Geology of the Pacific Northwest

Geology-208

Week 2: READING THE ROCKS OF WASHINGTON

Minerals

Mineral Characteristics

Minerals—the building blocks of rocks—are formed from elemental compounds made up of atoms, the building blocks of the universe. The following characteristics are true of minerals. They are:

natural
solid
inorganic
chemical compounds (a unique chemical makeup)
crystalline (a crystal structure):

atoms are arranged in a specific order
shapes are determined by atom-organization structure

Mineral Examples

Table salt, ice, snowflakes, and shells are mineral because they fit all of the criteria. Table salt is made up of alternating sodium and chloride atoms (NaCl, also known as halite). Snowflakes have 6 crystal points. Shells are made of calcite—a combination of calcium, carbon, and oxygen in the form of CaCO₃.

Non-Mineral Example

Glass is a chilled, thick, viscous liquid that runs too slowly to be seen by the human eye. Glass is not solid and cannot be classified as mineral.

Mineral Families

Native Elements (gold, silver, copper, platinum, diamond)
Silicates (75% of the Earth's crust: felsic, high in silica, and mafic, low in silica.) [The "-ates" ending means the mineral group includes oxygen.]
Carbonates (carbon and oxygen). Their carbon content causes them to bubble when hydrochloric acid is applied.
Sulfates (sulfur, oxygen)
Oxides (oxygen; i.e., iron oxide: magnetite)
Sulfides (sulfur)

Mineral Formation

Primary:

precipitate from water (evaporation)
crystallization

Secondary:

Every mineral is found in a certain environment (pressure, depth). A rock's shape is determined by the structure of its atoms. Its color is determined by its mineral makeup.

Rocks

Rock Forms

Three main forms of rock found in the Northwest are: igneous rocks (formed from cooled, crystallized molten rock), sedimentary rocks (formed from compacted, lithified fragments), and metamorphic rocks (changed mineralogically by increased temperature and pressure).

Rock Cycle

The life of a rock begins and ends as magma. Cooled by contact with the atmosphere or water, magma solidifies into igneous rocks like granite, obsidian, or pumice. Weathered igneous rocks erode and become sediment which is transported by glaciers, rivers, and winds and deposited in other places. Compacted sedimentary deposit is lithified into sedimentary rock such as coal, limestone, or sandstone. Sedimentary rock that is subjected to high pressures and temperatures becomes metamorphic rock like marble or slate. If rock temperatures and pressures increase, metamorphic rocks melt and return to magma.

Rock Properties

To identify a rock or mineral, gather information about its physical properties:

Shape

how it grows – smooth vs. grooved/ridged (crystal form: hexagonal (6) prisms, cubes, 6-sided cubes, 12-faced forms)
how it breaks – step-like cleavage; fracture; peeling (usually in the same form in which it grows [with the exception of quartz]). It breaks at the weakest place, at the chemical bonds. Mica peels off in layers when cleaved. Look for:

the number of planes
the angle between planes

Color

surface appearance
Streak (the color you get when you powder, or grind, the rock against a porcelain plate. Non-metallics leave colorless or white streaks.)

Luster (how it reflects light):

Metallic (silver, gold; shiny; reflects light)
non-metallic

vitreous (glassy)
pearly (shell-like; i.e., talc)
resinous (sap/resin-like)
silky (fibrous/asbestos-like
dull (earthy; i.e., hematite)

Strength (hardness, on the Mohs Hardness Scale, where "1" is softest and "10" is hardest):

1.0 – talc (fingernail can scratch it)
2.5 – fingernail
3.0 – calcite
3.5 – penny (from 1982 or earlier)
5.5 – glass (harder minerals can scrape glass; softer minerals cannot)
6.5 – porcelain ("streak" testing plate)
7.0 – quartz
10.0 – diamond

Density (weight)
Reaction To Acid (carbon reacts with bubbling)
Magnetism (magnetite is magnetic)
Taste – place your wet finger on the specimen and then taste your finger–so you will not get the full taste of a possibly poisonous substance. Do NOT taste-test Lab specimens, because acid has been applied to all of them.
Smell and Feel (i.e., soapy, greasy, etc.)
Striations (surface scratches or grooves)

Tools

fingernail
old copper penny (1982, pre-1982)
magnet
magnifying glass
glass tile
porcelain tile (streak-plate)
hydrochloric acid dropper (makes CO₂ gas bubbles)

RESOURCES

• The Rock Cycle (University of British Columbia; 4 January 2000)
• Introduction to Minerals (Gore, Pamela J.W.; July 17, 1999)
• Pellant, Chris, 1992, Rocks and Minerals (Dorling Kindersley, London, pp.256)

GLOSSARY

Igneous Rocks • Metamorphic Rocks • Sedimentary Rocks

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Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.