variety is schorl; notice hardness, striations on crystals, and triangular cross section of some crystals 7-7½ 

  C. Cannot be scratched by quartz 

   Tourmaline White Black variety is schorl; notice hardness, striations on crystals, and triangular cross section of some crystals 7-7½ 

11.

NONMETALLIC luster, GREEN color

  A. Can be scratched by a fingernail 

   Amphibole asbestos White Made up of slender, flexible fibers that can be pulled apart 1-2½ 

   Cerargyrite Shiny white or gray Light greenish color; appears waxy; knife cuts it smoothly; turns violet brown to black when exposed to light 1-1½ 

   Muscovite (white mica) White Light colored; splits into thin, flat, transparent sheets that will bend without breaking 2-2½ 

   Sulfur White or pale yellow Greenish; will burn with a blue flame; commonly found as crystals, crusts, or grains 1½-2½ 

   Talc White Light greenish color; knife cuts it smoothly; feels soapy or greasy; splits into thin flakes 1 

  B. Cannot be scratched by a fingernail but can be scratched by a copper penny 

   40-41, the Texas rocks described in this book are arranged in four major groups according to their texture.
 1. Glassy (the rocks are smooth, dark, and shiny) 2. Compact, dull, or stony (the rocks are smooth and dull, but the individual grains are too small to be recognized) 3. Granular (at least some of the individual grains of the rocks are large enough to be seen without a magnifying glass) 4. Fragmental (the rocks are made up of fragments that are either loose or cemented together) Consult Rock Chart 1, if the rock is glassy; Chart 2, if it is compact, dull, or stony; Chart 3, if it is granular; and Chart 4, if it is fragmental.
Two of the rock charts are subdivided. In Rock Chart 2, the compact, dull, or stony rocks are arranged according to hardness as follows:
 A. Rocks that can be scratched by a fingernail B. Rocks that cannot be scratched by a fingernail but can be scratched by an ordinary pocket knife C. Rocks that cannot be scratched by an ordinary pocket knife In Rock Chart 3, the granular rocks also are arranged according to hardness into:
 A. Rocks that can be scratched by an ordinary pocket knife B. Rocks that cannot be scratched by an ordinary pocket knife These harder rocks are subdivided into three groups: 1. Those that have grains of about equal size 2. Those with large easily seen grains that are scattered through a mass of finer grains 3. Those rocks whose grains are arranged in layers In the “remarks” column of the rock identification charts are included further tests that will aid you in identifying the rock.
For a more complete rock determination chart, you can consult a textbook, such as Rocks and Rock Minerals, by L. V. Pirsson and A. Knopf.
 Rock Identification Charts   Chart No. Rock Remarks 
 1. GLASSY appearance (rock is dark, smooth, and shiny) 
   Obsidian Entire rock is glassy 
   Vitrophyre Crystalline grains are scattered through the dark glassy mass 
 2. COMPACT, DULL, OR STONY appearance (individual grains too small to be recognized) 
  A. Can be scratched by a fingernail 
   Chalk Dilute hydrochloric acid fizzes on it 
   Clay Earthy odor when breathed on 
   Diatomite Crumbly 
   Rock gypsum Made up of the mineral gypsum 
   Soapstone Soapy or greasy feel 
  B. Cannot be scratched by a fingernail but can be scratched by ordinary pocket knife 
   Dolomite Dilute hydrochloric acid may fizz slightly on it; will fizz if rock is powdered 
   Limestone Dilute hydrochloric acid fizzes on it 
   Serpentine rock Commonly some shade of green 
   Shale Breaks in flat, thin flakes; earthy odor 
  C. Cannot be scratched by an ordinary pocket knife 
   Basalt Dark colored and heavy 
   Chert Hard, smooth, and porcelain-like 
   Rhyolite Light to dark colored; may show streaks or flow structure 
 3. GRANULAR appearance (at least some of the individual grains are large enough to be seen without a magnifying glass) 
  A. Can be scratched by an ordinary pocket knife 
   Limestone Dilute hydrochloric acid will fizz on it 
   Marble Dilute hydrochloric acid will fizz on calcite marble, and it may fizz slightly on dolomite marble 
   Rock gypsum Made up of the mineral gypsum 
   Rock salt Has a salty taste; made up of the mineral halite 
  
  B. Generally cannot be scratched by an ordinary pocket knife (some schist is softer) 
  1. Grains are of about equal size (equigranular) 
   Granite Quartz and feldspar grains interlock 
   Pegmatite Large interlocking grains; commonly feldspar, quartz, mica 
   Quartzite Rock breaks across the quartz grains 
   Sandstone Rock breaks through the cement but around the sand grains 
  2. Easily seen grains are scattered through a mass of finer grains 
   Basalt Dark colored, heavy rock 
   Llanite Rusty pink feldspar and blue quartz grains embedded in a brownish rock mass 
   Rhyolite porphyry Light to dark colored rock; may show streaks or flow structure 
  3. Grains are arranged in layers 
   Gneiss Interlocking grains are in straight or wavy bands 
   Schist Splits in thin layers; some schist is soft enough to be scratched by a knife 
 4. FRAGMENTAL appearance (rocks are made up of fragments that are either loose or cemented together) 
   Breccia Angular, gravel-size fragments that are cemented together 
   Conglomerate Rounded, gravel-size fragments that are cemented together 
   Coquina Shells and shell fragments that are cemented together 
   Gravel Loose fragments 
   Pulverulent limestone Loose, powdery fragments; dilute hydrochloric acid fizzes on them 
   Sand Loose fragments no larger than a pinhead 
   Sandstone Sand fragments that are cemented together 
   Volcanic ash Loose, fine, gritty particles 
 Physiographic outline map of Texas.
                                                                                                                                                                                                                                                                                                            

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