PHYSICAL PROPERTIES OF
MINERALS
Mineral Identification Basics

What is a Mineral?
There is a classic definition for
mineral.
Minerals must be:
Inorganic
Occurs Naturally on Earth
Always a Solid
Has Definite Chemical Composition
Crystalline Structure
Cubic Fluorite Crystal
Mineral Identification Basics

What is a Mineral?
Naturally Occurring
Minerals are not synthetic
or man made - they are
produced by the natural
geological processes working
on Earth.
Tourmaline Crystal from Brazil
Mineral Identification Basics

What is a Mineral?
Inorganic
Barite Rose - A flower like growth
of Barite crystals.
Minerals are NOT
produced by organic living
processes. As a result things
like pearls, coral, coal and
amber are NOT considered
minerals.
Mineral Identification Basics

What is a Mineral?
Crystalline Structure
Minerals are the result of
atoms joining together
through electrical bonds to
produce a definite internal
structure.
Crystalline Pattern of Halite
Halite (salt) from Searles Lake, CA
Red = Sodium
Green = Chlorine
(*)
Mineral Identification Basics

What is a Mineral?
Definite Chemical Composition
Halite - NaCl
For every atom of Sodium there is
an atom of Chlorine.
Minerals can be expressed by
a chemical formula. The
internal order of minerals means
that there is a definite
relationship in the number of
atoms that makes up the mineral.
Mineral Identification Basics

PHYSICAL PROPERTIES HARDNESS
HARDNESS is defined as the
resistance a mineral has to being
scratched - its “scratchability”.
Hardness tests are done by scratching
one mineral against another. The
mineral that is scratched is softer than
the other.
Pyrite Crystals
Hardness of 6.5
Mineral Identification Basics

PHYSICAL PROPERTIES HARDNESS
In this photo, a quartz crystal
will be rubbed across a glass
plate. The result is that the glass
plate will be scratched. The
quartz is therefore harder than
the glass.
Quartz is harder than glass.
Mineral Identification Basics

PHYSICAL PROPERTIES HARDNESS
Care must be taken on some minerals that
crumble easily. Remember that hardness is
the resistance a mineral has to being
scratched - NOT how easily it breaks apart.
Mineral Identification Basics

PHYSICAL PROPERTIES HARDNESS
MOH’S SCALE OF MINERAL HARDNESS
1. TALC
6. FELDSPAR
2. GYPSUM
3. CALCITE
4. FLUORITE
5. APATITE
7. QUARTZ
8. TOPAZ
9. CORUNDUM
10. DIAMOND
OTHER MATERIALS COMMONLY USED:
Moh’s scale is a list of minerals with increasing hardness.(*)
2.5 - FINGERNAIL
3 - COPPER PENNY
5.5 - GLASS
6-6.5 - STEEL FILE
Mineral Identification Basics

PHYSICAL PROPERTIES CLEAVAGE
CLEAVAGE is the property of a
mineral that allows it to break
repeatedly along smooth, flat
surfaces.
These GALENA cleavage fragments
were produced when the crystal was
hit with a hammer. Note the
consistency of the 90o angles along the
edges.
These are FLUORITE cleavage
fragments.
Mineral Identification Basics

PHYSICAL PROPERTIES CLEAVAGE
Within this crystalline pattern it is
easy to see how atoms will
separate to produce cleavage with
cubic (90o) angles. (*)
It is similar to tearing a piece of
paper that has perforations in it.
The paper has a tendency to tear
along the perforations. They are
zones of weakness. (*)
In this example the lines represent breaks between the atoms that make up the
mineral. Cleavage is guided by the atomic structure. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES CLEAVAGE
These pictures show different cleavage angles and the quality of cleavage.
Fluorite has cleavage in four
directions. (*)
Mica
A thin
has
sheet
perfect
of Muscovite
cleavage inseen
ONE
on
direction.
edge. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES CLEAVAGE
Common salt (the mineral HALITE) has very good cleavage in 3 directions. (*)
These 3 directions of cleavage are
mutually perpendicular resulting in
cubic cleavage. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES CLEAVAGE
Rhombohedral
Cleavage - 3 directions
CALCITE
Even these tiny fragments have rhombohedral cleavage. (*)
(*)
Mineral Identification Basics

