How to Use the Mineral Identification Key

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How to Use the Mineral Identification Key

• PHYSICAL SCIENCE 120
• GLENDALE COMMUNITY COLLEGE
• STAN CELESTIAN
• INSTRUCTOR

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For this lab you will need the following
materials from you lab kit.
Glass Plate
Copper Wire
Streak Plate
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This is the first page of the Mineral
Identification Key.
Note that the first property of the unknown
mineral that you need to identify is LUSTER.
Luster is divided into two main groups METALLIC
and NON METALLIC.
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The Key is like a flow chart you go through a
series of steps to narrow down the possibilities
This presentation will take you through 3
minerals using this Key.
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Mineral 2
Glass Plate
Copper Wire
Mineral 3
Streak Plate
Mineral 1
These are the minerals that will be used for this
example.
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The first thing to check is the luster. This
mineral has a metallic luster.
According to Table 1, the mineral is located on
Table 2.
The next step is to check the streak of the
mineral.
Streak Plate
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This mineral has a grayish to black colored
streak.
To take the streak of the mineral you rub it
across the streak plate.
Note that there is more than one mineral that has
a gray to black streak.
Streak Plate
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This one test has eliminated the last four
minerals on the list. Next you must read the
physical properties of the minerals that are
remaining.
Hefting the mineral in your hand reveals that it
is very heavy. The color of the specimen is also
a steel gray. It looks like it might be GALENA or
maybe MAGNETITE. The others are eliminated
because of their color.
This mineral does NOT scratch glass. Its
hardness is therefore less than 5.5. Magnetite
has a hardness of about 6 and would easily
scratch the glass. The mineral is therefore
GALENA. (It also displays cubic cleavage.)
Graphite is eliminated because of its greasy feel.
Streak Plate
No scratch on the glass plate
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This specimen is non-metallic and light colored.
So we proceed to tables 4A and 4B.
Next, is it harder or softer than glass?
It is determined that the specimen does NOT
scratch glass. This mineral is located on Table
4B.
The next unknown.
Streak Plate
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The next step is to check for cleavage. All of
the specimens that you have been given are of
broken pieces. Flat, parallel faces indicate
cleavage. This mineral possesses cleavage.
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From here it is a process of elimination.
Lets look at the cleavage. It is in 3 directions.
In addition to the two cleavage directions
indicated by the red lines, the third direction
is the side the specimen is resting on (bottom)
and the other side is the top. Note also that
the angles of cleavage are not at 90o.
Another useful aspect of this mineral is that it
does not scratch your fingernail. Its hardness
is less than 2.5.
Remember that cleavage faces are parallel.
Although there are 4 lines, they represent 2
different directions of cleavage.
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Starting from the top Halite may seem possible
but with perfect cubic cleavage (cleavage at
90o, it is ruled out.
Gypsum meets all of the physical properties
listed. In fact, it is gypsum. But lets see
how the others differ from gypsum.
Calcite will scratch your fingernail and it will
effervesce (fizz) in cold, dilute hydrochloric
acid. We are not able to supply you with this
acid but you could try vinegar. A piece of
calcite that has been powdered on a clean streak
plate will fizz slowly in vinegar.
Fluorite has cleavage in 4 directions and will
easily scratch your fingernail.
Muscovite is a type of mica. It cleaves
extremely easily producing thin sheets that are
elastic they spring back. Gypsum can produce
thin cleavage sheets but they do not spring back.
Also, muscovite has cleavage in only 1 direction
while gypsum cleaves in 3.
Finally there is Barite. Barite has a fairly
high specific gravity at 4.5. It feels heavy in
the hand. In addition, with a hardness of 3 it
would easily scratch your fingernail.
Talc is extremely soft with a hardness of 1. It
also cleaves so easily that it comes off in your
hands. Talc has a soapy feel.
Lets take a look at the last mineral.
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Just by looking at the specimen you should
classify it as non-metallic and light colored.
Then the question is Is it harder or softer
than glass?
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Rubbing this specimen across the glass plate
leaves a definite scratch. The mineral has a
hardness of greater than 5.5.
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From table 4A the next important property is
cleavage. Does the mineral have it or not?
Remember that all of the specimens that you will
be looking at are of broken minerals. If it has
smooth, flat surfaces, it has cleavage. This
specimen does have cleavage that can be seen when
the light angle changes. (move to next slide)
Note how the light reflects of the many parallel
faces. This is a good example of cleavage and
one way to look for it (without breaking the
specimen).
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So it appears that the only non-metallic, light
colored mineral with prominent cleavage on the
list is FELDSPAR. This is a specimen of
Orthoclase Feldspar, but for the lab, Feldspar is
good enough.
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THE FOLLOWING PAGES ARE THE MINERAL
IDENTIFICATION KEY FOLLOWED BY HELPFUL HINTS FOR
MINERAL IDENTIFICATION
Feel free to print these out for the
identification of the minerals in you kit.
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THE END