Geology of the Pacific Northwest
Week 2: READING THE ROCKS OF WASHINGTON
Mineralsthe building blocks of rocksare formed from elemental compounds made up of atoms, the building blocks of the universe. The following characteristics are true of minerals. They are:
- chemical compounds (a unique chemical makeup)
- crystalline (a crystal structure):
- atoms are arranged in a specific order
- shapes are determined by atom-organization structure
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 calcitea combination of calcium, carbon, and oxygen in the form of CaCO3 .
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.
- 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)
- precipitate from water (evaporation)
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.
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).
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.
To identify a rock or mineral, gather information about its physical properties:
- 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
- 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)
- 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 fingerso 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)
- old copper penny (1982, pre-1982)
- magnifying glass
- glass tile
- porcelain tile (streak-plate)
- hydrochloric acid dropper (makes CO 2 gas bubbles)
© 2001-2002, Rob Viens, Bellevue Community College. All rights reserved.
Page updated on 5 September 2002
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