Grade 6
Science
Unit: 02 Lesson: 02
Physical Properties of Minerals
There are many properties that can be used to identify minerals, but any one property is not enough
to identify an element. Some of the properties used in mineral identification are: fracture, cleavage,
specific gravity, crystal structure, magnetism, transparency, radioactivity, inclusion, reaction to acid,
color, streak, luster, and hardness. The following four properties will be studied:
Color
Color is the most obvious but least dependable property of mineral. Some minerals, such as gold,
only come in one color. Other minerals have a wide variety of colors such as calcite, which can be
pink, orange, blue, green, red, brown, white, or clear. Sometimes, other elements can cause a
change in color. For example, pure quartz is clear. If iron was present when the quartz formed, the
color of the quartz will be purple. The purple quartz is called amethyst. Some minerals will have a
different color if viewed under an ultraviolet light. These minerals are said to be fluorescent.
Streak
The color of crushed mineral powder is known as the mineral’s streak. A mineral is scraped across a
streak plate to see what color it leaves. The color of a mineral’s powder streak is important for
identifying minerals in two ways. First, the color of the streak may be different than the color of the
element. Second, nearly every mineral has a streak color that stays the same regardless of the color
of the mineral. For example, calcite comes in a wide range of colors, but its streak color will always be
white.
Streak plates are small squares of white, unglazed porcelain, like the underside of a ceramic
bathroom tile. Since the plate is white, dark color streaks will show up well. A mineral that streaks
white or clear cannot be tested this way. Streak is most useful in identifying dark-colored minerals. If
a streak plate is not available, a piece of fine grained sandpaper will work for the softer minerals.
Cleavage and Fracture
Minerals are made of crystals, and when the crystal is broken, it reveals a particular crystal shape.
This shape is known as cleavage. All minerals do not have cleavage. If the mineral does not have
any crystallized fragments, then it does not have cleavage.
Describing the way a mineral is broken is known as fracture. Different minerals break in different
ways. Fracture can be described as smooth, irregular, jagged, or splintery. Fracture can be used to
describe the way a mineral breaks when it does not have cleavage.
Luster
The way the surface of a substance reflects light is called luster. Luster is used to describe how a
mineral looks. It is impossible to identify minerals that have the same luster on this characteristic
alone.
There are several descriptors for luster. Some categories of luster are:
 metallic – opaque; reflects like metals; metal elements exhibited
 vitreous – glassy; applies to about 70% of all minerals
©2012, TESCCC
04/16/13
page 1 of 2
Grade 6
Science
Unit: 02 Lesson: 02




adamantine – brilliant or diamond-like; high shine
silky – reflect like silk cloth; minerals have fine fibrous structure
pearly – similar to a white shirt button, mother-of-pearl
dull – poor reflectors; minerals have rough, porous texture
Hardness
Hardness describes how a substance will resist being scratched by another substance. Frederick
(Friedrich) Mohs, an Austrian mineralogist, developed a scale to measure hardness in 1822. It is
known as Mohs’ Scale of Mineral Hardness, and it is the standard scale for measuring the hardness
of minerals. The scale rates minerals 1 through 10, with 1 being the softest and 10 being the hardest.
A different mineral is represented by each number, with each one being harder than the previous one.
The scale is relative in measurement. A mineral can be more than 3 and less than 4 in hardness. The
hardness can be written as a whole number or in half number increments for in-between hardness.
Mohs’ Scale of
Mineral Hardness
1
talc
2
gypsum
3
calcite
4
fluorite
5
apatite
6
feldspar
7
quartz
8
topaz
9
corundum
10
diamond
©2012, TESCCC
Common Items to Use to Test for Mineral Hardness
1
soft, pencil lead
2–2½
fingernails
3–3½
penny dated prior to 1982
4–4½
10-penny nail
5–5½
penknife blade or glass
6-6½
steel file
8
whet stone (sharpening stone)
04/16/13
page 2 of 2