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Geological Sciences 101
Lab #7 – Introduction to Rocks and Minerals
PART I
One key to understanding how the Earth works is to be able to recognize what you see around
you. One way to begin this is to group items that share common features, in an attempt to
understand the relationships between them. We begin today by developing a classification scheme
for a group of Earth materials. The boxes labeled “Classification Tray” each contain 28 samples.
Your team has the job of developing a classification scheme for these samples. You may devise
any system that makes sense to you—don’t worry if it is “right” or not. Your goal is to design a
system that conveys information about the objects in the tray—that allows us to better understand
them in some way. Think about the criteria you would like to use. Spend about 15 minutes
working with your group, and then be prepared to explain your work to the other teams. Use the
space below to diagram your system.
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PART II – INTRODUCTION TO MINERALS
Minerals are all around you. Minerals make up the rocks of the Earth’s lithosphere and mantle;
they are the nutrients in soil necessary for plant growth; they form your bones and teeth; they are
crushed and processed to extract metals for all purposes; they are used to dye fabric and color
paint; minerals make glass, plaster, concrete, ceramics--even kitty litter; and they are prized as
gems and jewelry.
In order to make use of minerals, we must first be able to identify them. This means
understanding a little about how minerals are formed, what they’re made out of, and how their
constituent atoms are put together.
MINERAL STRUCTURE AND COMPOSITION
To a geologist a mineral is a naturally occurring solid with a fixed chemical composition and an
ordered arrangement of atoms that form a crystal structure. For example, the mineral Galena has
the chemical formula PbS, and this crystal structure:
Pb
S
S
Pb
Pb
S
Pb
S
The atoms of lead (Pb) and sulfur (S) are present in a 1:1 ratio and are arranged in a cubic pattern.
The diagram shown here is only a small part of the Galena crystal structure. You should try to
imagine a crystal of Galena made of billions and billions of atoms extending in a cubic structure in
all directions.
(1) Locate a crystal of galena in the sample trays (look in the Breaking Box).
(a) Pick it up and feel it with your hands. What property(s) of galena suggest to you that it is
made of lead?
(b) Put a small drop of hydrochloric acid on the galena sample. What tells you that there is also
sulfur in the galena crystal?
While all crystals of galena are shaped like cubes, not all cube-shaped crystals are galena. Halite
(salt) and Pyrite are two other cubic minerals. Halite has the chemical formula NaCl and Pyrite is
FeS2 .
(2) Locate and examine a crystal of halite (NaCl) from one of the sample trays.
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(a) Make a sketch of the halite crystal.
(b) Use one of the binocular microscopes to examine crystals of table salt. Sketch these crystals.
(c) Having seen the macroscopic structure of halite, make a sketch of its atomic structure.
There are many other crystal shapes in addition to cubes. For example, quartz crystals are
columns with a hexagonal in cross-section. Some crystals are needle-like, others are double
pyramids, some are dodecahedrons (12-sided), some are blade-shaped, columnar, sheets,
rhombohedrons, or pinacoids (a rectangle in 3-D).
Columnar
Rombohedron
Bi-pyramid
Calcite is a good example of a mineral with a rhombohedral crystal structure. Because the shapes
of minerals reflect the internal arrangement of their atoms, a mineral will often retain its shape
even when broken. This is because the planes of weakness within the mineral are also determined
by the arrangement of atoms.
(3) From the “Samples for Breaking” box choose either calcite, halite, or galena. Break the
sample with a hammer. What is the relationship between the crystal shape before and after
breaking? A sketch might help.
The minerals calcite and galena come from two of the major families of minerals. Mineral families
are grouped according to chemical composition. Because galena (PbS) has sulfur in it, is belongs
to the Sulfide family. Pyrite (FeS2 ) is another sulfide. Calcite (CaCO3 ) is a Carbonate mineral
(CO3 is carbonate). There are Oxide minerals such as magnetite (Fe3 O4 ), and Native Elements
such as Gold (Au), Silver (Ag) and Copper (Cu). The largest mineral family is the Silicate
minerals, such as Quartz (SiO2 ) and Olivine (Mg2 SiO4 ).
