MINERAL DETECTIVE
This exercise is designed to allow students to use their observational skills in conjunction
with basic scientific methods in order to identify minerals. To identify a mineral, you must
first look at it carefully! You may want to use a magnifying glass or a hand lens to see more
detail in the mineral, and you may see some structures that you could miss with the naked
eye. You can then run through a list of properties that will help you narrow down your
search, with the ultimate goal of correctly identifying the unknown mineral. It is important to
record the properties that you observe in order to compare them with the known properties
of the mineral.
One thing to remember: To get an accurate identification of a mineral you must look at as
many properties as you can. In combination, color, luster, streak, hardness and crystal
shape will give you a fair idea of what mineral you are looking at. Never rely on one
property alone - try as many tests as you need to be sure of your identification.
How can we tell minerals apart?
Remember the definition of a mineral: Minerals are inorganic, naturally occurring
homogenous solids, with definite chemical compositions, and ordered (crystalline)
atomic arrangements.
Every mineral has a set of physical and chemical properties that allow us to distinguish
them. These properties include:

Color: most minerals have a distinctive color or range of colors

Hardness: how easily the mineral is scratched.

Cleavage: how minerals break apart - some minerals break on smooth planes in
certain directions, some minerals never break on smooth planes, but break on rough,
irregular fractures.

Luster: a description of the way light interacts with the surface of a crystal

Streak: the color of the powder of a mineral.

Transparency: minerals are either transparent (you can see clearly through them);
translucent (light passes through, but it’s more like frosted glass); or opaque (you
can’t see through them at all)

Specific gravity (density): The weight of a mineral compared to the weight of an
equal volume of water.

