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Atoms Elements and Minerals Lab Resource 1
http://education.jlab.org/glossary/abund_ele.html
10 Most Abundant Elements in the Earth's Crust
Source: CRC Handbook of Chemistry and Physics, 77th Edition
Element
Abundance
Abundance
percent by weight parts per million by weight
Oxygen
46.1%
461,000
Silicon
28.2%
282,000
Aluminum
8.23%
82,300
Iron
5.63%
56,300
Calcium
4.15%
41,500
Sodium
2.36%
23,600
Magnesium 2.33%
23,300
Potassium 2.09%
20,900
Titanium
0.565%
5,650
Hydrogen
0.14%
1,400
What is a mineral?
When you see a glittering crystal in our Museum you would probably say it
was a mineral - and you'd almost certainly be right. But can you say what a
mineral really is?
Defining a mineral
A mineral is a naturally occurring inorganic solid, with a definite chemical
composition, and an ordered atomic arrangement. This may seem a bit of a
mouthful, but if you break it down it becomes simpler.
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Minerals are naturally occurring
They are not made by humans
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Minerals are inorganic
They have never been alive and are not made up from plants or animals
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Minerals are solids
They are not liquids (like water), or gases (like the air around you)
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Minerals have a definite chemical composition
Each one is made of a particular mix of chemical elements
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Minerals have an ordered atomic arrangement
The chemical elements that make up each mineral are arranged in a particular way - this is why
minerals 'grow' as crystals
What Is A Rock Exactly?
What is a rock? We all know what rocks are right? The are lying on the ground everywhere! They are the
mountains, and canyons that you can’t help but notice. We have all thrown them, sat on them, and dug
them out of our gardens. However, to pin down an exact definition… that is something not all of us
could do.
A rock is:
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A solid
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Naturally occurring
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Is made up of minerals or mineral like matter.
Rock Composition
Some rocks are composed of just one mineral. Pyrite and quartz are two common rocks that fit this
category. Most rocks are a solid mixture of several minerals like granite.
Rock Classification
Rocks are classified by how they are formed. There are three basic groups, igneous, sedimentary, and
metamorphic.
In each group, distinctions are made for texture or grain size and chemical or mineral content.
That is pretty straightforward isn’t it? That is the textbook definition. However, there are some gray
areas.
Exceptions to the What Is A Rock definition
What about mercury? It is not a solid at normal temperatures but if it gets cold enough it becomes solid.
Then there is coquina. Coquina is a sedimentary rock made of seashells. The shells are made of minerals
but they are not minerals.
Coal is considered a rock but it is not made of minerals it comes from organic matter plants.
These last two are called biogenic rocks.
Petrology is the study of rocks.
What Is A Rock? Perhaps the more important question is “why study rocks?”
Rocks are the pages in the history book of the earth. They tell stories of times long past. From rocks, we
have learned of dinosaurs, trilobites and thousands of other life forms that existed on earth once but are
now no more.
We have learned that the earth is constantly moving, shifting, and changing. Rock is continually being
created. It is heated, squeezed, and weathered into new rocks. Finally, it is remelted to start the process
anew. By studying rocks, we learn to read the pages of earth’s history.
Silicates
The silicates are the largest, the most interesting and the most complicated class of minerals
than any other minerals. Approximately 30% of all minerals are silicates and some geologists
estimate that 90% of the Earth's crust is made up of silicates, SiO44- based material. Thus,
oxygen and silicon are the two most abundant elements in the earth's crust.
Silicates are based on the basic chemical unit SiO44-, the silicate tetrahedron. It is a tetrahedron shaped anionic
group (negative ion). The central silicon ion has a charge of positive four while each oxygen has a charge of
negative two (-2) and thus each silicon-oxygen bond is equal to one half (½ ) the total bond energy of oxygen. This
condition leaves the oxygens with the option of bonding to another silicon ion and therefore linking one SiO44tetrahedron to another.
In the extreme case, the tetrahedra are arranged in a regular, orderly fashion forming a
three-dimensional network. Quartz is such a structure (see the diagram), and its formula is
SiO2. If silica in the molten state is cooled very slowly it crystallizes at the freezing point. But
if molten silica is cooled more rapidly, the resulting solid is a disorderly arrangement which
is called glass, often also called quartz.
How are Minerals Identified?
Color
Color is not always a good clue to the identity
of a mineral. Many minerals are found in
several colors and many minerals have extra
chemicals in them that give them an
unexpected color. Also, weathering may
change the color of the mineral.
Example-quartz comes in various colors: rose, milky, clear
Luster
Luster is the way a mineral reflects light.
Minerals can be described as metallic, pearly,
glassy, silky, greasy, brillant, or dull.
For example, diamonds are brillant. Talc has a greasy luster, quartz has
a glassy luster, and gypsum has a pearly luster. Pyrite or fool's gold, as
it is sometimes called, has a metallic luster.