SILICATE MINERALS
Silicon and oxygen comprise 75% of the mass of the Earth’s crust. Thus the silicate minerals are
by far the most abundant minerals on the surface of the Earth. The relative sizes of the silicon and
oxygen atoms determine their arrangement in a silicate crystal structure. Four large oxygen anions
fit around the smaller silicon cation. This arrangement is in the shape of a tetrahedron. A silicate
tetrahedron, however, would carry a charge of -4, if it were not bonded in some way to other
ions. There are two different “end member” solutions to this problem of excess negative charge.
One would be to add more cations—Olivine is formed in this way, with two Mg ions attached to
each tetrahedron (Mg2 SiO4 ). The second technique for balancing charges is to link tetrahedra to
each other. Quartz is an example of this solution, where all four corners of each tetrahedron are
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linked to others, producing the overall mineral formula SiO2 . Most silicate minerals are
constructed through a combination of tetrahedral linking and the addition of cations—for example,
pyroxene (MgSiO3 ). As we will see (or have seen) in class, the amount of tetrahedral linking
provides a useful way to organize the silicate mineral groups, and is important in controlling their
behavior and properties.
(4) Using the materials provided in lab, build a model of a silica tetrahedron. Have your TA
approve your design, then continue to add ions to build a model of the pyroxene structure—
just the silicate tetradedra, you don’t need to worry about the metal cations. Sketch the
pyroxene, using the “triangle shorthand” for representing tetrahedra. You may eat your model
when you’ve finished.
CRYSTAL GROWTH
Most minerals form by crystallizing from a molten solution—a magma. Some form by
precipitation from a solution (e.g. halite), and others precipitate with help from a biological catalyst
(e.g. calcite). Natural crystallization is a slow process and difficult to observe, so today we will
take a shortcut and use an organic compound (thymol, “thyme oil”) and attempt to grow crystals
big enough and quickly enough for us to be able to watch while it happens.
(5) Follow the instructions for growing thymol crystals (or watch the instructor).
(a) Sketch a single large thymol crystal as it grows.
(b) What happened when you quenched your magma with ice? How are the iced crystals
different from those grown without ice? Form a hypothesis to explain this behavior.
DETERMINATIVE PROPERTIES
Identifying minerals is a little like solving a murder mystery. We examine the problem carefully
and then systematically test and eliminate various possibilities until we are left with only one
possible solution. The tests that we use are the various characteristics of minerals--their
“determinative properties.”
You have already encountered two determinative properties: (1) crystal shape and (2) cleavage-the shape a mineral has when it breaks.
CLEAVAGE
Many minerals have a characteristic cleavage. For example, a calcite crystal breaks along three
different planes, each cleavage fragment in the shape of a rhombohedron. Galena also has three
planes of cleavage; these form a cube. Other minerals cleave at an angle of 120°, for example,
amphibole. Others cleave into sheets, for example, mica. Some minerals have no cleavage at all.
This means that the crystal structure is equally strong in all directions and it simply fractures, like
glass. Minerals with no cleavage are said to have “conchoidal fracture,” among them quartz and
olivine.
Most mineral samples have been broken off of larger pieces of rock, thus the presence or absence
of cleavage is often (but not always) easily observed. Compare the broken surface of a sample of
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feldspar (either orthoclase or plagioclase) with a piece of quartz. Move the samples back and
forth in the light to see the difference between cleavage (feldspar) and fracture (quartz).
HARDNESS
Hardness is a measure of how well a mineral resists physical damage, such as a scratch on the
surface. Ten common minerals make up a reference scale referred to as Mohs’ Hardness Scale.
From softest to hardest these are:
(1) Talc, (2) Gypsum, (3) Calcite, (4) Fluorite, (5) Apatite, (6) Orthoclase (7) Quartz,
(8) Topaz, (9) Corundum, (10) Diamond.
If you had a topaz and a diamond, the diamond would scratch the topaz, but the topaz would not
scratch the diamond. Thus all the Mohs minerals will scratch any that are below them on the
hardness scale. It is often convenient to use common materials rather than mineral specimens to
determine hardness. Your fingernail has a hardness of 2.5; a penny has a hardness of 3.5; glass
has a hardness of 5.5; a steel knife blade has a hardness of 6.5. Thus you can scratch talc and
gypsum with your fingernail, but not calcite or fluorite, nor any other mineral with a hardness of 3
or greater.
