Part 3

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 14 violet colorless 15

Mineral Identification Charts

_Chart _Mineral_ _Streak_ _Remarks_ _Hardness_ No._

1. METALLIC luster, WHITE color

A. Does not readily leave mark on paper but can be scratched by ordinary pocket knife Native silver Shiny silver Silver-white 2½-3 white, unless color that tarnished tarnishes to gray, black, or yellowish brown; heavy; can be flattened when hit with hammer

2. METALLIC or SUBMETALLIC luster, GRAY color

A. Will leave mark on paper Argentite Shiny, blackish Lead-gray color 2-2½ to lead gray that tarnishes to dull black; knife cuts it smoothly; heavy; may occur as masses and coatings Galena Grayish black Shiny lead-gray 2½ color; heavy; cube-shaped fragments and crystals Graphite Black Steel-gray color; 1-2 greasy feel; very soft; splits into thin flakes B. Does not readily leave mark on paper but can be scratched by ordinary pocket knife Chalcocite Grayish black Shiny lead-gray 2½-3 color that tarnishes to dull black; knife cuts it smoothly; may have black sooty coating; commonly occurs as compact or granular masses Hollandite Black Silvery-gray 4-6 color; may occur as rounded masses C. Cannot be scratched by ordinary pocket knife Braunite Steel gray or Dark steel-gray 6-6½ black color and submetallic luster Hematite Dark reddish brown Steel-gray color; 5½-6½ commonly occurs (may be as granular or softer) compact masses; shiny, scaly variety is _specular_ hematite; notice streak Hollandite Black Silvery-gray 4-6 color; may occur as rounded masses

3. METALLIC luster, YELLOW color

A. Does not readily leave mark on paper but can be scratched by ordinary pocket knife Chalcopyrite Greenish black Brass-yellow or 3½-4 golden-yellow color that may tarnish and show rainbow-like colors; commonly massive; notice streak Gold Shiny golden Shiny yellow 2½-3 yellow color; extremely heavy; flattens when hit with hammer; notice streak B. Cannot be scratched by ordinary pocket knife Pyrite Black, greenish Shiny, pale 6-6½ black, or golden-yellow or brownish black brass-yellow color that may tarnish; occurs as grains, as masses, or as cubes or other crystal shapes; notice hardness and streak

4. METALLIC or SUBMETALLIC luster, BROWN color

A. Does not readily leave mark on paper but can be scratched by ordinary pocket knife Limonite Rusty yellowish Dark-brown color; 5-5½ brown some specimens have a shiny black surface; notice streak B. Cannot be scratched by an ordinary pocket knife Cassiterite Pale brown, pale Brown; 6-7 yellow or white submetallic; heavy; notice streak Hematite Dark reddish brown Dark brown color; 5½-6½ commonly occurs (may be as granular or softer) compact masses; notice streak Limonite Rusty, yellowish Dark brown color; 5-5½ brown some specimens have a shiny black surface; notice streak

5. METALLIC or SUBMETALLIC luster, BLACK color

A. Will leave mark on paper Argentite Shiny, blackish Lead-gray color 2-2½ to lead grey that tarnishes to dull black; knife cuts it smoothly; heavy; may occur as masses and coatings Graphite Black Greasy feel; very 1-2 soft; splits into thin flakes Pyrolusite Black Very soft; will 1-2 soil fingers; may be powdery B. Does not readily leave mark on paper but can be scratched by an ordinary pocket knife Chalcocite Grayish black Shiny lead-gray 2½-3 color that tarnishes to dull black; knife cuts it smoothly; may have a black sooty coating; commonly occurs as compact or granular masses Hollandite Black May occur as 4-6 rounded masses Limonite Rusty, yellowish Some specimens 5-5½ brown have shiny black surface; notice streak C. Cannot be scratched by an ordinary pocket knife Braunite Steel gray or Luster is 6-6½ black submetallic Cassiterite Pale brown, pale Submetallic 6-7 yellow, or white luster; heavy; notice streak Hematite Dark reddish brown Notice streak; 5½-6½ commonly occurs (may be as granular or softer) compact masses Hollandite Black May occur as 4-6 rounded masses Limonite Rusty yellowish Some specimens 5-5½ brown have shiny black surface; notice streak Magnetite Black Fragments cling 6 to a magnet Pitchblende Brownish black Brownish black, 5½ greenish black, or black; radioactive; heavy; may appear dull or greasy

6. NONMETALLIC luster, WHITE color

A. Can be scratched by a fingernail Cerargyrite Shiny white or Appears waxy; 1-1½ gray knife cuts it smoothly; turns violet brown to black when exposed to light Gypsum White Soft; occurs as 2 crystals or as fibrous, granular, compact, or earthy masses Talc White Knife cuts it 1 smoothly; feels soapy or greasy; splits into thin flakes B. Cannot be scratched by a fingernail but can be scratched by a copper penny Anhydrite White Commonly occurs 3-3½ as sugary-looking masses Barite White Rather heavy; 3-3½ cleavage fragments may be flat and slab-like Calcite White Dilute 3 hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments Celestite White Not quite as 3-3½ heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like Halite White Salty taste; 2½ dissolves in water; cube-shaped cleavage fragments C. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife Anhydrite White Commonly occurs 3-3½ as sugary-looking masses Barite White Rather heavy; 3-3½ cleavage fragments may be flat and slab-like Celestite White Not quite as 3-3½ heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like Dolomite White Commonly occurs 3½-4 as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes Opal White Curved, 5-6 conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Feldspar White Glassy or pearly 6 luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks Opal White Curved, 5-6 conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors Quartz White Curved conchoidal 7 fracture; occurs as milky quartz, chert, and chalcedony; crystals commonly 6-sided prisms with pyramid-like ends

7. NONMETALLIC luster, GRAY color