PHYSICAL PROPERTIES FRACTURE
FRACTURE is defined as the way a
mineral breaks other than cleavage. (*)
This is a piece of volcanic glass called
OBSIDIAN. Even though it is NOT a
mineral, it is shown here because it has
excellent conchoidal fracture. (*)
If you try this yourself, use caution.
Conchoidal fracture in obsidian can
produce extremely sharp edges. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES FRACTURE
This Quartz crystal will be
struck with a hammer to
show how that the external
form of the crystal does not
repeat when broken. (The
flat crystal faces are not
cleavage faces.)
This is a good example of
conchoidal fracture. (*)
Note the smooth
curved surfaces. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES STREAK
STREAK is defined as the
color of the mineral in powder
form. (*)
Streak is normally obtained by rubbing a
mineral across a “streak plate”. This is a
piece of unglazed porcelain. The streak plate
has a hardness of around 7 and rough texture
that allows the minerals to be abraded to a
powder. This powder is the streak. (*)
Hematite on Streak Plate
Hematite has a reddish brown streak. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES STREAK
Sphalerite is a dark mineral,
however, it has a light colored
streak. Next to the reddish brown
streak of hematite is a light yellow
streak. This is the streak of the
sphalerite. (*)
Light colored streaks are often
difficult to see against the white
streak plate. It is often useful to rub
your finger across the powder to see
the streak color. (*)
Sphalerite has a light yellow streak. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES LUSTER
LUSTER is defined as the quality
of reflected light. Minerals have
been grossly separated into either
METALLIC or NON-METALLIC
lusters.
Following are some examples:
Native Silver has a Metallic Luster.
(*)
Mineral Identification Basics

PHYSICAL PROPERTIES LUSTER METALLIC
Stibnite
Galena
The basic idea for Metallic Luster is that the minerals look like metals. (*)
Pyrite (*)
Marcasite
Mineral Identification Basics

NON-METALLIC LUSTER VITREOUS
Vitreous Luster means that the mineral has a “glassy” look.
Normally we think of glass as being clear, but there are many
different colors of glass and they are all very “glassy” looking. Even
china plates and glazed porcelain are vitreous. Here are some
examples: (*)
Olivine - Peridot
Spinel
Wulfenite
Quartz (*)
Mineral Identification Basics

NON METALLIC LUSTER
Miscellaneous Lusters
Asbestos - Silky
Apophyllite – Pearly (*)
Graphite has a greasy or submetallic luster
and easily marks paper. (*)
Limonite - Dull or Earthy
Sphalerite - Resinous
Mineral Identification Basics

PHYSICAL PROPERTIES COLOR
The COLOR of a mineral is usually
the first thing that a person notices
when observing a mineral. However, it
is normally NOT the best physical
property to begin the mineral
identification process. (*)
Following are some examples of color
variation within mineral species
followed by minerals that have a
distinctive color: (*)
Various colors of CALCITE. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES COLOR
Amethyst
Quartz comes in a wide range of colors. It is very easily colored
by even trace amounts of impurities. (*)
Ionic Iron
Clear - Without Impurities
Hematite Inclusions
Various colors of Quartz.
Chlorite inclusions
Mineral Identification Basics

INDICATIVE COLOR
Some minerals do have a certain
color associated with them. Here are
some examples: (*)
Turquoise
Malachite
Rhodochrosite
Azurite (*)
Sulfur
Mineral Identification Basics

PHYSICAL PROPERTIES SPECIFIC GRAVITY
The SPECIFIC GRAVITY
of a mineral is a measure
of the mineral’s density. It
is related to the types of
elements that make up the
mineral and how they are
packed into the mineral’s
atomic structure.
Gold in Quartz
Mineral Identification Basics

PHYSICAL PROPERTIES TASTE
IT IS NOT RECOMMENDED
THAT A TASTE TEST BE
PERFORMED ON MINERALS
AS A STANDARD PROCESS.
SOME MINERALS ARE TOXIC.
However, the mineral HALITE is common
salt and has a unique taste. (*)
Halite cubes from Trona, CA (*)
Mineral Identification Basics

PHYSICAL PROPERTIES MAGNETISM
MAGNETISM is the ability of
a mineral to be attracted by a
magnet. This most commonly
is associated with minerals rich
in iron, usually magnetite.
This is a piece of MAGNETITE with a magnet
adhering to it. Magnetite is a mineral that is
strongly magnetic in that a magnet will easily be
attracted to it. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES MAGNETISM
This is a sample of “black sand”
from Lynx Creek, Arizona. Its dark
color is due to its high
concentration of magnetite. See
what happens when a magnet is
place beneath the bottom right
portion of the paper. (*)
This technique is used to separate out much of the unwanted material in the search
for gold in placer deposits. (*)
Mineral Identification Basics

PHYSICAL PROPERTIES MAGNETISM
LODESTONE is a variety
of Magnetite that is
naturally a magnet. (*)
Mineral Identification Basics

DOUBLE REFRACTION
DOUBLE
REFRACTION: Is a
property shared by many
minerals ( but not those
in the isometric crystal
system). It is produced
by the separating of a
beam of light as it passes
through the crystal
structure. It is best
displayed in the mineral
CALCITE. This image
clearly shows the double
image below the calcite.
Mineral Identification Basics

CHEMICAL PROPERTIES
REACTION TO ACID
Some minerals, notably the
carbonates, react to cold
dilute HCl. In this
illustration a piece of
CALCITE is shown to
react (fizz) after HCl is
applied.
Calcite Reacts to HCl (*)
This ends the basic introduction to
the Physical Properties of Minerals.