COLOR
Color is the most interesting, and sometimes deceptive, property of minerals. Color results from
the interaction of light with the atoms in the crystal structure of the mineral. Sunlight (or “white
light”) is comprised of the colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo,
Violet. When this light strikes any object it may behave in one of five ways:
Transmitted
Reflected
Refracted
Scattered
Absorbed
For example, green leaves are green because the green light is reflected and all other colors are
absorbed. The sky is blue because small particles in the atmosphere scatter blue light.
When light interacts with particular elements in minerals we see characteristic colors. This is true
even if the elements are not a normal part of the mineral, but present only in trace amounts. The
most important color-producing elements are:
Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn),
Iron (Fe), Cobalt (Co), Nickel (Ni), and Copper (Cu).
For example, in the normally gray mineral Corundum (Al2 O3 ) small amounts of Cr turn the color
bright red, producing the mineral Ruby. Alternatively, small amounts of Ti & Fe turn it deep blue,
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resulting in the mineral Sapphire. Cr in Beryl produces green Emeralds (and in general gives a
green color to minerals). Fe tends to give minerals a dark coloring, red, black or brown, and Fe in
quartz makes purple Amethyst; Mn can make minerals pink; Cu gives a blue or green color; V
yellow; and Co deep blue.
Because the combination of crystal structure, chemical composition, and various impurities can
produce so many different colors, color can be a deceptive guide to identification. In general
however, a few basic color guidelines hold true.
1. Minerals containing Fe and Mg (“mafic” minerals) are dark in color. Examine the mafic
minerals tray.
2. Minerals with Al, K, Na (“felsic” minerals) tend to be light in color. Compare the mafic and
felsic mineral trays.
3. Some minerals have characteristic and unvarying colors: turquoise is green/blue; malachite is
green; sulfur is yellow; hematite is red; biotite mica is black; muscovite mica is clear/gray;
olivine is green; galena is silver; pyrite is gold.
4. Other minerals occur in a limited number of colors almost all the time: pyroxene is almost
always black, brown, or green; amphibole is mostly black or green, Ca-plagioclase is black or
dark gray, orthoclase is pink, white, or gray; quartz is mostly clear, white, or gray; calcite is
clear, white, or gray.
(6) Look through all the sample trays for quartz crystals. How many different colors of quartz
are present? List them.
LUSTER & STREAK
Related to color are the properties of luster and streak. The way light reflects off the surface of
the mineral is its luster; for example, metallic, glassy, dull etc. Streak is the color that a mineral
has when it is powdered or scratched. Oddly, this is not always the same as the color of the
crystal. Streak is determined by scratching the mineral on a white ceramic plate.
UNUSUAL PROPERTIES
Some minerals have quite unusual but very distinctive properties. For example the mineral
Magnetite is magnetic. Sulfur smells like, well, sulfur. Halite tastes like salt. Calcite has an
unusual property called DOUBLE REFRACTION. This is seen in clear crystals of calcite.
(7) Place a clear calcite crystal on top of the words printed on this page. Make sketch of what
you see when you look through the crystal.
Calcite (CaCO3 ) will also EFFERVESCE in the contact with an acidic solution. Some minerals
posses a property called TWINNING. This occurs when two crystals growing side-by-side
intergrow with one another. Sometimes the crystals form a small cross, but most often the
intergrowth is seen as tiny parallel lines that look like the grooves on an old vinyl record. Calcite,
pyrite and plagioclase feldspar often have finely striated surfaces.
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MINERALS ARE EVERYWHERE
(8) Stroll through the Snee atrium to the mineral display. Find the sample that is derived from the
locality closest to your home town. Give the mineral name and its location (and tell us your home
town too).
PART III – MINERALS COMBINE TO FORM ROCKS
Rock can be defined as any naturally occurring aggregate of minerals. Just as letters combine to
form words, minerals combine to form rocks. Rocks are the history book of the planet.
Meteorite impacts, collisions and rifting of ancient continents, erosion of long vanished mountain
ranges, ice ages, and the life itself have all left records in this book. Our job is to interpret it.
