physical properties minerals a ian kenyon

The Physical Properties Of Minerals

WJEC AS Geology
I.G.Kenyon


Colour 1
• Determined by the
chemical composition of
the mineral
• Minerals rich in Al, Ca,
Na, Mg, Ba and K are
often light coloured
• Minerals rich in Fe, Ti,
Ni, Cr, Co, Cu and Mn
are often dark in colour

8cm

Haematite, Kidney Ore


Colour 2
5cm

• Determined by the atomic
structure of the mineral
• Atomic structure controls
which components of white
light are absorbed or reflected
• White minerals reflect all

components of white light
• Black minerals absorb all
components of white light
• Green minerals reflect green
light and absorb the others

Pyrite Cubes with Striated Faces


Colour 3
•Colour is not particularly useful
as a diagnostic property
•Some minerals show a wide variety of colours
•Quartz can be transparent, white, pink,
brown, purple, yellow, orange and even black
•Many minerals show very similar colours
•Calcite, gypsum, barytes, fluorite,
plagioclase feldspar and halite are
commonly grey or white in colour


Colour 4

Examples of colour variation in Fluorite


Colour 5
Plagioclase feldspar

Barytes

Quartz

Fluorite

Calcite

Gypsum

All these minerals are grey or white in colour


Transparency
2cm

• When outlines of objects

seen through it appear
sharp and distinct
•A good examples is Iceland
Spar, a variety of calcite that
is used for optical lenses
•Iceland Spar also shows the
remarkable property of
double refraction
Calcite – Iceland Spar

• Determined by the atomic
structure and chemical
composition of the mineral

Translucency
1 cm
•The ability for a mineral to
let light pass through it
•Many minerals if cut thin
enough will show some
degree of translucency
•Controlled by atomic
structure and chemical
composition

Fluorite

•All transparent minerals
are also translucent


Lustre
2cm

The way in which a
mineral reflects light
Controlled by the atomic
structure of the mineral
Main types of lustre are
Vitreous
Metallic
Pearly

Resinous
Adamantine

Quartz – Vitreous Lustre

Dull/Earthy


Vitreous Lustre
Fluorite

Dog-Tooth Calcite

The mineral reflects light like glass
Sometimes glassy lustre is used instead of vitreous


Metallic Lustre
Malachite

Galena

Minerals reflect light like metals.
Metallic lustre often tarnishes to a dull lustre


Pearly Lustre
Biotite Mica

Muscovite Mica

The lustre of a pearl
or mother of pearl
Shows clearly on the
cleavage surfaces
of biotite and
muscovite mica
Also shown by Talc
and selenite (a variety
of gypsum)


Silky Lustre
1cm

The lustre of silk
Occurs in minerals with
a fibrous structure
Satin spar (a fibrous
form of gypsum) shows
this to good effect

Gypsum (Satin Spar)


Resinous Lustre
1cm

The lustre of resin
The mineral has a

grainy appearance

Sphalerite (Zinc Blende)

Sphalerite, opal
and amber show
resinous lustre


Adamantine Lustre
5mm

The lustre of a diamond


Dull or Earthy Lustre
The mineral does not
reflect light and has the
same appearance as soil.

1cm

Minerals such as galena
have metallic lustres on
freshly broken surfaces
but they tarnish to dull
with prolonged exposure
to the atmosphere

Limonite has a dull or earthy lustre

Streak
The colour of a mineral’s powder
Obtained by rubbing a mineral
specimen on an unglazed white
porcelain tile
Useful for identifying
metallic ore minerals
Silicates generally do not mark
the tile and have no
streak
White minerals streaked on a
white tile will have a white streak
Haematite gives a
cherry red streak

Any minerals harder than the tile
(6) will scratch it


Streak 2

Malachite – pale green

Galena – lead grey

Haematite – cherry red

Iron Pyrite – greenish black

Sphalerite – pale brown

Limonite – yellowish brown


Metallic Ore Minerals – Characteristic Streaks


Relative Density
Measured relative to an equal volume
of distilled water at 4 degrees centigrade.
1 litre = 1000g (1kg) 1 cubic centimetre = 1g
Controlled by the atomic weight of the constituent atoms
(chemical composition) and the packing (atomic structure)
A useful property for identifying metallic ore minerals,
these usually have relative densities over 5.0.
The only non-metallic mineral which
is quite dense is barytes (4.5)
Most of the silicate minerals have
densities between 2.5 and 3.2


Relative Density- Some Examples

Kyanite 3.5-3.7

Gold 12.0-20.0

Fluorite 3.2

Iron Pyrite 4.9-5.2

Haematite 4.9-5.3

Gypsum 2.3


Hardness
Measured on Moh’s scale from 1.0 (softest) to 10 (hardest)

Talc 1.0

Diamond 10.0

Scale was devised by measuring the amount of noise and
powder produced from rubbing a mineral on a metal file


Moh’s Scale of Hardness

10 Diamond
9 Corundum
8 Topaz
7 Quartz
6 Orthoclase Feldspar
Note diamond is over 30 x harder than corundum


Moh’s Scale of Hardness

10. Diamond

7. Quartz

9. Corundum

8. Topaz

6. Orthoclase Feldspar


Moh’s Scale of Hardness

5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
1 Talc
From 1 through to 9 on the scale, hardness increases in equal steps