SLE: _________
_________
_________
Name _____________________________ # ___
P-__ Sci – Rock Talk
Date ___________________________________
Objective: To learn the difference between minerals and rocks and to see some gem
specimens from local mines.
A mineral is a naturally-occurring substance formed through geological processes that has
a characteristic chemical composition, a highly ordered atomic structure and specific
physical properties. A rock is a naturally occurring aggregate of minerals and/or
mineraloids. Rocks do not have a definite chemical composition.
Chart Comparing Rocks to Minerals
Minerals
Study
Chemical composition
Mineralogy
Has a definite chemical composition;
minerals are inorganic compounds.
Examples Gold, silver, fluoride etc.
color
shape
fossils
Function in biology
color is usually the same
usually have a shape
no fossils
Minerals help in bone and tooth
formation, blood coagulation and
muscle contraction.
Nutritional requirement Only some minerals are required by the
for the human body human body for nutrition.
Rocks
Petrology
Does not have a definite
chemical composition
Limestone, Basalt, Coal,
Claystone
color is not the same
no definite shape
some have fossils
shelter and foundation
little to none
Scientific Study
Petrology is the scientific study of rocks while the study of minerals is called mineralogy.
Composition
Rocks are generally made of two or more minerals. A main determining factor in the
formation of minerals in a rock mass is the chemical composition of the mass, for a certain
mineral can be formed only when the necessary elements are present in the rock. Calcite is
most common in limestones, as these consist essentially of calcium carbonate; quartz is
common in sandstones and in certain igneous rocks like granite which contain a high
percentage of silica. Two rock masses may have very much the same bulk composition and
yet consist of entirely different assemblages of minerals.
1
Composition of minerals varies from one mineral to another. Some may have a high
content of carbonate while others may have a high content of oxides. Even their physical
characteristics may vary. While one may be known for its hardness, another may have a
characteristic luster. All these variations also form standards for classifying them in broad
categories.
Classification
Rocks are classified by their mineral and chemical composition, by the texture of the
constituent particles and by the processes that formed them. These indicators separate
rocks into igneous, sedimentary and metamorphic. Igneous rocks are formed when molten
magma cools and are divided into two main categories: plutonic rock and volcanic.
Sedimentary rocks are formed by deposition of eithe clastic sediments, organic matter, or
chemical precipitates (evaporites), followed by compaction of the particulate matter and
cementation during diagenesis. Metamorphic rocks are formed by subjecting any rock type
(including previously-formed metamorphic rock) to different temperature and pressure
conditions than those in which the original rock was formed.
Minerals may be classified according to chemical composition, for example - the silicate
class, the carbonate class, the element class etc. A mineral can be identified by several
physical properties such as crystal structure, hardness, color, luster, specific gravity, etc.
Even though there are 92 elements that are naturally found, only eight of them are
common in the rocks that make up the Earth’s outer layer, the crust. Together, these 8
elements make up more than 98% of the crust and the minerals that exist there.
The 8 most common elements in Earth’s crust (by mass):
46.6% Oxygen (O)
27.7% Silicon (Si)
8.1% Aluminum (Al)
5.0% Iron (Fe)
3.6% Calcium (Ca)
2.8% Sodium (Na)
2.6% Potassium (K)
2.1% Magnesium (Mg)
The picture on page 3 shows where these elements are located within the periodic table.
Together, the elements oxygen and silicon make up most of the Earth’s crust including
silicate minerals such as quartz and feldspar.
2
There are currently more than 4,000 known minerals, according to the International
Mineralogical Association, which is responsible for the approval of and naming of new
mineral species found in nature. Of these, perhaps 150 can be called "common," 50 are
"occasional," and the rest are "rare" to "extremely rare."
Formation of Rare Minerals and Gems
Minerals form in many different environments in the Earth. Most gemstones form in the
earth's crust, the top layer of the earth, with a depth of 3 to 25 miles. Only two gemstone
varieties -- diamond and peridot -- form in the earth's mantle, which represents 80% of the
earth's volume. The mantle consists mostly of melted rock called magma with a solid
upper layer.
3
While a few gemstones originally
formed in the mantle, all gems are
mined in the crust. The crust is
made up of three kinds of rocks,
known in geology as igneous,
metamorphic and sedimentary.
These technical terms refer to the
way in which rocks formed. Some
gemstones are associated
especially with one kind of rock;
others with multiple types.
The igneous process involves the
solidification of magma. Magma
from the mantle can rise to the
crust, usually through volcanic
pipes. If it reaches the surface of
the earth, it solidifies as lava. But if
Rock Cycle
the magmatic mass cools slowly in
the crust, it can crystallize and
form minerals. Increases in pressure can also cause this pegmatitic fluid to infiltrate
surrounding rocks, often making chemical exchanges with them. The long list of gemstones
formed from igneous rock include the chrysoberyl group, all of the quartzes (including
amethyst, citrine and ametrine), the beryls (emerald, morganite and aquamarine), the
garnets, moonstone, apatite, diamond, spinel, tanzanite, tourmaline, topaz and zircon.
Once the igneous rock reaches the surface of the earth, the forces of erosion and
weathering produce smaller particles which accumulate on the surface or are moved by
wind and water. As time passes, layers of these sediments build up on land or under water.
The pressure from upper layers causes compaction in the lower layers along with various
chemical and physical changes such as lithification, which lead to the creation of
sedimentary rock. Evaporation is an another process which also produces sedimentary
rocks, as when dripping mineral-laden waters leave behind stalactites or stalagmites.
Gemstones associated with sedimentary rock include jasper, malachite, opal and zircon.
The presence of intrusive magma in an area (known as contact metamorphism), or of
tectonic plate interactions on a larger scale (known as regional metamorphism) puts the
igneous and sedimentary rocks and minerals under heat or pressure which may cause
changes in their chemistry and crystal structure. The result is the creation of metamorphic
rocks. Gemstones associated with metamorphic rock include the beryls, jade, lapis lazuli,
turquoise, spinel, ruby, sapphire and zircon.
Rocks and minerals are in a constant state of change, referred to as The Rock Cycle.
Igneous rock can change into sedimentary or metamorphic rock. Sedimentary rock can
4
change into metamorphic or igneous rock. And metamorphic rock can change into igneous
or sedimentary rock. But you have to be patient.
Pegmatite - The Rock with Large Crystals
Pegmatites are extreme igneous rocks that form during the final stage of a magma’s
crystallization. They are extreme because they contain exceptionally large crystals and they
sometimes contain minerals that are rarely found in other types of rocks. To be called a
“pegmatite,” a rock should be composed almost entirely of crystals that are at least one
centimeter in diameter. The name “pegmatite” has nothing to do with the mineral
composition of the rock. Most pegmatites have a composition that is similar to granite
with abundant quartz, feldspar and mica.
Pegmatites are sometimes sources of valuable minerals such as spodumene (an ore of
lithium) and beryl (an ore of beryllium) that are rarely found in economic amounts in other
types of rocks. They also can be a source of gemstones. Some of the world’s best
tourmaline, aquamarine and topaz deposits have been found in pegmatites.
Large crystals in igneous rocks are usually attributed to a slow rate of crystallization.
However, with pegmatites, large crystals are attributed to low-viscosity fluids that allow
ions to be very mobile.
During the early states of a magma’s crystallization, the melt usually contains a significant
amount of dissolved water and other volatiles such as chlorine, fluorine and carbon
dioxide. Water is not removed from the melt during the early crystallization process, so its
concentration in the melt grows as crystallization progresses. Eventually there is an
overabundance of water, and pockets of water separate from the melt.
These pockets of superheated water are extremely rich in dissolved ions. The ions in the
water are much more mobile than ions in the melt. This allows them to move about freely
and form crystals rapidly. This is why crystals of a pegmatite grow so large.
The extreme conditions of crystallization sometimes produce crystals that are several
meters in length and weigh over one ton. For example: a large crystal of spodumene at the
Etta Mine in South Dakota was 42 feet long, 5 feet in diameter and yielded 90 tons of
spodumene!
Uses
Rocks are very useful in making roads, buildings, polishing materials, industrial work etc.
Rocks like granite and marble are now used in home improvement for aesthetic reasons.
Different minerals are useful for various purposes like the gem industry, for carving, as
abrasives and for the formation of other elements or rocks.
5
References