Rocks are classified into three major groups, depending on their mode of formation
• Igneous – crystallize from a cooling liquid magma
• Sedimentary – an aggregate of loose, accumulated particles
• Metamorphic – have undergone solid state recrystallization due to high temperature and/or
pressure
The mode of formation of rocks produces characteristic textures. Texture is the key to placing an
unknown rock into one of the three rock groups. Within each group, chemical composition—
expressed as mineral assemblage—as well as texture, allow us to classify individual rocks.
IGNEOUS ROCKS
The texture of an igneous rock depends on the rate of cooling. Slow cooling produces large
crystals and a coarse texture, while quick cooling produces small crystals and a fine texture.
Igneous rocks erupted from volcanoes cool quickly on the surface of the Earth, while others cool
slowly in the Earth’s interior. As you saw in the thymol experiment, crystals nucleate and grow,
producing a crystalline solid of irregular, interlocking texture. This is the characteristic texture of
igneous rocks. The slower the cooling, the larger the crystals. Occasionally, crystals begin to
form within the magma, growing to large size, when suddenly the magma is erupted and
subsequently cools very quickly. This produces a very distinctive “porphyritic” texture,
characteristic of volcanic rocks. Volcanic rocks may also have a simple, fine-grained texture if
there are no early-formed crystals. Often the crystals are so small that a microscope is required to
see the texture of a sample and to identify its constituent minerals. Igneous rocks are divided into
two types:
• Volcanic (or extrusive) – erupt and cool on the surface
• Plutonic (or intrusive) – cool within the interior
Within each of these two groups, the rocks are organized by their chemical composition. Rock
chemistry can be determined in the lab by grinding the rock up for analysis, but it is also reflected
in the mineral assemblage that makes up the rock. The chart below illustrates the mineral make-up
of 8 different igneous rocks. For example, granite and rhyolite have the same chemistry and the
same minerals, but because they cool at different rates, they have a different texture.
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Ø Examine the labeled samples of igneous rocks, as well as the colored hand-outs, and make
sure you can recognize coarse-grained, fine-grained, and porphyritic textures.
SEDIMENTARY ROCKS
Sedimentary rocks can be one of two types:
• Clastic – particles are transported and deposited by wind or water
• Chemical – particles precipitate from solution and accumulate on sea/lake bottom
The texture of a chemical sedimentary rock is very similar to an intrusive igneous rock, but the
mineral composition is quite different. The texture of clastic sedimentary rocks in unmistakably
one of rounded grains of various sizes cemented together.
Ø Examine the labeled samples of sedimentary rocks, as well as the colored hand-outs, and
make sure you can recognize clastic and chemical rocks.
METAMORPHIC ROCKS
Metamorphic rocks are rocks that have undergone changes as a result of elevated temperature or
pressure or reaction with a fluid. A variety of changes may occur. In some cases, change is
limited to an increase in grain-size. In other cases, the original minerals are replaced by ones
stable at the temperature and pressure at which metamorphism occurred. An important concept in
classifying metamorphic rocks is that of metamorphic grade. Grade corresponds more or less to
the highest temperature at which metamorphism occurred. Hence low grade rocks were
metamorphosed at low temperature and high grade ones at high temperature. In general, as one
goes from low to high grade, grain size increases. Usually, increasing pressure accompanies
increasing temperature, so rocks metamorphosed at high temperatures have also experienced
moderate to high pressures, though there are exceptions (as noted below).
For fine-grained sediments such as shales, mudstones, etc., the sequence of metamorphic
rocks formed going from low grade to high grade is:
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Slate
Phyllite
Schist
Gneiss
Granulite
For (mafic) igneous rocks, the sequence is a bit simpler:
Greenschist
Amphibolite
Granulite
There are a few other metamorphic rock names you should be familiar with:
1.Marble is formed by metamorphism of limestone.
2.Quartzite is formed by metamorphism of quartz sandstone.
At low grade, sheet silicates such as chlorite and mica, are often the dominant minerals,
sometimes giving the rock a shiny appearance. These minerals are often aligned. If so, the rock
is said to be foliated. At higher grade, this foliation is often replace by segregation of light and
dark minerals (usually quartz and feldspar and amphibole and mica respectively) into bands.
