Notes Unit 2:
Minerals are made from elements. Elements are substances that cannot be broken down by normal chemical means. All elements are listed in the Periodic Table of Elements. Some common elements in minerals are Oxygen (O), Silicon (Si), Calcium (Ca) and Sodium (Na).
All minerals have four properties:
1. All minerals are naturally occurring
2. All minerals are inorganic (that means non-living)
3. A different chemical formula can be written for each mineral and that formula is constant for that mineral.
ex. Galena is PbS- that is it has one lead atom (Pb) and one sulfur (S) atom in each molecule.
4. Most minerals have an unique crystalline shape given the right growing conditions (like in a cave).
Compounds are substances that has two or more elements chemically combined. When the compound is formed, it has different properties than the elements it is made from.
For example, NaCl is a substance made from Na- Sodium and Cl- Chlorine. Sodium explodes when it is combined with water, and chlorine is a poison. When these elements are combined, they form NaCl, what we call table salt (halite).
Minerals have many different properties, and geologists use these properties to determine what kind of mineral is before them. Scientists use a combination of properties to distinguish between these minerals, using the process of elimination. More on the physical properties of minerals. The following is a description of each of these properties:
1.Color-
Color is usually the first property observed when handling a specimen. Color alone is not reliable, but it can help us to eliminate possibilities. We must be sure we have a freshly exposed surface, because weathering can change the color of a mineral. Some minerals, such as sulfur, always have the same color(yellow). Other minerals may have different colors. One example is quartz, which may be clear, purple, pink and even gray!
2.Luster-
Luster is the way light reflects from a mineral's surface. Luster is either metallic or nonmetallic. Metallic minerals look like.... well, a metal. Nonmetallic minerals can be glassy, dull, brilliant, waxy, earthy, or pearly.
3. Streak-
The color of the powder of a mineral when scraped on a porcelain plate. Oddly sometimes the powder of a mineral can be different from color of the mineral itself. Pyrite (fool's gold) is gold in color, but its streak color is dark gray or black! Weird, huh?
4.Hardness-
Hardness is a mineral' s ability to resist being scratched. Talc, the softest mineral, can easily be scratched with a finger nail. Diamond is so hard that it can't be scratched by another mineral. Hardness does not describe a substance's ability to resist being broken. Even though diamond is the hardest mineral, it can be broken if you smash it with a hammer. To show the hardness of mineral, we use:
Moh's Scale of Hardness.
Talc- 1 Softest
Gypsum- 2
Calcite- 3
Fluorite- 4
Apatite- 5
Orthoclase- 6
Quartz- 7
Topaz- 8
Corundum- 9
Diamond- 10 Hardest
To determine the hardness of a mineral, scratch it with these minerals, and find out which minerals will and won't scratch it, and you know that mineral's hardness falls between those minerals. For instance, if a mineral is not scratched by Quartz, but is scratched by Topaz, the hardness must be between 7 and 8. Get it?
4.Cleavage-
Cleavage is the tendency of a mineral to break parallel to atomic planes in its crystalline structure. Perhaps the best example of cleavage is Muscovite mica, which is a mineral that breaks in parallel sheets. Mica always flakes off in thin sheets, like pieces of paper off of a giant stack of papers. Its atoms line up in sheets, and this is what provides a plane of weakness. Another example is Halite, that has cleavage in three directions at angles of 90 degrees from each other. When a piece of Halite is broken, it breaks into many smaller cubes.
Fracture is when a mineral breaks into jagged edges. Some minerals have no planes of weakness, so they have no cleavage.
Some minerals have a distinct fracture. Obsidian always fractures in a pattern that looks like a clam shell. Obsidian is also called volcanic glass.
5. Crystals-
Crystal Shape is the natural geometric shape of a mineral. Minerals grow into different shapes because their atoms form a unique pattern. Each layer is built up much like layers of chocolate would on an ice cream cone dipped again and again into melted chocolate sauce.
ex. Quartz crystals form hexagonal shapes, salt and pyrite are cubic, and calcite is rhombohedral.
Animated Movie on Crystals
5. Other Properties-
Some minerals have special properties. HCl,
or hydrochloric acid, can be used to identify calcite or limestone.
Calcium Carbonate bubbles furiously when acid is applied.
Halite (NaCl) is tested by using your sense of taste. If you lick it, it tastes very salty, because salt is actually made from Halite.
Double refraction is a dead give away property of Iceland spar calcite. When this mineral is placed on paper you see double!
Magnetic minerals, such as magnetite, give a magnetic tuck when close to magnetites. In fact, early compasses where made by floating magnetite on a piece of wood in water.
