CLASS SET
What is a Mineral?
A mineral is a homogeneous inorganic substance which has a more or less specific chemical composition
and fairly definite physical properties. In a restricted sense, substances resulting from the processes of
plant and animal life, such as coal, petroleum, animal bones, and pearls are not classed as minerals. Most
minerals are characterized by crystal faces, the flat smooth surfaces of a crystal. Of particular importance
are the physical properties of minerals because familiarity with them facilitates a rapid identification of
the mineral specimen that may be collected almost anywhere.
PHYSICAL PROPERTIES USED IN THE IDENTIFICATION OF MINERALS
1. Hardness
Hardness is expressed as the resistance that a smooth surface of a given mineral offers to
scratching. See Figure 1: The horizontal axis of the graph is divided into 10 equal units based on
Moh’s scale of hardness. The vertical axis, which is based on absolute units, emphasizes how the
interval of hardness between different pairs of in the scale varies. The scale means that any
mineral can scratch the mineral listed below it. It is advisable to try to scratch mineral A with
mineral B, and then reverse the procedure. A fresh surface must be used for this test because
mineral B may leave a mark on mineral A which appears to be permanent. However, the mark
may only be a powered streak of the softer mineral which can be rubbed off.
Figure 1
Tools that are commonly used in field to measure the hardness of a mineral specimen are:
 Fingernail
2.5



Copper Coin
Iron Nail
Glass Plate
3.5
5.5
6.0
2. Streak
The streak of a mineral is the color obtained by finely powdering the mineral.
Powdering is usually achieved by rubbing a piece of the mineral on an unglazed porcelain plate
known as a streak plate.
3. Color
The attractive colors which minerals display constitute one of the quickest aids to
identification. Some caution must be exercised, however, for the following reasons:
 Certain minerals occur in a wide variety of colors
 Impurities often give a mineral a false color which is not characteristic of it.
 A change in color is often produced by a change in composition
 Exposure to the atmosphere may alter the surface of a mineral, tarnishing it or changing its
color. In this case the mineral should be broken thus exposing a fresh surface which shows
the true color.
4. Luster
The way a mineral reflects light is known as luster. The luster of minerals is divided into
two major types – Metallic and non-metallic. Metallic minerals, such as galena usually have a
dark appearance and a dark streak in contrast to non-metallic minerals. The luster of non-metallic
minerals is further subdivided into several types. For example quartz has a glassy or vitreous
luster while talc has a pearly luster.
5. Cleavage
Some minerals exhibit the property of breaking along smooth, flat surfaces, which are
called cleavage planes. The number and direction of cleavage planes are diagnostic physical
properties for a given mineral. Minerals that do not have good cleavage exhibit rough and
irregular broken surfaces known as fracture surfaces. Biotite mica is an excellent example of
cleavage in one direction. The thin sheets of the mineral can literally be peeled apart. Calcite has
perfect cleavage in three directions which results in rhomboid shaped fragments. No matter how
small the pieces are, they always exhibit the same form depending upon the type of cleavage.
6. Specific Gravity
Specific gravity is an expression of the weight of a mineral compared with the weight of
an equal volume of water. Thus, galena, which has a high specific gravity, feels “heavy”,
whereas halite, with a relatively low specific gravity feels “light”. In some mineral identification
tables the specific gravity is simply categorized as high, medium, or low. In general, ore minerals
have high specific gravities and rock-forming minerals have low specific gravity.
7. Magnetism
Some minerals, such as magnetite, are magnetic and can be easily tested with a magnet.
8. Taste
Although this test is not recommended for every mineral, it is used to easily identify the
salty mineral halite.
9. Double Refraction
Some varieties of the mineral calcite can split light into two rays creating a double image.
10. Acid Test
The acid test is used to identify the mineral calcite, which contains calcium carbonate
(CaCO3). When dilute hydrochloric acid is dropped onto calcite a reaction occurs and
the mineral effervesces or fizzes. Carbon dioxide (CO2) gas is being released from the
(CO3) calcium carbonate.
11. Double Refraction
Some varieties of the mineral calcite can split light into two rays creating a double image.
12. Acid Test
The acid test is used to identify the mineral calcite, which contains calcium carbonate
(CaCO3). When dilute hydrochloric acid is dropped onto calcite a reaction occurs and
the mineral effervesces or fizzes. Carbon dioxide (CO2) gas is being released from the
(CO3) calcium carbonate.
Types of Cleavage in One to Four Directions
Conchoidal Fracture – (literally, shell-like) - A type of fracture that produces a smooth,
curved surface. It is characteristic of quartz, obsidian, glass and flint.
It is 7:00am and you are awakened by calls to get up and get ready to go to school. You hop out of bed
and head for the bathroom. After a quick shower with your favorite scented anti bacterial body wash you
brush your teeth with the latest whitening toothpaste and blow your hair dry. Back in your room you
quickly dress and then rush to the kitchen for a bowl of Iron Man muscle building fortified cereal, a bacon
and egg sandwich with salt and pepper, and a glass of juice. As you are eating you glance at the battery
powered clock hanging on the newly painted kitchen wall, and realize that you are right on schedule for
the bus. Before heading out the door you check your book bag to make sure that you have pens, pencils
and paper. Wrapping a rubber band around some flash cards you toss them into the bag, and head out
the door. In order to be at the bus stop a little early, you hurry out the door and down the concrete
sidewalk to the corner.
As you approach the corner, you slow your stride to a casual, but oh, so cool, walk and give her that
gleaming white, you can’t resist me smile. She looks great this morning, with just the right touches of
make-up. The dramatic eye shadow and lipstick look just “killer”. You also notice that this morning she is
wearing the cool grey crystal and gold colored metallic bead earrings that you gave her for her birthday
last week. You know in your heart that, at least in part, because of the minerals in your life it is going to
be a really great day.
She smiles back and compliments you on your cool new iPod…
In less than the time it took to board the school bus, you have encountered or utilized dozens of minerals.
Do you how and when? Can you identify them? Don’t you think the minerals in your life deserve more
respect? After all, she does look really cool wearing those earrings! Realizing all that minerals do for
you, you make a commitment. Today is the day to begin to know your minerals better.
Product
Toothpaste
Mineral
Pencil
Concrete
Jewelry
Glass
salt
cosmetics
drywall
Rubber