Banding is characteristic and diagnostic of gneisses, i.e., if the rock is banded it is called a gneiss.
Ø Examine the labeled samples of metamorphic rocks, as well as the colored hand-outs, and
make sure you can recognize metamorphic textures.
(9) Now its time to put all of your skills to work. Return to the tray that you worked with at the
beginning of the lab. Reclassify the samples into the following groups:
•
•
•
•
Minerals
Igneous rocks
Sedimentary rocks
Metamorphic rocks
(a) For each of the mineral samples, use the determinative properties, as well as the labeled
samples in the other trays to identify the mineral.
(b) For the igneous rock samples, determine which are volcanic and which are plutonic. Try to
identify one or two minerals in each sample and use the attached table to identify the rock.
(10) When you’ve identified each mineral, reclassify the minerals based on their chemical
composition. Make a table showing your work.
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CHARACTERISTICS OF COMMON MINERALS
Galena – PbS – most important ore of lead.
Characteristics:
Perfect cleavage in three directions
Metallic luster
High density
Hardness between 2.5 and 5.5
Pyrite – FeS2 – “fool’s gold”
Characteristics:
Lacks cleavage, but usually occurs in cubes
Metallic luster and gold color
Parallel striations
Hardness of 6-6.5
Gypsum – CaSO4 + H2 O
Characteristics:
Perfect cleavage in one direction
Curved and splintery fracture
Commonly found in arrow-head twins
Colors include colorless, white, gray, yellow, red, and brown
Hardness of 2
Calcite – CaCO3 – common mineral for invertebrate shells
Characteristics:
Perfect cleavage in three directions forming rhombohedrons
Transparent to translucent
Effervesces when HCl is added to its surface
Colors include white, gray, yellow and red
Hardness of 3
Magnetite – Fe3 O4 – common ore of iron
Characteristics:
Magnetic!!!!
No cleavage
Iron black color
Hardness of 4
Quartz – SiO2 – popular gemstone, including amethyst
Characteristics:
Lacks cleavage and has a curved fracture
May be prismatic
Colors include clear, purple, pink, gray, and yellow
Hardness of 7
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Potassium-Feldspar – KalSi3 O8 – this is a major component of many granites!
Characteristics:
Two good perpendicular cleavages
Striations can be found on crystal faces
Color can be pink, blue, green, white and pale yellow
Hardness of 6
Mica – (chemical composition can vary depending on type)
Biotite – K(Mg,Fe) 3 (AlSi3 O10 )(OH) 2
Muscovite – KAl2 (AlSi3 O10 )(OH) 2
Characteristics:
One perfect direction of cleavage
Typically looks like sheets of paper in a book
Colors are dark for biotite, and lighter for muscovite...and we have both!
Hardness of 2-2.5
Amphibole – (and what a complicated chemical composition!) – frequently a component of granite
Perfect cleavage in two directions in the shape of a diamond
Fracture is typically uneven to splintery
Crystals generally prismatic
Colors are dk green, dk brown, and black
Hardness of 5-6
Olivine – (Mg,Fe) 2 SiO4 – this mineral is a huge component of the green beaches in Hawaii
Cleavage indistinct
Curved and uneven fracture
Transparent to translucent
Colors are olive-green, yellowish-brown, and reddish
Hardness of 6.5-7
Serpentine - Mg6 (Si4 O10 )2 (OH) 2 - This mineral is usually found in large masses of fine grained
crystals and is often used as a decorative rock. One of the most interesting forms is chrysotile
which occurs long fibers and is the commonest type of asbestos.
Usually green because of the substitution of iron for the magnesium.
Massive and fine grained when not fibrous.
Hardness 2.5 - 3.
Garnet - Mg3 Al2 (SiO 4 )3 - This mineral is often used to make abrasive sand papers but when
optically pure can be used for gems of fine quality.
Usually red but can be green or yellow.
Crystals often have so many faces they look like balls.
Hardness 6 - 7.5.