http://www.rocksforkids.com/RFK/identification.html#difference
http://en.wikipedia.org/wiki/Rock_(geology)
http://en.wikipedia.org/wiki/Minerals#Mineral_definition_and_classification
http://msnucleus.org/membership/html/jh/earth/minerals/lesson4/minerals4c.html
http://www.windows2universe.org/earth/geology/crust_elements.html
http://geology.com/rocks/pegmatite.shtml
You will NEED the following items for this lab:




Safety Goggles
Pencil and eraser
Long hair MUST be secured back into a bun prior to entering the lab.
This lab preview
You will NOT need the following items:




Food or drinks (including gum/mints and bottled water)
Loose fitting jackets/sweaters/sweatshirts: ALL shirts MUST be tucked in prior to
entering the lab. Long sleeves MUST be able to be rolled up and securely stay rolled for
the duration of the lab or sweatshirts/sweaters will not be permitted to be worn during
lab. Additionally, drawstrings from hoods must be tucked in and stay inside your
hoodie. Jackets and outerwear are NEVER permitted to be worn in the lab.
Extra books, bags/purses, etc. EVERYTHING MUST FIT INTO YOUR ZIPPERED
BACKPACK. UPON ENTERING THE CLASSROOM, CHECK THE BOARD AND YOUR PRELAB FOR ANY UPDATES TO THE LAB PROTOCOL AND PREPARE FOR LAB.
Please review your SAFETY CONTRACT if you have any questions as to what is
expected of you during science lab. Infractions will be issued for any behavior
(including but not limited to uniform/clothing violations) that pose any safety hazard
or violate the school dress code.
6