Crystal Habit: a description of the shapes and aggregates that a certain mineral is
likely to form.
For a more complete explanation of mineral properties, turn this page over.
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Hardness
A scratch test developed by a German mineralogist Fredriech Mohs in 1822 is used to
determine mineral hardness. He developed a hardness scale that helps to identify mineral
properties. The scale measures hardness on a scale of 1-10. One (1) being the softest
mineral (talc) and 10 being the hardest mineral (diamond). Common objects of known
hardness can be used to determine mineral hardness. These common objects are: your
fingernail (2.5), a penny (3), a piece of glass (6) and a knife blade or nail (5). For example, if
your fingernail can scratch the mineral, it has a hardness of less than 2.5, which is quite soft.
If the mineral can scratch glass it has a hardness of greater than 6, which is very hard.
Color
Color can sometimes be helpful when identifying minerals. However, some minerals have
more than one color, like quartz. Quartz can be blue, brown, pink, red, purple, and almost
any other color, or it can be totally colorless. Therefore, geologists have developed a better
way of using color as an identifying property. This property is called a streak.
Streak
Streak is the name given to the colored residue left by scratching a mineral across an
abrasive surface, such as a tile of unglazed porcelain. The streak may not always be the
same color you see in the hand specimen. A mineral with more than one color will always
leave a certain color of streak. Hematite is a mineral that can be red, brown, or black, but it
will always leave a characteristic reddish brown streak.
Luster
Another mineral property that geologists use to identify minerals is luster. Luster is the way
in which the surface of a mineral reflects light. There are two main types of luster: metallic
and nonmetallic. A metallic luster is shiny and similar to the reflection from a metal object,
such as a faucet. A mineral that does not shine like metal has a nonmetallic luster. For
example, the wall has a nonmetallic luster. There are many types of nonmetallic luster. A
glassy luster is bright and reflects light like a piece of glass. A greasy luster has an oily
appearance. An earthy luster is a very dull and looks like dirt. Waxy luster looks like the
shininess of a crayon.
Cleavage
Cleavage is another property used to distinguish minerals. Cleavage is the tendency for
minerals to break along flat planar surfaces. Cleavage is rated as good, fair and poor
depending on the quality of the flat surface produced. Mica, for example, is a mineral that
has good cleavage. It breaks into very flat sheets. Minerals that have very poor cleavage will
only break along irregular surfaces. Quartz, for example, will break into pieces that have a
seashell-like fracture plane. Others, like garnet, shatter with no distinguishable pattern.
These are considered to have no cleavage at all.
**Chemical Reaction (make sure you have eye protection and gloves if you do this test)
A weak acid, usually dilute hydrochloric acid, is used to tell if rocks or minerals contain
calcium carbonate (CaCO3). If the specimen fizzes (giving off CO2) when it comes in contact
with acid, it is considered carbonate rich. If it does not contain calcium carbonate, it will not
fizz. Calcite and aragonite are two minerals that will always fizz.
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ACTIVITY - MINERAL DETECTIVE
STEP 1: Using your sense of smell, smell the specimens.
 Do they have different smells?
 Group the samples that have a similar smell in groups across the page next to
the “smell” category.
 For each group, describe and label the smell. Is it earthy, “rotten-egg”-like, sweet,
sour, etc.?
STEP 2: Using your sense of sight, arrange the rock and mineral specimens by color
groups. Put the brown ones together, the white ones together, and so on. Circle and
label the color groups.
STEP 3: Using your sense of sight once again, look at a fresh surface of each sample.
Some of the samples are shiny like glass (glassy or vitreous). Some are dull (earthy
or chalky). Some look or feel like metal (metallic). Some look waxy, pearly, or silky.
The way a rock or mineral shines or reflects light is called luster.
• Put the glassy specimens in a group, the earthy or chalky specimens in a
group, the metallic specimens in a group, and make groups for those specimens
that look waxy, silky, or pearly. Circle and label each group.
STEP 4: Using your sense of touch, rub your fingertips over the surface of each specimen.
Some will feel sandy or gritty, others will feel powdery (earthy or chalky), others
will feel smooth or glassy, some may feel waxy or metallic.
 Group, circle, and label the specimens by how they feel.
STEP 5: Use the magnet to find those specimens that are magnetic or attracted to the
magnet, and those that are not magnetic. Circle and label the two groups.
STEP 6: Next, pick up the white unglazed porcelain tile and scratch each specimen across
it. The powdery mark that is left on the tile is called the streak. Each rock or mineral
may have several different colors, but the streak of a rock or mineral is its true
color, and it can be very different from the color of the specimen.
 Group the specimens by streak color, circle and label.
STEP 7: Now we will test the hardness (or the resistance to scratching) or each specimen
using our fingernails (for the softest samples) and additional tools (for increasingly
harder samples).
Line up the samples in the following order:
 On the first line, place the specimens that can be scratched with your fingernail.
 On the next line, place the specimens that can be scratched with the penny.
 On the next line, place the specimens that can be scratched with the steel blade.
 On the next line, place the specimens that can be scratched with the glass plate
or quartz crystal.
 On the last line, place the specimens that can scratch the steel blade or the
glass plate/quartz crystal.
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STEP 8: • Finally, let’s again use our sense of touch, and compare the weights of the rock
and mineral samples. (NOTE: This is a measure of the specific gravity or each
sample. Specific gravity is the weight of any material in relation to, or proportion to,
the weight of an equal volume of water.)
 Place the specimens in three different weight groups: Light, medium, and heavy.
 Circle and label each group.
We have just learned some basic scientific ways that we can distinguish rocks and minerals
from one another!
There are many more tests that mineral detectives, also known as mineralogists, use to
identify rocks and minerals. Some of these include: Reactions to acids, fluorescence,
microscopic examination, and sophisticated X-ray diffraction techniques. We have also seen
distinctive characteristics and properties that can help us to identify it. There are many
published that list the physical characteristics and properties of the thousands of rocks and
mineral types found on the earth. More new minerals are still being discovered as we
develop more sophisticated scientific equipment to measure and observe their
characteristics and properties.
So, tonight when your spouse asks how the soup tastes you can reply, "variable color,
greasy surface, low specific gravity, texture smooth with bits of ductile material."
Where Do Minerals Come From?
Minerals "grow," or crystallize, from many types of solutions. They may precipitate from
evaporating seawater, or crystallize from magmas when lava cools. Ice is a mineral that
forms as pure water cools below 0C.
So, what the difference
between a rock and a mineral?
Minerals are made from one or more elements, and rocks are combinations of one or
more minerals. There are many analogies that might help you to remember the difference:
You can think of minerals as words, made up from letters (elements). Just as letters have to be
put together in the right order for the word to make sense, elements have to be bonded
together in the right structure to make a particular mineral. Some words only have one letter (I,
a) just as some minerals contain only one element (diamonds, gold). Rocks are a combination
of minerals like sentences are a combination of words. Some sentences are very short, while
some are very long, just like some rocks contain a lot of minerals while some contain very few.
Some people think of minerals as the “ingredients” that go into a rock, just like ingredients in a
cookie. The flour, sugar, chocolate chips could be considered the minerals, and the finished
cookie the rock. Certainly, some people make rock-like cookies!
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