Fluorescent minerals glow under UV (black) light. Some of the colors are amazing!
Minerals are the building blocks of rocks. Nearly one hundred of the thousands of known minerals make up ninety five percent of the earth's rocks. Twenty of these minerals are most likely to be found in rocks found in the field.
Igneous Rocks
Igneous rocks form by the crystallization of
molten material inside or outside the surface of the earth.
When molten rock is inside the earth it is called Magma,
and when it is outside the earth, it is called Lava. The
crystallization of igneous rocks is a complex and time consuming
process. The amount of time that a crystal has to form, the larger
the crystal will be.
If a mineral has a short amount of time to
cool, like lava that pours into water, no crystals will form,
and
obsidian, or volcanic glass will be the result. If cooling takes
more time, crystals will start to form, but most often will cool
too quickly to form crystals large enough to see the crystals
without the help of a magnifying glass.
If given a long enough time to cool, crystals
will begin to form that are large enough to see with the naked
eye. Magma that is stuck underground will cool very slowly, because
it is insulated by the surrounding rock. As this material cools,
crystals will form that can be very large.
Some quartz crystals have been known to grow to a length of more
than seven feet. Some magma chambers cool only a few degrees per
century.
Igneous rocks are identified based on their texture and mineral composition. Texture is the size and arrangement of mineral crystals. Mineral composition is simply the minerals that comprise an igneous rock. Felsic or light colored igneous rocks, like granite, are listed in your reference table as having lots of quartz and feldspar in it. Mafic, or dark, rocks like basalt is heavy in density and made of proxene and amphibole (hornblende).
Sedimentary Rocks-
Sediments (broken bits of rock) are materials that have been transported by air, water, or ice. Most sediments are rock, but some can be pieces of animals or plants, and even molecules dissolved in water. When these materials are deposited, they form loose layers , and are then changed into rock layers.
Types of Sedimentary Rock.
Clastic- sediments are cemented (glued) together and compacted by burial. Clay sediments make shale, sand makes sandstone and conglomerate rocks are made of pebbles.
Clastic sedimentary rocks can be recognized by their layers of sediments. Take a virtual field trip to a Arizona to see the ultimate in sedimentary layers.
Organic- animal or plant remains make up this rock. Coal is made of squashed plants and Fossil Limestone is formed when animal skeletons sink to the bottom of the sea, and collect. New York has lots of cool fossils. In 2000 they found two mastodon skeletons!
Chemical Evaporite- The evaporation of water can leave behind a combination of minerals, such as halite and other salts. Salts in a rock are not surprisingly named- Rock Salt.
Most sedimentary rocks are formed by deposition in shallow seas, oceans and lakes. They are laid down in flat layers (a great identifying feature) and sometimes contain fossils.
Metamorphic Rocks-
Metamorphic rocks are formed by changing either sedimentary or igneous rocks. Many things can cause the formation of metamorphic rocks.
Heat-
Most minerals expand when heated, causing atoms
to move apart, stretching the bonds that hold them together. If
heated enough, the bonds will break, and the mineral melts. In
a metamorphic rock,
some bonds break, but not all, so some changing takes place. Contact
with magma and deep burial can cause a rock to undergo metamorphism.
Metamorphic rocks have a different set of characteristics than
its original form. One example is limestone. When it is "baked"
by the contact with magma, it turns into marble.
Pressure-
Pressure has the opposite effect of heat. It pushes the atoms of rocks This pressure will result in some sort of deformation. Often, rocks seem to have bands that are twisted and turned, resulting from metamorphosis of sedimentary rocks.
A good thing to look for in identifying a metamorphic rock are layers of flattened crystals, distorted structure and the appearance of exotic crystals.
Change to Crystalline Texture-
During metamorphosis, the size, spacing, and shape of the grains are changed. They may fuse due to heating and create brand new, exotic crystals. In New Hampshire they found a single crystal of Beryl had grown to be 250 feet long! Garnet schist is a good example of a rock that grew bizarre crystals with help from metamorphosis.
Increased Density-
Pressure which forces the grains closer together can cause the volume to decrease, while the mass remains the same. The density of the rock increases.
Foliation and Banding-
Sometimes, heat and pressure can produce a thin layer of flattened crystals. Banding occurs when minerals recrystallize and separate out according to density, like oil and water. Gneiss is banded.
Distortion of Layers-
When a sedimentary rock is metamorphosed, its layers become distorted (that means the layers are all wavy).
The Rock Cycle
The rock cycle is the constant changing of rocks from one form to another. Any rock type can form from any other rock other type.
Check out this funny cartoon on the Rock Cycle
