Part 8

A. 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 B. 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 Azurite Blue Bright, intense 3½-4 blue color; dilute hydrochloric acid will fizz on azurite 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 Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cube-shaped 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 C. 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° 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 crystalline quartz and as bluish chalcedony D. Cannot be scratched by quartz Topaz White Perfect basal 8 cleavage gives flat, plate-like fragments; notice hardness

13. NONMETALLIC luster, RED or PINK color

A. Can be scratched by a fingernail Gypsum White Reddish; soft; 2 occurs as crystals or as fibrous, granular, compact, or earthy masses Hematite Dark reddish brown Brownish-red 1+ color; soft and earthy Sulfur White or pale Reddish; will 1½-2½ yellow burn with blue flame; commonly found as crystals, crusts, or grains B. Cannot be scratched by a fingernail but can be scratched by a copper penny Anhydrite White Pinkish tint; 3-3½ commonly occurs as sugary-looking masses Barite White Pinkish tint; 3-3½ rather heavy; cleavage fragments may be flat and slab-like Calcite White Pink color; 3 dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments Cinnabar Dark red Dark red or 2½ bright yellowish-red color; shiny, brilliant luster when pure; dull and earthy when impure; heavy Halite White Reddish tint; 2½ salty taste; dissolves in water; cube-shaped cleavage fragments Sulfur White or pale Reddish; will 1½-2½ yellow burn with blue flame; commonly found as crystals, crusts, or grains C. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife Anhydrite White Pinkish tint; 3-3½ commonly occurs as sugary-looking masses Barite White Pinkish tint; 3-3½ rather heavy; cleavage fragments may look flat and slab-like Dolomite White Pink color; 3½-4 commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite Fluorite White Pink color; 4 cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes Opal White Reddish color; 5-6 curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull 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° Garnet White Commonly occurs 6½-7 as crystals Opal White Reddish color; 5-6 curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull Quartz White Curved, 7 conchoidal fracture; occurs as rose quartz, as pink chert, and as agate and jasper

14. NONMETALLIC luster, PURPLE or VIOLET color

A. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes B. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Quartz, variety: White Curved, 7 amethyst conchoidal fracture; amethyst crystals commonly 6-sided prisms with pyramid-like ends

15. NONMETALLIC luster, COLORLESS

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 Transparent 2 selenite variety commonly occurs as flat, diamond-shaped crystals; splits into thin, flat sheets that will not bend without breaking Muscovite (white White Splits into thin, 2-2½ mica) flat, transparent sheets that will bend without breaking B. Cannot be scratched by a fingernail but can be scratched by a copper penny 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 Muscovite (white White Splits into thin, 2-2½ mica) flat, transparent sheets that will bend without breaking C. Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife 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; transparent hyalite variety resembles ice D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Opal White Curved, 5-6 conchoidal fracture; transparent hyalite variety resembles ice Quartz White Curved, 7 conchoidal fracture; rock crystal quartz commonly 6-sided prism with pyramid-like ends E. Cannot be scratched by quartz Topaz White Perfect basal 8 cleavage gives flat, plate-like fragments; notice hardness

How To Use the Rock Identification Charts

In the rock identification charts on pages 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 Rock Remarks No. 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) Grains are of about equal size (equigranular) 1. 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 Easily seen grains are scattered through a mass of finer grains 2. 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 Grains are arranged in layers 3. 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 Loose, powdery fragments; dilute limestone 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

DESCRIPTIONS OF SOME TEXAS ROCKS AND MINERALS

The pages that follow contain descriptions of Texas rocks and minerals. The descriptions are given in alphabetical order, except that related varieties are described together. For example, agate, amethyst, chert, flint, jasper, onyx, and chalcedony are discussed under quartz, because they are varieties of quartz. The descriptions include the properties of the rock or mineral that will help you identify it and also include information on where the rock or mineral can be found in Texas, some of its uses, and how it may have formed. The chart on page 99 lists chemical composition, specific gravity, and hardness of various Texas minerals.