Streak
This is the color of the streak made by rubbing
a mineral across a hard, rough surface like a
bathroom tile.
An example is pyrite which is a brassy, yellow color. However, it
makes a greenish, black streak when rubbed across a rough surface.
You will need a field guide to rocks and minerals to tell you what color
streak a mineral makes.
Cleavage
Cleavage is when certain minerals break in a
definite way.
For example, galena cleaves into little cubes. Mica cleaves into thin
sheets. Calcite cleaves into slanting bricks. Feldspar breaks into little
steps. Quartz cleaves into irregular chunks.
Hardness
Hardness is measured by seeing how easy it is
to scratch a mineral.
For example, talc, the softest mineral known, can be scratched with a
fingernail. Diamond, the hardest mineral, can scratch all the other
minerals but cannot be scratched by them. A German mineralogist,
Friedrich Mohs, developed a standard scale of hardness in 1822. His
scale is so easy to use that it is still in use today.
Specific Gravity
Specific gravity of a mineral is its relative
weight compared to the weight of an
equal volume of water. It determines the
density of the mineral.
Two minerals may be the same size, but their weight may be
very different. Water has a specific gravity of 1. If a mineral has
a specific gravity less than 3, it is considered a "light" mineral,
between 3 and 5 - "average", and more than 5 - "heavy".
Minerals with a metallic luster are usually "heavy".
Common Rock-forming Minerals
While rocks consist of aggregates of minerals, minerals themselves are made up of one or a number of chemical
elements with a definite chemical composition. Minerals cannot be broken down into smaller units with different
chemical compositions in the way that rocks can. More than two thousand three hundred different types of minerals
have been identified. Luckily many are rare and the common rocks are made up of a relatively small number of
minerals.
Identifying the common minerals
Since minerals are the building blocks of rocks, it is important that you learn to identify the most common varieties.
Minerals can be distinguished using various physical and/or chemical characteristics, but, since chemistry cannot be
determined readily in the field, geologists us the physical properties of minerals to identify them. These include
features such as crystal form, hardness (relative to a steel blade or you finger nail), colour, lustre, and streak (the
colour when a mineral is ground to a powder). More detailed explanations of these terms and other aspects of mineral
identification may be found in field handbooks or textbooks. Generally the characteristics listed above can only be
determined if the mineral grains are visible in a rock. Thus the identification key distinguishes between rocks in which
the grains are visible and those in which the individual mineral components are too small to identify.
The six commonest minerals
The six minerals olivine, quartz, feldspar, mica, pyroxene and amphibole are the commonest rock-forming minerals
and are used as important tools in classifying rocks, particularly igneous rocks. Except for quartz, all the minerals
listed are actually mineral groups. However, instead of trying to separate all the minerals which make up a group,
which is often not possible in the field, they are dealt with here as a single mineral with common characteristics.
Quartz: Quartz is a glassy looking, transparent or translucent mineral
which varies in colour from white and grey to smoky. When there are
individual crystals they are generally clear, while in larger masses quartz
looks more milky white. Quartz is hard - it can easily scratch a steel knife
blade. In many rocks, quartz grains are irregular in shape because crystal
faces are rare and quartz does not have a cleavage (ie, it does not break on
regular flat faces).
Feldspar: Feldspar is the other common, light-coloured rock-forming
mineral. Instead of being glassy like quartz, it is generally dull to opaque
with a porcelain-like appearance. Colour varies from red, pink, and white
(orthoclase) to green, grey and white (plagioclase). Feldspar is also hard
but can be scratched by quartz. Feldspar in igneous rocks forms well
developed crystals which are roughly rectangular in shape, and they cleave
or break along flat faces. The grains, in contrast to quartz, often have
straight edges and flat rectangular faces, some of which meet at right
angles.
The feldspars include orthoclase (KAlSi3O8) and plagioclase
((Ca,Na)AlSi3O8).
Mica: Mica is easily distinguished by its characteristic of peeling into many
thin flat smooth sheets or flakes. This is similar to the cleavage in feldspar
except that in the case of mica the cleavage planes are in only one direction
and no right angle face joins occur. Mica may be white and pearly
(muscovite) or dark and shiny (biotite).
Mica is a name given to aluminum-containing silicates which
cleave in thin sheets. The micas include muscovite
(KAl2(AlSi3O10)(OH)2) and biotite (K(Mg,Fe)3(AlSi3O10)(OH)2).
Pyroxene: The most common pyroxene mineral is augite. Augite is
generally dark green to black in colour and forms short, stubby crystals
which, if you look at an end-on section, have square or rectangular crosssections.
Pyroxene is a silicate mineral with the general composition
(Mg,Fe)SiO3.
Amphibole: The most common amphibole is hornblende. Hornblende is
quite similar to augite in that both are dark minerals, however hornblende
crystals are generally longer, thinner and shinier than augite and the
mineral cross-sections are diamond-shaped.