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Pyroxene - (Ca,Mg,Fe)SiO 3 – Common mineral in mafic and ultramafic rocks such as basalt or
peridotite
Usually black, brown, dark green, but can also be bright green, white, pink
Crystals typically form stubby prisms
Two cleavage planes at right angles (87-93°)
Hardness 5-6
Halite - NaCl - This is used for common table salt and for de-icing roads. It is mined just north
of Ithaca along the east shore of Lake Cayuga.
Has good cleavage in three directions to make cubes
Colorless in most cases.
Hardness 2.
Strong taste (often used to test the solubility of a mineral).
Talc - Mg2 Si4 O10 (OH) 2 - This mineral has a slippery feel due to very weak vander Waals bonding
between sheets and is sometimes known as soapstone. It is used in cosmetics, as filler for paints,
and in many other products.
Talc often occurs in sheets much like mica, but these sheets are usually warped.
It also occurs in fine-grained masses.
Color ranges from white to gray to green.
Hardness 1
Graphite - C – This mineral is black but can look almost metallic when well crystallized. It is used
as the "lead" in pencils and is a good lubricant
It often occurs as sheets due to the very weak cleavage in one direction
It is slippery much as talc is.
Opaque
Black or submetallic
Hardness 1 - 2
Sphalerite – ZnS – This mineral is the main ore of zinc. The western Adirondacks is one of the
main sources of this mineral in the US.
Cleavage occurs in six directions giving broken fragments a kind of sparkle.
Its high density makes chunks feel heavy
Color ranges from yellow-green to brown to deep red
Hardness 3.5 - 4
Corundum – Al2 O3 – Cr impurities make red rubies while Ti or Fe impurities make blue sapphires.
Emery is black, granular corundum.
Color normally gray or brown.
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Hexagonal crystals.
Hardness 9
Hematite – Fe2 O3 – This mineral is the most abundant and important ore for iron. In the US it is
mined around Lake Superior. Oxidized hematite gives many rocks a red color (such as the red
rocks of the Grand Canyon and environs).
Tabular crystals, also shiny flakes (specular hematite), round blobs (botryoidal or oolitic)
Color black, red, or silver
Luster – metallic or earthy
Streak – red
Hardness 5.5 – 6.5
Malachite – Cu2 CO3 (OH) 2
Azurite – Cu3 (CO3 )2 (OH) 2 – Copper carbonates, brightly colored copper ores, blue (azurite) and
green (malachite)
Effervesce in dilute HCl
Hardness 3.5 -4
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DESCRIPTIONS OF COMMON IGNEOUS ROCKS
Basalt - a dense, black rock, aphanitic in texture and mafic in composition. Iceland and the
Hawaiian Islands are composed entirely of basalt. Sometimes the upper surface of a lava flow has
a glassy texture, due to rapid chilling when exposed to the air. Incidentally, don't be fooled by
rocks that contain some larger crystals in a finer- grained "matrix" -- clearly this matrix must have
been cooled quickly and so the rocks are classified as extrusive.
Scoria - a sponge-like volcanic foam, of basaltic composition; it is spongy from the large number
of vesicles left by escaping gas in the melt.
Obsidian - volcanic glass. Obsidian is usually felsic in composition. It looks dark because of
finely disseminated particles or impurities in the glass. The specimen you are examining in lab
may have brown streaks, showing compositional banding.
Andesite - a reddish to grayish-black, fine-grained, dense rock. Andesite grades into basalt, and
often cannot be distinguished without chemical analysis in the laboratory.
Rhyolite - powdery white or pink. You can also see some tiny crystals of feldspar, quartz and
perhaps biotite. This is similar to the "pumice" used in some soaps and scouring powders.
Peridotite - an ultramafic (mantle) rock composed almost entirely of olivine, with some pyroxene.
Similar rocks are eclogites, which contain olivine, pyroxene and garnet.
Gabbro - dense and dark, with coarse crystals of pyroxene and Ca-plagioclase.
Diorite - usually gray in color and intermediate in composition between felsic and mafic,
composed largely of feldspar and amphiboles.
Granite - Two granite samples have been provided, to illustrate some of the observable variation
possible within this important rock type. This one has pink orthoclase feldspar grains, quartz, and
biotite (black mica).
Granite - This one is white, with biotite, plagioclase feldspar and quartz. The minerals are in
smaller grains, and so are difficult to identify.
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