Amphibole is a name given to silicate minerals with the general
composition Ca2(Fe,Mg)5Si8O22(OH)2. The hornblende sample
below is the most common example of this grouping.
Olivine: Olivine, or peridot in the jewellery trade, is yellow-green,
translucent and glassy looking. Crystals are not common; it usually occurs
as rounded grains in igneous rocks or as granular masses. Olivine is almost
as hard as quartz; it does not have a well-developed cleavage.
Olivine is a silicate mineral with the general composition
(Mg,Fe)2SiO4. The chemical formula (Mg,Fe)2SiO4 implies any
mixture of iron and magnesium. It is said to be a "family" of
minerals rather than one which has a definite composition.
Other common rock-forming minerals
Calcite: Calcite is a very common mineral in sedimentary rocks. It is commonly white to grey in colour. Individual
crystals are generally clear and transparent. Calcite is softer than quartz and can be scratched easily by a steel knife
blade. In a rock, calcite grains are often irregular to rhomb-like in shape. Calcite's major distinguishing characteristic
though is its vigorous reaction with dilute hydrochloric acid. Dolomite is very similar to calcite but does not react well
with acid unless powdered first.
Clays: Clay minerals are very fine grained and difficult to tell apart in the field. They can vary in colour from white to
grey, brown, red, dark green and black. Clays are plastic and often sticky when wet; they feel smooth when smeared
between the fingers.
Magnetite: Magnetite is common in igneous and metamorphic rocks, and some sediments, though usually in only
small amounts (1 - 2 %). It is black in colour with a metallic lustre, occurring in small octahedra (like two pyramids
stuck together). Easily recognized by its strongly magnetic character.
Talc: Talc occurs in granular or foliated masses sometimes known as soapstone. It is white to green,
sometimes grey or brownish. It is very soft and will be scratched by a finger nail. It has a greasy feel.
Chalcopyrite
Chalcopyrite (or copper pyrite), looks like, and is easily confused with Pyrite, FeS2. Chalcopyrite is one of
the minerals refered to as "Fool's Gold" because of its bright golden color. But real gold is a more buttery
yellow and is ductile and malleable.
PHYSICAL CHARACTERISTICS:
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Color is brassy yellow, tarnishes to irredescent blues, greens, yellows and purples.
Luster is metallic.
Transparency: Crystals are opaque.
Crystal System is tetragonal; bar 4 2m
Crystal Habits are predominantly the disphenoid which is like two opposing wedges and resembles a tetrahedron. Crystals
sometines twinned. Also commonly massive, and sometimes botryoidal.
Cleavage is rather poor in one direction.
Fracture is conchoidal and brittle.
Hardness is 3.5-4
Specific Gravity is approximately 4.2 (average for metallic minerals)
Streak is dark green.
Other Characteristics: Some striations on most crystal faces.
Associated Minerals are quartz, fluorite, barite, dolomite, calcite, pentlandite, pyrite and other sulfides.
Notable Occurances include Chile, Peru, Mexico, Europe, South Africa, several USA sites and many others around the world.
Best Field Indicators are crystal habit, tarnish, softness and brittleness.
Pyrite
Pyrite: The commonest of the sulphide minerals, i.e. those minerals containing sulphur
as a principle component. It occurs in all rock types, though usually only in small
amounts. It is a pale brassy yellow in colour with a metallic lustre and often forms cubeshaped crystals. Also known as "fool's gold".
PHYSICAL CHARACTERISTICS:
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Color is brassy yellow.
Luster is metallic.
Transparency: Crystals are opaque.
Crystal System is isometric; bar 3 2/m
Crystal Habits include the cube, octahedron and pyritohedron (a dodecahedron with pentagonal faces) and crystals with
combinations of these forms. Good interpenetration twins called iron crosses are rare. Pyrite is commonly found in nodules. A
flattened nodular variety called "Pyrite Suns" or "Pyrite Dollars" is popular in rock shops. Also massive or reniform and replaces
other minerals and fossils forming pseudomorphs or copies.
Cleavage is very indistinct.
Fracture is conchoidal.
Hardness is 6 - 6.5
Specific Gravity is approximately 5.1+ (heavier than average for metallic minerals)
Streak is greenish black.
Other Characteristics: Brittle, striations on cubic faces caused by crossing of pyritohedron with cube. (note - striations on cube
faces also demonstrate pyrite's lower symmetry). Pyrite (unlike gold) is not malleable.
Associated Minerals are quartz, calcite, gold, sphalerite, galena, fluorite and many other minerals. Pyrite is so common it may
be quicker to name the unassociated minerals.
Notable Occurrences include Illinois and Missouri, USA; Peru; Germany; Russia; Spain; and South Africa among many others.
Best Field Indicators are crystal habit, hardness, streak, luster and brittleness. s